Solar proposal evaluation methodology for homeowners

Comparing solar proposals with different degradation rates and production guarantees requires looking past the first-year estimates and evaluating the long-term financial and operational reliability of each system.

Here is a structured, scannable guide to comparing these critical metrics:

• Understand Projected Degradation Rates : Degradation is the natural loss of efficiency over time as solar panels age.
  • Industry Standards : Standard monocrystalline panels typically degrade between 0.5% and 0.7% per year. Premium panels might claim 0.3% to 0.4%.
  • The Impact : A lower degradation rate means the system produces more electricity in years 10, 20, and 25. When evaluating proposals, check if the installer uses a flat rate or a compounding annual loss formula. A difference of 0.2% per year adds up to significant kilowatt-hours (kWh) lost or gained over a 25-year lifespan.
  • Realism Check : Be wary of proposals boasting unrealistically low degradation rates unless backed by top-tier manufacturer warranties.
• Analyze Production Guarantees : A production guarantee ensures that if the system underperforms relative to its contractual promise, the installer will compensate you for the shortfall.
  • Scope and Duration : Look at how many years the guarantee covers (e.g., 5, 10, or 25 years). A guarantee that covers the first 10 years is much stronger than one covering only the first year.
  • Financial Backing : A guarantee is only as good as the company standing behind it. Check if the guarantee is provided directly by the local installer (who might go out of business) or backed by a third-party manufacturer or insurance policy.
  • Measurement and Payout : Read the fine print to see how shortfalls are measured and paid out. Does the installer monitor the system remotely and automatically issue a check if production falls below the threshold? Is there a minimum dollar amount required before they pay out?
• Normalize the Financial Models : Installers often project savings using their own assumptions about degradation and future utility rate increases.
  • Re-run the numbers using a standardized degradation rate (e.g., use 0.5% for all proposals) to see how the lifetime savings actually compare on an apples-to-apples basis.
  • Calculate the levelized cost of energy (LCOE) or total cumulative savings over 25 years rather than focusing solely on the year-one utility bill offset.
• Evaluate Equipment Quality : The hardware itself dictates both degradation and warranty backing.
  • Compare the specific panel and inverter brands across proposals. Look up the manufacturer's official linear performance warranty to see if it matches or contradicts the installer’s projected degradation rate.

If you'd like, let me know:

• The degradation rates and guarantee lengths listed in your current proposals
• The equipment brands (panels and inverters) being offered

I can help you break down how these differences will impact your 25-year financial return.

The most important principle is to compare the actual electricity each proposal is expected to deliver over 25 years and, more importantly, what the installer is contractually obligated to do if production falls short.

A proposal with a lower projected degradation rate isn't automatically better. Likewise, a higher production guarantee isn't necessarily stronger if it has extensive exclusions or weak compensation provisions.

Here's how I would evaluate competing proposals.

1. Separate the three different performance figures

Solar proposals often combine three different concepts that should be evaluated independently.

For example, an installer might project 0.4% annual degradation but offer no system-level production guarantee.

Another might project 0.6% degradation but guarantee annual system production and compensate you if it falls short.

The second proposal could provide better financial protection.

2. Normalize the degradation assumptions

Ask both installers to provide:

• First-year production in kWh.
• First-year degradation, if applicable.
• Annual degradation after year one.
• Expected production in years 5, 10, 15, 20 and 25.
• Manufacturer's guaranteed panel output at year 25.

For a simplified example, assume both systems produce 10,000 kWh in year one.

ILLUSTRATIVE COMPARISON · SAME INITIAL PRODUCTION

Simplified model using constant annual compounding, with no additional first-year loss.

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The difference becomes more meaningful over time.

However, make sure both companies are using the same degradation convention. Some models include an initial first-year reduction followed by a separate annual degradation rate.

Also, don't apply degradation twice if the production estimates already account for it.

3. Evaluate the actual production guarantee

This is arguably the most important part of the comparison.

Ask each installer:

A. What exactly is guaranteed?

• Is it annual production in kWh?
• Is it a percentage of modeled production?
• Is it a cumulative production guarantee?
• Does the guarantee decline each year?
• Does it cover the entire system or just the panels?

B. What happens if production falls short?

Look for specific contractual remedies:

• Cash reimbursement for missing production.
• Repair or replacement of defective equipment.
• Additional compensation based on the production shortfall.
• A combination of repairs and financial compensation.

A promise to investigate underperformance is not equivalent to a promise to compensate you.

C. What exclusions apply?

Pay particular attention to:

• Unusually cloudy weather.
• Utility outages.
• Equipment failures.
• Shade from growing trees.
• Homeowner maintenance obligations.
• Monitoring-system failures.
• Changes to the solar array.

A guarantee with numerous exclusions may provide substantially less protection than its headline percentage suggests.

4. Compare the guarantees side by side

Here's an example of how I would organize competing proposals.

(Potentially stronger protection)

Proposal B may be more attractive despite its higher degradation assumption because its production guarantee is stronger and includes financial compensation.

However, I would not select it until confirming the guarantee's calculation methodology, exclusions, and payment terms.

A 95% guarantee with broad exclusions could be weaker than a 90% guarantee with a clearly enforceable remedy.

5. Calculate lifetime financial value, not just lifetime electricity

A useful comparison should consider:

1. Total projected lifetime electricity production.
2. Value of electricity consumed directly by the household.
3. Compensation or credits for exported electricity.
4. Utility rate assumptions.
5. Financing costs and escalation clauses.
6. Equipment replacement and maintenance expenses.

One important distinction: A production guarantee is not a savings guarantee.

Even if the system generates the promised electricity, your actual savings depend on utility rates, fixed charges, net-metering arrangements, and when you consume electricity.

The Department of Energy and Virginia Energy both emphasize examining system design, production estimates, warranties, and financial assumptions when evaluating solar proposals.www.energy.gov+2

6. Questions I would insist each installer answer in writing

Before signing, request answers to these questions:

1. Is the degradation rate based on the specific panel model being installed?
2. Does the production forecast incorporate degradation and first-year losses?
3. What are the expected annual kWh figures for years 1–25?
4. Is the production guarantee based on modeled production or actual annual kWh?
5. Does the guarantee cover inverter failures and other system components?
6. What compensation is payable for every kWh of guaranteed production missed?
7. Who is financially responsible for honoring the guarantee?
8. What happens if the installer or financing provider goes out of business?
9. Is the production guarantee transferable if I sell my home?
10. Does the guarantee require the homeowner to pay for monitoring or maintenance?

Get the answers incorporated into the contract or an enforceable written addendum, not just a salesperson's email or presentation.

7. My recommended decision framework

I would rank the comparison criteria in this order:

For additional independent guidance, consult:

• U.S. Department of Energy – Consumer Solar Guide
• Virginia Energy – Solar Buying Guidance
• NREL – Homeowners Guide to Leasing Solar

8. A practical next step

If you have two or three actual solar proposals, I can compare them using a standardized 25-year analysis.

I can calculate:

• Projected annual and cumulative electricity production.
• The financial impact of different degradation rates.
• The actual value of each production guarantee.
• The differences in contractual protection.
• Which proposal offers the strongest overall financial value.

You can upload the proposals (PDFs are fine), and I'll build a side-by-side comparison rather than relying on the installers' advertised performance figures.

The standard method is to compare solar bids on an apples-to-apples, 25-year (or longer) lifecycle basis, separating the system’s expected energy production from the legal guarantees that protect you if it underperforms.

The key principle: don't automatically choose the lowest degradation percentage or the highest production guarantee. A credible production estimate, a clearly defined warranty, and an enforceable remedy matter more than an attractive headline number.

The National Renewable Energy Laboratory (NREL) and U.S. Department of Energy provide useful benchmarks for understanding degradation, production uncertainty, and solar-system performance. NREL's research puts median observed system performance loss at about 0.75% annually, while actual results vary by system and conditions.www.energy.gov+1

1. Separate the three numbers in each bid

One important distinction: a panel performance warranty is not necessarily a whole-system energy guarantee. A panel manufacturer might guarantee a percentage of original panel power after 25 years, while an installer might promise a certain amount of annual electricity. Those are different commitments.www.energysage.com+2

2. Normalize degradation over the same time period

A quoted degradation rate is only comparable if each bidder uses the same starting point, time period, and calculation method.

For a simple model of annual compounding:

P_n=P_1(1-d)^{n-1}

where P_1 is first-year production, d is the annual degradation rate, and n is the year.

Here's what different rates mean over 25 years, assuming the same first-year production:

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These are illustrative calculations, not predictions for any specific product. Actual manufacturer warranties may use a different structure, such as an initial first-year loss followed by a lower annual degradation rate.

For context, NREL reports that solar modules often lose less than 1% of their performance annually, and its residential PV modeling assumptions use 0.7% per year.www.nlr.gov+1

Practical takeaway: A 0.3% rate is more favorable than 0.75% all else being equal, but only if the equipment specification and warranty substantiate it. Don't confuse a manufacturer's guaranteed maximum degradation with the installer's forecast of actual system performance.

3. Put all bids on a common production basis

Ask each installer for the following:

• First-year expected AC electricity production in kWh.
• Expected annual production for years 1–25, including degradation.
• System size in kW DC and, where available, inverter capacity in kW AC.
• Shading, orientation, tilt, equipment-loss and downtime assumptions.
• The underlying production model and weather data.
• A conservative production estimate, if available, such as P90.

The U.S. Department of Energy recommends using NREL's PVWatts Calculator as an independent starting point for checking production estimates. Use consistent system inputs when comparing proposals; PVWatts is a screening tool, not a substitute for a site-specific engineering analysis.www.energy.gov+1

P90 is also useful when available: it represents an annual energy estimate expected to be met or exceeded in 90% of modeled outcomes, given the model's assumptions. It is not a guarantee that your system will produce that much every year.samrepo.nlr.gov+1

4. Evaluate the production guarantee as a contract

A strong-looking guarantee can be weak protection if it has restrictive terms. Compare these items side by side.

Read the exact warranty documents, not just the sales proposal. Manufacturer warranties, workmanship warranties, and whole-system production guarantees cover different risks. NREL also notes that warranty conditions and the cost of enforcing claims can affect their practical value.www.nrel.gov+1

5. Compare the lifetime economics

Use the same assumptions for utility-rate escalation, self-consumption, export compensation, maintenance, inverter replacement, financing, and system life.

A useful metric is the levelized cost of electricity (LCOE):

\text{LCOE}=
\frac{\text{Lifetime costs, discounted}}
{\text{Lifetime electricity, discounted}}

This estimates the cost per kWh over the system's life. Also compare net present value and expected utility-bill savings under the same rate assumptions. A bid with slightly higher degradation could still be the better investment if it costs substantially less or has better warranty protection.

6. Use a standardized bid scorecard

Customize your bid evaluation

Suggested weights; adjust them to reflect your priorities. Total weight: %.

[Copy scorecard weights]

Score each bid from 1 to 5 in each category, based on verified proposal details rather than marketing claims. Multiply each score by its category weight and add the results. Treat a missing or ambiguous contractual commitment as a reason to request clarification, not as proof that the bid is equivalent to a better-documented offer.

Recommended next step

Ask every installer to provide the same 25-year production table and the complete terms of any production guarantee. Then independently check the production assumptions using NREL PVWatts and compare the expected lifetime cost per kWh.

The best bid is usually the one with credible production, reasonable degradation assumptions, transparent and enforceable guarantees, and the strongest overall economics—not necessarily the one advertising the lowest degradation rate.

Sources

• U.S. Department of Energy,Homeowner's Guide to Solar
• National Laboratory of the Rockies,Photovoltaic Lifetime Project
• NREL,System Advisor Model: P50/P90 Simulations
• U.S. Department of Energy,Choosing a Solar Installer