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Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
Comparing solar proposals with different degradation rates and production guarantees requires looking past the glossy first-year estimates to see how the system performs over 25 to 30 years.
Here is a clear, structured way to evaluate and compare them:
Degradation Rate Analysis : This is the annual percentage by which a solar panel's energy production drops over time. Premium panels typically degrade at 0.3% - 0.5% per year, while budget panels can degrade at 0.7% - 1.0% or more. Calculate the cumulative loss over a 25-year lifespan—a 0.5% rate leaves you with roughly 88% of original production at year 25, whereas a 1.0% rate drops you to 75%.
Production Guarantee Terms : Check what the installer actually promises. A manufacturer's linear performance warranty guarantees a certain output level at year 25 (e.g., 84.8%), but a workmanship or production guarantee from the installer means they will write you a check or fix the system if it underperforms relative to their contract estimate. Prefer proposals where the installer backs their modeling with a real performance guarantee.
First-Year Production Estimates (kWh) : Dishonest or overly optimistic modeling can make a proposal with a high degradation rate look better than it is. Compare the estimated Year 1 kWh production across quotes using independent third-party tools like PVWatts Calculator to see if one installer is artificially inflating the baseline generation numbers.
Financial Net Present Value (NPV) : Plug the real degradation-adjusted production curves into your financial payback calculation. A cheaper system with higher degradation might underperform financially in years 15–25 compared to a slightly more expensive, high-efficiency system that holds its output steady.
Warranty Inclusions & Labor : A warranty is only as good as the company backing it. Look closely at whether the production guarantee covers both parts (the panels/inverters) and the labor required to swap them out if a failure or accelerated degradation occurs.
If you'd like, share the specific numbers from your competing proposals (like Year 1 production, proposed degradation percentages, and panel brands), and I can help you calculate the 25-year output difference between them.
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
ChatGPT Search· Sep 25, 2026
When comparing solar proposals, don’t compare only the headline “degradation rate” or “production guarantee.” Treat them as two different promises:
Degradation rate = the assumed or warranted decline in panel output over time.
Production guarantee = the installer’s or system provider’s promise that the whole system will produce a certain amount of electricity, usually under stated assumptions.
A good comparison process looks like this:
1. Put every proposal on the same timeline
Ask each installer for a year-by-year production table (for example, years 1, 10, 20, and 25 or 30).
A panel warranty might say something like:
Year 1: 98% of rated output
Years 2–25: maximum 0.4% annual degradation
Another might use a different structure. The important question is:
“How many kilowatt-hours (kWh) will this system actually produce over 25–30 years?”
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
ChatGPT Search· Sep 21, 2026
The key is to separate the proposal’s modeling assumption from the contractual guarantee. A lower projected degradation rate is valuable only if the rest of the proposal is comparable and the installer is actually guaranteeing enough production to matter.
1. Put both proposals on the same degradation basis
Ask each installer for:
First-year expected production in kWh.
Degradation in year 1, if any.
Annual degradation from year 2 onward.
Expected production in years 5, 10, 20, and 25/30.
The exact module model and its manufacturer performance warranty.
Whether degradation is modeled as linear or uses a stepped schedule.
Don't compare simply “0.25% vs. 0.5% degradation.” A proposal might assume, for example, a 2% first-year loss followed by 0.3% annually, while another uses a different first-year assumption.
As a reality check, DOE says module degradation is often around 0.5% per year, while NREL's large U.S. fleet analysis found a median performance loss of about 0.75%/year. More recent NREL lifetime testing has found some newer module technologies degrading at roughly 0.25–0.55%/year, so a particularly low quoted rate isn't automatically implausible—but you should ask what evidence and warranty support it.
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
Google AI Mode· Sep 17, 2026
Comparing solar proposals with different degradation rates and production guarantees can feel like comparing apples to spaceships. To make an apples-to-apples evaluation, look at the following structured approach:
Degradation Rate : This is the percentage by which a solar panel's electricity production drops each year (typically between 0.3% and 0.7% per year). A lower degradation rate means the system produces more energy over time. Calculate the cumulative production over 25 years using𝑃𝑛=𝑃0×(1−𝑟)𝑛 (where P₀ is initial year-one production, r is the degradation rate, and n is the year) to see how much total lifetime energy each proposal actually yields.
Production Guarantee vs. Product Warranty : Distinguish between a production performance guarantee (where the installer or manufacturer promises the system will produce a specific minimum amount of kWh, often backed by insurance or a cash payout if it underperforms) and a standard equipment warranty (which only covers manufacturing defects). A robust production guarantee shifts the financial risk of underperformance to the installer.
Year-One Baseline Production Estimates : Scrutinize the estimated first-year output. Some installers artificially inflate projected production or use optimistic weather models to make their lifetime savings look better on paper, even if their degradation rate looks good. Compare the specific solar insolation data and shading assumptions each company used.
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
ChatGPT Search· Sep 13, 2026
The key is to separate “what the proposal predicts” from “what the contract actually guarantees.” A low modeled degradation rate is valuable only if the equipment warranty and production guarantee support it.
1. Normalize the degradation assumptions
Ask each installer to provide:
First-year degradation.
Annual degradation after year one.
Guaranteed output at years 10, 20, 25, and 30.
Whether the rate is linear or a stepped warranty.
The exact panel model and manufacturer warranty supporting the assumption.
This matters because the first year can have a larger loss than subsequent years. For example, a warranty might assume 2–3% loss in year one and then 0.3–0.5% annually afterward.
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
Question: How should a homeowner compare solar proposals that differ on projected degradation rates and production guarantees?
ChatGPT Search· Sep 9, 2026
When comparing solar proposals, do not look at the degradation rate or production guarantee alone. Treat them as different promises:
Degradation rate = the assumed decline in panel output over time.
Production guarantee = the installer’s or provider’s promise that the installed system will generate a certain amount of energy (usually measured in kWh/year), often with remedies if it falls short.
A good comparison process:
1. Put every proposal on the same production timeline
Ask each installer for:
Year 1 estimated production (kWh)
Year 10 production
Year 25 production
Year 30 production (if applicable)
The assumptions used: - roof orientation and tilt
shading losses
weather dataset
A lower degradation rate only matters if it produces a meaningful lifetime energy difference.
2. Compare guaranteed end-of-life output, not just the annual degradation number
A difference between 0.25% and 0.50% annual degradation sounds small, but it compounds.
For example, after many years:
A system degrading at 0.25%/year retains more output than one degrading at 0.50%/year.
However, the value depends on the system size, electricity rates, and whether the higher-performing equipment costs more.
Look at:
Guaranteed output at year 25 (or year 30)
Whether the warranty is linear or stepped
The first-year degradation allowance
Many modern performance warranties specify a minimum retained output after 25–30 years; the exact warranty terms matter more than the marketing label.
3. Separate panel warranty from installer production guarantee
These are often confused.
Panel performance warranty
Usually covers that the panels will not fall below a certain output level.
Questions:
Who backs it—the panel manufacturer or installer?
How is output measured?
Who pays for testing?
Are labor and shipping covered if panels fail?
Production guarantee
Usually covers the entire installed system.
Questions:
Is the guarantee based on a simulation?
What assumptions are used for shading, weather, roof angle, and utility rules?
What happens if production falls short?
Is compensation a credit, refund, or system repair?
A strong panel warranty does not automatically mean a strong production guarantee, and vice versa.
4. Check whether the proposals use the same assumptions
Two installers may quote different annual production because they modeled:
Different shading losses
Different roof layouts
Different inverter efficiency assumptions
Different weather datasets
Different system sizes
Request:
System size (kW DC)
Expected first-year production (kWh)
Production assumptions
Shading analysis
Software/model used
Then compare kWh per installed kW:
kWh per kW=annual productionsystem size\text{kWh per kW} = \frac{\text{annual production}}{\text{system size}}
This helps normalize proposals.
5. Calculate the lifetime value difference
A simple comparison:
Lifetime energy = sum of annual production over the warranty period
Then estimate:
Lifetime value difference=(extra kWh)×(electricity price)\text{Lifetime value difference} = (\text{extra kWh}) \times (\text{electricity price})
Example:
Proposal A produces 5,000 extra kWh over 25 years
Electricity costs $0.18/kWh
Value difference:
5,000 × $0.18 = $900
If Proposal B costs $4,000 more because of a better degradation claim, the math may not justify it.
6. Look beyond degradation
Other factors can matter as much or more:
Installer reputation and workmanship warranty
Inverter warranty and replacement cost
Battery warranty (if included)
Roof condition before installation
Panel temperature performance
Monitoring and service terms
Transferability if you sell the home
7. Ask these questions before signing
“Show me the guaranteed kWh production for each year.”
“What degradation curve did you use?”
“Is the production guarantee backed by a contract with compensation?”
“What exactly happens if production misses the guarantee?”
“Who handles warranty claims 10 years from now?”
“Are labor costs covered if equipment must be replaced?”
A practical way to compare proposals is to make a spreadsheet with columns for:
Installed price
System size (kW)
Year-1 kWh estimate
25-year cumulative kWh
Degradation assumption
Year-25 guaranteed output
Production guarantee terms
Warranty coverage
That will usually reveal whether a better degradation number represents meaningful value or just a better-looking specification.
6. Look beyond degradation
Other factors can matter as much or more:
Installer reputation and workmanship warranty
Inverter warranty and replacement cost
Battery warranty (if included)
Roof condition before installation
Panel temperature performance
Monitoring and service terms
Transferability if you sell the home
7. Ask these questions before signing
“Show me the guaranteed kWh production for each year.”
“What degradation curve did you use?”
“Is the production guarantee backed by a contract with compensation?”
“What exactly happens if production misses the guarantee?”
“Who handles warranty claims 10 years from now?”
“Are labor costs covered if equipment must be replaced?”
A practical way to compare proposals is to make a spreadsheet with columns for:
Installed price
System size (kW)
Year-1 kWh estimate
25-year cumulative kWh
Degradation assumption
Year-25 guaranteed output
Production guarantee terms
Warranty coverage
That will usually reveal whether a better degradation number represents meaningful value or just a better-looking specification.
Comparing solar proposals with different projected degradation rates and production guarantees requires looking past the first-year estimates and modeling the long-term financial reality . Because solar panels lose efficiency over time, a minor difference in the assumed degradation percentage compounds over a 25-year lifespan.
Here is a structured, step-by-step framework to evaluate and compare these conflicting claims:
Normalize the First-Year Production (kWh)
Check if the estimated annual generation (kWh) per kW of installed capacity matches realistic local solar irradiance (use tools like PVWatts Calculator to benchmark estimates).
Ensure an aggressive production guarantee isn't masking an inflated first-year production estimate.
Scrutinize the Projected Degradation Rate (%/year)
Standard vs. Premium: Traditional panels degrade around 0.5% to 0.7% per year, while premium panels (like Maxeon/SunPower) claim as low as 0.2% to 0.3% per year.
Do the math: Over 25 years, a 0.5% linear degradation means the system operates at roughly 87.5% capacity by year 25, whereas a 0.3% rate retains around 92.5%. Calculate the cumulative lifetime kWh difference using:
T o t a l k W h≈Y e a r 1 k W h×∑2 5𝑡=1(1−𝑟)𝑡−1
where r is the degradation rate. Multiply that difference by your utility's projected escalation rate for electricity.
Evaluate the Teeth of the Production Guarantee
Read the fine print on what is actually guaranteed. A "25-year production guarantee" is useless if the remediation threshold is set too low or if the company goes out of business.
Check the baseline: Does the guarantee promise a specific cumulative output over 25 years, or a minimum percentage of the installer's (often inflated) initial estimate?
Verify how shortfalls are paid out (cash reimbursement vs. system repair/replacement) and whether labor is included.
Investigate Manufacturer vs. Installer Warranties
A guarantee backed solely by a local installer ( workmanship/performance guarantee) only has value if the installer stays in business. Look for a manufacturer-backed linear power performance warranty (typically 25 years) combined with a strong local reputation.
Cross-reference the panel manufacturer's tier ranking or financial stability.
Run a Levelized Cost of Energy (LCOE) or Net Present Value (NPV) Comparison
Plug the true lifetime generation (adjusted for each proposal's specific degradation rate) and total net cost (after incentives) into a financial comparison.
A proposal with a higher upfront cost but a lower degradation rate (0.3%) often yields a better Net Present Value over 25 years than a cheaper proposal that degrades quickly (0.7%).
If you can share the quoted degradation rates (e.g., 0.5% vs 0.3%) and first-year production estimates for each proposal, I can help you calculate the exact lifetime energy and financial difference.
2. Calculate the actual long-term production difference
Suppose two otherwise identical systems both produce 10,000 kWh in year 1:
Annual degradation
Year 10 production
Year 25 production
0.25%
~9,777 kWh
~9,414 kWh
0.50%
~9,558 kWh
~8,866 kWh
0.75%
~9,350 kWh
~8,355 kWh
The important number isn't the percentage itself; it's how many additional kWh the assumption produces over the life of the system and what those kWh are worth to you.
For a purchase decision, I'd ask both installers to provide a 25- or 30-year annual production table using identical assumptions. That makes optimistic modeling much easier to spot.
3. Treat a production guarantee differently from a degradation assumption
A projected degradation rate is essentially part of the installer's forecast.
A production guarantee is a contractual obligation—but only to the extent that the contract actually says what happens when production falls short.
Look for:
Guaranteed kWh in each year, or a clearly defined cumulative guarantee.
Guarantee duration.
What baseline/model is used to determine expected production.
Whether the guarantee accounts for weather.
How production is measured.
Minimum uptime/availability requirements.
What exclusions apply: shading, utility outages, force majeure, equipment failures, etc.
What you receive if the system misses the guarantee: cash, additional equipment, service, replacement, or something else.
Who pays for labor associated with a warranty claim.
DOE specifically recommends documenting system performance and having warranty arrangements that cover repair/replacement and the labor involved in addressing underperformance.
4. Check whether the guarantee actually covers system production
This is an easy place to get misled.
A panel manufacturer's performance warranty might guarantee that an individual module retains, say, a specified percentage of its original rated power after a certain number of years. That is not necessarily a guarantee that your rooftop system will produce a specified number of kWh.
System production is affected by orientation, shading, weather, inverter losses, wiring, downtime, soiling, and other factors. DOE notes that actual system performance depends on factors including location-specific solar availability and equipment age/degradation.
So ask:
“Is your production guarantee based on module nameplate power, modeled system output, or actual AC energy delivered by the system?”
The last one is generally much easier for a homeowner to understand and verify.
5. Look closely at the manufacturer's performance warranty
Compare the actual warranty documents—not just the sales proposal.
Some warranties use a linear degradation curve, while others have a higher initial loss followed by a lower annual rate. NREL has noted typical performance warranties historically around 90% at 12 years and 80% at 25 years, although newer products and warranties can differ substantially.
Also determine who stands behind the guarantee. A theoretically excellent warranty is less useful if making a claim is difficult or the installer is responsible for a contractual guarantee but has limited resources to honor it.
6. Compare the proposals using “guaranteed dollars,” not marketing percentages
A useful spreadsheet has columns for each proposal:
System
System size (kW)
Panel model
Inverter model
Battery, if applicable
Production
Year-1 kWh
Year-10 kWh
Year-25 kWh
25-year cumulative kWh
Assumed degradation
Guarantee
Guaranteed year-1 kWh
Guaranteed year-10 kWh
Guaranteed year-25 kWh
Guarantee duration
Remedy for shortfall
Exclusions
Measurement method
Economics
Net system cost
25-year electricity savings
Cost per expected kWh
Cost per guaranteed kWh
That last comparison is particularly useful: don't give full economic credit to production that exists only in the salesperson's projection.
One question I'd send to both installers
“Please provide the annual production guarantee and annual modeled production for years 1–30, including all first-year and subsequent degradation assumptions, and identify exactly what compensation or remediation I receive if actual AC production falls below the guaranteed amount.”
If you paste the two proposals' system size, year-1 production, degradation assumptions, guarantee language, price, and panel models, I can put them into a side-by-side comparison and calculate the long-term production difference.
A useful spreadsheet has columns for each proposal:
Financial Net Present Value (NPV) : Convert the differing lifetime energy outputs into financial terms. Divide the total net cost of the system (after incentives) by the total guaranteed or projected lifetime kilowatt-hours (kWh) to find the true levelized cost of energy (LCOE) or cost per kWh delivered over the life of the system.
Company Longevity and Stability : A 25-year production guarantee is only as good as the company backing it. Check whether the guarantee is provided directly by a local installer (who might go out of business) or backed by a tier-one manufacturer with robust third-party insurance backing.
If you'd like, share the specifics of your proposals:
The first-year production (kWh) and degradation rates for each option
Whether the guarantees are manufacturer-backed or installer-backed
I can help you calculate the lifetime cost per kWh for each proposal.
The key is to separate the panel’s assumed degradation from the installer’s contractual production guarantee. They answer different questions.
1. Normalize the degradation assumptions
Ask each installer to show the same production forecast using the same starting-year production.
For an annual degradation rate dd, approximate year-nn production as:
Pn=P1(1−d)n−1P_n=P_1(1-d)^{n-1}
For example, starting at 10,000 kWh/year:
0.3%/yr degradation: about 9,142 kWh in year 30
0.5%/yr: about 8,660 kWh
0.7%/yr: about 8,192 kWh
The differences compound over decades. NREL's residential-PV modeling currently uses 0.7%/year as a baseline assumption, while noting lower rates for some future scenarios.
But don't automatically treat a lower salesperson-provided degradation rate as superior. Ask what panel warranty and manufacturer documentation support it. Historical NREL material describes common module warranties around 80% of original power at 25 years and annual degradation assumptions around 0.5–0.7% in the cited context.
2. Focus heavily on what the production guarantee actually guarantees
A proposal might say:
"Estimated production: 12,000 kWh/year"
That's not necessarily a guarantee.
A contractual production guarantee should tell you:
The guaranteed kWh in each year or measurement period.
Whether the guarantee is based on AC electricity actually delivered, or some modeled value.
How production is measured.
What exclusions apply—shading, outages, grid failures, extreme weather, soiling, equipment failure, etc.
What happens if production falls short.
Whether the remedy is cash, a bill credit, replacement equipment, additional electricity, or something else.
Who pays for diagnosing and fixing an underperforming system.
California's CPUC specifically advises homeowners to ask whether there is a minimum energy guarantee and how they will be compensated if promised production isn't achieved.
3. Don't compare guarantees without comparing their baseline
Suppose:
Proposal A
Proposal B
Year-1 estimate
12,000 kWh
Assumed degradation
0.5%
Year-25 modeled output
~10,641
Contractual guarantee
10,000
Proposal A's lower degradation assumption doesn't necessarily make it the better contractual proposition. B's actual guarantee is more protective if its guarantee is enforceable and its measurement/remedy provisions are favorable.
In other words, compare guaranteed kWh, not just the advertised degradation percentage.
4. Ask for the manufacturer's actual warranty
Get the exact panel model number and request its current power-warranty document. Look for language such as:
Initial power/degradation allowance.
Guaranteed output at years 10, 20, 25 or 30.
Whether degradation is linear or has a first-year step-down plus subsequent annual degradation.
Who administers the warranty.
Whether labor/shipping/removal/reinstallation is covered.
A "30-year warranty" doesn't necessarily mean the manufacturer guarantees 30 years of labor or system-wide production.
5. Calculate cumulative production
This is often more useful than looking at year 25 alone.
Have both companies provide:
Year 1, Year 5, Year 10, Year 15, Year 20, Year 25 and Year 30 production, plus the cumulative kWh over the contract period.
Then compare:
Cost per guaranteed kWh=total amount you paycumulative guaranteed kWh\text{Cost per guaranteed kWh} = \frac{\text{total amount you pay}}{\text{cumulative guaranteed kWh}}
For a purchase, you can additionally calculate cost per projected kWh. For a lease/PPA, incorporate the actual payment structure and escalators rather than comparing the equipment price alone.
6. Watch for differences unrelated to degradation
Two proposals can have different production forecasts even with identical panels and degradation because of assumptions about:
Roof orientation and tilt
Shading
Inverter efficiency
DC-to-AC ratio
System downtime
Soiling
Temperature
Snow, if applicable
Panel layout and obstructions
Future tree growth
So ask both installers for their underlying production-model assumptions, not merely their headline annual kWh. California's CSLB specifically advises consumers to review the calculations used to determine how much energy the proposed system will generate.
A practical way to compare two proposals
I'd make a spreadsheet with these columns:
Item
Proposal A
Proposal B
System size (kW DC)
Year-1 projected kWh
Degradation assumption
Year-25 projected kWh
The most important question to put to each salesperson is:"Show me exactly where in the contract you guarantee this amount of electricity, what happens if you miss it, and calculate the cumulative guaranteed kWh over the entire contract."
If you're in California, the CPUC currently recommends getting multiple qualified bids and emphasizes that promises about performance and savings should be understood from the written contract rather than the sales presentation.
If you paste the two proposals' degradation rates, year-1 production, guarantee language, system sizes, prices, and warranty terms, I can put them into a side-by-side calculation and show exactly how much the degradation assumptions and guarantees differ.
Year 1, Year 5, Year 10, Year 15, Year 20, Year 25 and Year 30 production, plus the cumulative kWh over the contract period.
Then compare:
Cost per guaranteed kWh=total amount you paycumulative guaranteed kWh\text{Cost per guaranteed kWh} = \frac{\text{total amount you pay}}{\text{cumulative guaranteed kWh}}
For a purchase, you can additionally calculate cost per projected kWh. For a lease/PPA, incorporate the actual payment structure and escalators rather than comparing the equipment price alone.
6. Watch for differences unrelated to degradation
Two proposals can have different production forecasts even with identical panels and degradation because of assumptions about:
Roof orientation and tilt
Shading
Inverter efficiency
DC-to-AC ratio
System downtime
Soiling
Temperature
Snow, if applicable
Panel layout and obstructions
Future tree growth
So ask both installers for their underlying production-model assumptions, not merely their headline annual kWh. California's CSLB specifically advises consumers to review the calculations used to determine how much energy the proposed system will generate.
Don't compare "0.3% degradation" from one proposal directly with "0.5%" from another without checking those details.
2. Recalculate lifetime production yourself
Suppose two otherwise identical proposals start at 12,000 kWh/year:
Proposal A: 0.3% annual degradation
Proposal B: 0.7% annual degradation
Using a simple compound model, after 25 years the annual production would be roughly:
A: 11,130 kWh/year
B: 10,080 kWh/year
That's about a 1,050 kWh/year difference by year 25. Over the entire period, the cumulative difference is much larger.
But don't let the installer simply hand you a 25-year kWh number. Ask for the year-by-year production schedule and use the same degradation methodology for every proposal.
3. Give more weight to the production guarantee than the modeled degradation rate
This is the most important distinction.
A panel manufacturer's performance warranty generally promises that the panel will retain at least some percentage of its original rated output after a specified period. It isn't necessarily a promise that your whole solar system will generate a particular number of kWh.
Meanwhile, an installer may offer a system-level production guarantee, such as:
"The system will produce at least X kWh during year one, with compensation if it doesn't."
That's potentially much more valuable to the homeowner because it addresses actual system production, not merely panel nameplate output.
Ask exactly:
Is the guarantee in kWh, dollars, or percentage of estimated production?
How is the baseline production calculated?
Does it apply every year or only the first year?
Does the guaranteed amount decline with panel degradation?
What compensation do you receive for a shortfall?
Who pays for troubleshooting or repairs?
Are there exclusions for weather, utility outages, shading, inverter failures, monitoring failures, etc.?
What happens if the installer goes out of business?
4. Don't automatically choose the lowest degradation rate
Real-world PV-system degradation is influenced by more than the panel's warranty. A large U.S. fleet analysis from DOE/NREL found a median system performance loss of about 0.75% per year, while individual modules can have lower warranted rates.
So a proposal assuming 0.25% degradation isn't necessarily wrong—but I'd want to know why the proposal uses that number and whether it's consistent with the exact panel warranty.
Likewise, a 0.5–0.7% assumption isn't necessarily a bad proposal. NREL has historically cited roughly 0.5–0.7% annual degradation as a typical warranty assumption after the initial loss.
5. Compare the economics, not just kWh
Put each proposal into the same spreadsheet with:
Metric
Proposal A
Proposal B
System size (kW)
Year-1 production (kWh)
Year-1 degradation
Annual degradation thereafter
Year-25 guaranteed output
25-year cumulative production
System-level production guarantee
Compensation for shortfall
Panel performance warranty
Inverter warranty
Labor/workmanship warranty
Total installed cost
Price per expected lifetime kWh
Then calculate cost per lifetime kWh and, separately, the expected electricity savings under the same utility-rate assumptions.
My rule of thumb
If Proposal A says 0.25% degradation and Proposal B says 0.5%, but B has a substantially stronger, clearly written system-level production guarantee, I would not automatically pay extra for A's lower degradation assumption.
I'd favor the proposal where the numbers are:
Transparent — exact panel/model and calculation method.
Contractually backed — not merely a sales projection.
System-level where possible.
Comparable — same shading, weather data, azimuth, tilt, losses, and utility assumptions.
Economically meaningful — the extra lifetime production actually justifies any higher price.
One particularly useful question to send both installers is: “Please show me your guaranteed production in kWh for each year through year 25, the degradation formula used to calculate it, and the exact compensation I'll receive if actual production falls below that guarantee.”
That tends to expose very quickly whether you're comparing two genuinely different systems or simply two different sets of optimistic assumptions.
Metric
Proposal A
Proposal B
System size (kW)
Year-1 production (kWh)
Year-1 degradation
Annual degradation thereafter
Year-25 guaranteed output
25-year cumulative production
System-level production guarantee
Compensation for shortfall
Panel performance warranty
Inverter warranty
Labor/workmanship warranty
Total installed cost
Price per expected lifetime kWh
Then calculate cost per lifetime kWh and, separately, the expected electricity savings under the same utility-rate assumptions.
My rule of thumb
If Proposal A says 0.25% degradation and Proposal B says 0.5%, but B has a substantially stronger, clearly written system-level production guarantee, I would not automatically pay extra for A's lower degradation assumption.
Comparing solar proposals with different projected degradation rates and production guarantees requires looking past the first-year estimates and evaluating the long-term financial and performance risk . Because solar panels degrade over time, a minor difference in the assumed degradation percentage or warranty backing can compound into thousands of dollars over a 25-year lifespan.
Here is a step-by-step framework to compare these critical metrics effectively:
Normalize the baseline production: Request that all competing installers run their production models using the same third-party software (like PVWatts) and local weather data. This ensures differences in projected production come down to panel specs and layout design, rather than overly optimistic software settings by one vendor.
Evaluate the degradation rate percentage: Traditional panels degrade by about 0.5% to 0.7% per year, while premium panels (like Panasonic or Maxeon/SunPower) drop by 0.2% to 0.4% per year. Calculate the cumulative output at year 25 using the formula Y₂₅ = Year 1 Output × (1 - r)²⁴ (where r is the annual degradation rate) to see how much electricity each system actually produces by the end of its warranty.
Scrutinize the production guarantee (performance warranty): Check what percentage of original production the manufacturer or installer guarantees at year 25 (e.g., 80% vs. 92%). A lower degradation rate means little if the manufacturer's performance floor allows for steep drops without triggering a claim.
Distinguish between manufacturer vs. installer warranties: A performance guarantee backed by a Tier-1 panel manufacturer is safer than one backed solely by a local installer, who might go out of business in 10 years. Look for a 25-year product, labor, and performance warranty bundle.
Calculate the lifetime cost per kilowatt-hour (kWh): Divide the net system cost (after incentives) by the cumulative total kWh produced over 25 years factoring in the specific degradation rate. This reveals the true cost of energy, showing whether a higher upfront price for a lower-degradation system pays off over time.
If you'd like, let me know:
The degradation rates and year-25 guarantees listed in your current proposals
The cash or loan costs for each option
I can help you calculate the lifetime value and true cost per kWh for each system.
Maxeon Solar Technologies
Comparing solar proposals with different projected degradation rates and production guarantees can feel like comparing apples to oranges . To make an apples-to-apples evaluation, you need to look past the first-year estimates and analyze the long-term financial and performance risk.
Here is a step-by-step framework to evaluate and normalize these competing offers:
Normalizing Production Estimates: Ask both installers for their exact modeling software (such as PVWatts or Helioscope) and weather file assumptions. A company projecting higher production might just be using an overly optimistic weather year or shading assumption rather than superior equipment.
Evaluating Degradation Curves: Premium panels (like Panasonic or Maxeon) typically degrade at 0.25% - 0.3% per year, while budget panels might degrade at 0.5% - 0.7% per year. Calculate the cumulative loss over 25 years using the formula𝑃𝑛=𝑃1×(1−𝑑)𝑛 (where d is the annual degradation rate and n is the year). A steeper degradation curve means significantly less electricity production—and fewer solar renewable energy certificates (SRECs) or bill offsets—by year 15 through 25.
Scrutinizing the Production Guarantee: Read the fine print on what the guarantee actually covers. A true performance guarantee means the installer compensates you if the system falls short of the promised kWh output over a set period (e.g., 5 or 10 years). Check if the guarantee covers both equipment failure and lower-than-expected environmental generation, and verify whether the installer is backing it or if it relies entirely on a manufacturer's warranty (which rarely covers labor costs for replacement).
Calculating Lifetime Financial Impact: Multiply the projected annual production degradation curve by your current (and forecasted) utility electricity rate. Often, a slightly more expensive proposal with a lower degradation rate and a stronger production guarantee yields a higher lifetime net present value (NPV) and better return on investment than a cheaper upfront system that loses juice rapidly.
If you'd like, share the specifics of the proposals:
The equipment brands and models proposed
The offered degradation rates and guarantee terms for each
Your local utility rate
I can help you run a quick side-by-side comparison of their long-term value.
inverter efficiency
system availability assumptions
degradation assumption
A proposal that starts with higher production but assumes faster degradation may not be better than one with slightly lower Year 1 output and a stronger long-term curve.
2. Compare the actual end-of-life output, not just the degradation percentage
A difference like:
Proposal A: 0.25% annual degradation
Proposal B: 0.50% annual degradation
sounds small, but over decades it compounds. Also check the guaranteed output percentage at a specific year (for example, “at least 90% of original output after 25 years”). Performance warranties commonly specify both the degradation curve and the minimum retained output.
A useful question:
“How many more kWh does this lower degradation rate actually produce over 25–30 years?”
A premium degradation rate may only be worth a small amount if the upfront price difference is large.
3. Separate panel performance guarantees from system production guarantees
A panel warranty might say:
“The modules will retain at least X% of their rated power after 25 years.”
That does not mean:
“Your roof will produce X kWh per year for 25 years.”
The roof-level production guarantee depends on the entire system: inverter, installation quality, orientation, shading, monitoring, and maintenance.
Ask:
Is the guarantee for DC panel output or AC energy delivered to the home/grid?
Is it measured annually or averaged over multiple years?
What happens if production falls short?
Is compensation cash, credits, repairs, or just troubleshooting?
4. Examine the assumptions behind optimistic proposals
Some proposals look better because they assume:
unusually low shading
aggressive production modeling
low degradation
high system uptime
Ask for the modeling report or software output. A conservative proposal with realistic assumptions can outperform an optimistic one after installation.
5. Compare warranties beyond the degradation number
Look at:
Item
Questions to ask
Panel performance warranty
What output is guaranteed at year 25 or 30?
Equipment warranty
How long are panels, inverter, and batteries covered?
Labor coverage
Who pays for diagnosis and replacement labor?
Installer warranty
How long is workmanship covered?
Warranty process
Who handles claims if the installer disappears?
A strong panel warranty is helpful, but it does not replace a reliable installer and good workmanship coverage.
6. Convert differences into dollars
For a fair comparison:
Estimate lifetime energy from each proposal.
Multiply annual kWh by your expected electricity value (utility rate or export compensation).
Subtract the installed cost difference.
Example:
Proposal A produces 5,000 extra kWh over 30 years.
Your electricity value is $0.20/kWh.
Lifetime value difference = about $1,000.
If Proposal A costs $5,000 more, the better degradation rate alone probably does not justify the premium.
A simple scorecard
Factor
Weight
Realistic Year 1 production
High
Lifetime kWh estimate
High
Installer reputation and workmanship warranty
Very high
Equipment warranties
Medium-high
Degradation rate difference
Medium
Small efficiency differences
Low-medium
In many residential systems, installer quality, accurate shading analysis, and a realistic production estimate matter as much as (or more than) a few tenths of a percent difference in degradation rate.
If you have the two proposals’ numbers (system size, Year 1 kWh, degradation rate, guarantee terms, price), I can help compare them side by side.
PVWatts Calculator to see if one installer is artificially inflating the baseline generation numbers.
