Solar Panel Production Estimates 2026: How to Verify Installer Claims

Your installer handed you a proposal showing your new 10 kW solar system will produce 14,200 kWh per year and save you $1,900 on your electric bill. Before you sign that contract, you need to know: is that number accurate, optimistic, or deliberately inflated?

Production estimates are the single most manipulable number in a solar proposal. A 15–20% overestimate on a $30,000 system might cost you $3,000–$5,000 in expected savings over 25 years that never materialize. Unlike a panel efficiency spec or a cost-per-watt figure, production estimates are calculated using software and assumptions that differ significantly from installer to installer—and from optimistic to realistic.

This guide gives you the tools to verify any installer's production estimate before you sign, and to hold them accountable afterward. It is specifically distinct from the solar panel performance testing guide, which covers how to diagnose underperformance once your system is operating. This guide covers the pre-installation evaluation of whether installer estimates are reasonable in the first place.

Why Production Estimates Vary So Much

A solar production estimate is not a measurement—it's a calculation based on several inputs that vary by geography, roof, and installer choice:

Solar resource data: How many peak sun hours per day does your roof receive? Different datasets give different answers. PVWatts uses NASA satellite data (NSRDB). Aurora Solar uses lidar-derived shading analysis. Some installers use simplified regional averages. The difference between datasets can be 5–10% for the same address.

Derate factor: Real-world systems never achieve nameplate efficiency. Wiring losses, temperature losses, inverter efficiency, soiling, and shading all reduce output below the theoretical maximum. A realistic derate factor is 0.75–0.82. Some installers use 0.85–0.90 to show higher projected output. Every point difference affects the annual kWh estimate by approximately 1–1.5%.

Shading assumptions: Does the estimate account for the trees on the south side of your house? The chimney shadow in the afternoon? Shading is the most common source of overestimation, and it's invisible if the installer uses a simplified shade analysis rather than a proper sun-path tool.

Panel degradation: Year 1 production is always higher than Year 25. A realistic 25-year average should account for the 0.5%/year degradation rate, which means Year 1 is roughly 10–12% higher than the Year 25 figure. If an installer gives you a single production number without showing the year-by-year trajectory, ask which year it represents.

System losses beyond derate: Inverter clipping (when panels produce more DC power than the inverter can process), soiling losses not captured in the derate factor, and interconnection losses can each add another 1–4%.

Step 1: Run Your Own PVWatts Estimate

The NREL PVWatts Calculator is the industry's free, government-backed production modeling tool. It uses the same NASA NSRDB data that most professional software references. Running it yourself takes 5 minutes and gives you a credible independent benchmark.

How to use PVWatts:

1. Go to pvwatts.nrel.gov and enter your address
2. Click "Go to System Info"
3. Enter your system size in DC watts (e.g., 10 kW = 10.0)
4. Module type: Select "Monocrystalline Silicon" for standard TOPCon/HJT panels, or "Thin film" for First Solar CdTe
5. Array type: Select "Fixed (roof mount)" for a roof system
6. System losses: Start with 14% (corresponding to a 0.86 derate factor), which is realistic for a new clean system. Increase to 18–20% if significant shading exists.
7. Tilt: Use your actual roof pitch in degrees (12:12 = 45°, 6:12 = 26.6°, 4:12 = 18.4°)
8. Azimuth: 180° = true south, 90° = due east, 270° = due west

Interpreting the output: PVWatts gives you annual AC energy production in kWh. If your installer's estimate is within 5–8% of PVWatts, it's in the reasonable range. If the installer's estimate is more than 10–12% higher than PVWatts, ask them to explain every assumption that accounts for the difference.

Important caveat: PVWatts uses default loss assumptions. If your installer has done a proper shade analysis showing your system will have minimal shading and excellent sun exposure, their estimate can legitimately exceed PVWatts by a few percent. But the explanation should be specific, not vague.

Step 2: Check the Specific Yield Benchmark

Specific yield is production normalized by system size—annual kWh per kilowatt of installed DC capacity (kWh/kWp/year). It eliminates system size from the comparison, letting you evaluate whether the production intensity is reasonable for your location regardless of how large the system is.

Regional specific yield benchmarks for new systems (Year 1):

Location	Specific Yield Range
Phoenix, AZ	1,680–1,780 kWh/kWp
Las Vegas, NV	1,640–1,740 kWh/kWp
Albuquerque, NM	1,700–1,800 kWh/kWp
Los Angeles, CA	1,560–1,660 kWh/kWp
Dallas, TX	1,500–1,600 kWh/kWp
Denver, CO	1,480–1,580 kWh/kWp
Atlanta, GA	1,350–1,460 kWh/kWp
Charlotte, NC	1,310–1,430 kWh/kWp
Chicago, IL	1,200–1,320 kWh/kWp
New York, NY	1,180–1,300 kWh/kWp
Boston, MA	1,150–1,280 kWh/kWp
Portland, OR	1,000–1,120 kWh/kWp
Seattle, WA	960–1,080 kWh/kWp
Minneapolis, MN	1,180–1,300 kWh/kWp
Miami, FL	1,500–1,600 kWh/kWp
Honolulu, HI	1,580–1,700 kWh/kWp

How to calculate specific yield from your proposal: Proposed annual production (kWh) ÷ System size in kWp = Specific yield

If a Boston installer is claiming 1,500 kWh/kWp for a south-facing roof with no shading, that's materially above the benchmark and warrants scrutiny. If they're claiming 1,200 kWh/kWp for a partially shaded east-facing roof in Boston, that's reasonable.

Step 3: Verify the Shade Analysis

Shade is the most common source of inflated estimates. Even a system that shows 100% solar access in a simplified analysis can lose 8–15% of annual production to partial shading from chimneys, dormers, or neighboring trees.

What a proper shade analysis includes:

• A sun-path diagram showing the annual sun arc at your latitude
• Shade factor by month (winter shading is more severe at lower sun angles)
• Total shade-adjusted solar access percentage (TSRF)
• Which panels are affected and which are not

Tools that produce credible shade analyses:

• Aurora Solar (the industry standard): Creates a 3D site model from aerial imagery and calculates shade-adjusted production hour by hour. Ask for your Aurora shade report.
• SunEye/Solmetric: A handheld fisheye lens device that takes a photo from each panel location and calculates TSRF precisely.
• PVWatts shade calculator: The free alternative — you can manually input monthly shading percentages.

If an installer claims "no shading" for a roof with trees 30 feet to the south, that's a red flag. For anything but a completely unobstructed south-facing roof, request the specific TSRF percentage and ask which tool was used to calculate it.

Step 4: Understand the Year-Over-Year Trajectory

A single production number in a proposal is incomplete. Request the year-by-year production table that shows:

• Year 1 production
• Year 25 production
• The assumed degradation rate (should be 0.5%/year for standard panels, 0.25–0.35%/year for premium HJT/IBC)
• The 25-year lifetime production total

Why this matters: If an installer quotes you a single "12,500 kWh/year" figure, they might be using Year 1 production. Over 25 years at 0.5% degradation, the 25-year average is about 10.7% lower—roughly 11,160 kWh/year. For a $0.15/kWh electricity rate, that difference amounts to approximately $500–$600 in annual savings variance and $12,000–$14,000 over the system's life.

Also check: Does the proposal show electricity rate escalation assumptions? Most proposals assume 3–4% annual rate inflation. Ask what happens to the financials at 2% inflation vs. 5% inflation. If the payback only works at one specific rate scenario, the proposal is optimistic.

Step 5: Ask About the Production Guarantee

The key distinction is between an energy production guarantee and an equipment performance warranty. Most solar contracts include only equipment warranties—not energy production guarantees.

Equipment performance warranty: The panels will produce X% of their rated output after Y years (e.g., 87.4% after 25 years). This is a panel manufacturer guarantee, not an installer guarantee. It doesn't mean your system will hit a specific kWh target.

Energy production guarantee: The installer contractually commits to a minimum kWh production level, measured by the monitoring system, with a financial remedy if underperformance is documented over a defined period (typically 1 year or a rolling average).

Questions to ask:

"If my system produces 10% less than your estimate over the first 3 years after weather adjustment, what is your remedy?"

A reputable installer should be willing to offer either:

• A production guarantee with financial backstop (e.g., credit for missing kWh)
• A NABCEP-certified engineer's signature on the production estimate, indicating professional liability for the calculations

If an installer refuses to discuss underperformance remedies at all, that's a warning sign. The performance testing guide covers how to document a formal claim once the system is operating.

Step 6: Red Flags in a Solar Production Estimate

The estimate is 15%+ above PVWatts without explanation: Occasionally legitimate (e.g., exceptional south orientation, minimal shading, bifacial panels with high albedo ground surface), but always requires explanation. If the installer can't cite the specific inputs that account for the difference, assume the estimate is optimistic.

"We use our proprietary production model": While proprietary tools exist, refusing to provide PVWatts or Aurora-equivalent outputs is a flag. Reputable installers can cross-reference any estimate against standard tools.

A single production number with no year-by-year table: Ask for the full trajectory. One number without the degradation model is incomplete.

Offset percentage instead of kWh: "This system will offset 110% of your electricity" is a marketing statement, not a production estimate. Convert it to kWh and run the specific yield check.

Installer uses utility bill instead of measured usage: If your electric bill includes fixed charges, taxes, and fees that solar can't eliminate, using total bill as the baseline overstates the savings. Solar offsets the energy consumption portion of your bill—typically 55–75% of the total.

Summer-only installation timing bias: Installers sometimes quote from summer peak performance data. A system installed in July and showing "your system produced 1,500 kWh last month" is not representative of annual production.

The estimate went up during negotiation: If a production estimate increases in response to your negotiation on price (e.g., "we'll also revise the estimate up to 13,000 kWh"), that's a direct sign of estimate manipulation.

State-Specific Production Estimate Considerations

California (NEM 3.0): With export rates as low as $0.03–$0.08/kWh, the value of production depends heavily on when you consume it, not just how much you produce. Verify that the savings estimate reflects self-consumption optimization, not simple gross production multiplied by retail rate. The California solar incentives guide covers this in detail.

Massachusetts (SMART program): SMART pays per kWh produced, regardless of whether you consume it. This means the production estimate directly drives SMART income. A 10% overestimate means 10% less SMART income than projected over 10 years—a real dollar difference. Verify SMART income calculations separately from bill savings. See the Massachusetts solar guide.

Illinois (Illinois Shines): REC payments are based on actual measured production over 15 years. Overestimated production = overestimated REC income. The Illinois solar guide covers Shines payment calculation.

Indiana, Tennessee, Alabama, Mississippi (avoided-cost NEM): In states where exported solar earns only $0.03–$0.06/kWh, the production estimate's value depends on the self-consumption split. If an installer assumes 70% self-consumption and your actual self-consumption is 50%, the financial model breaks down significantly more than in retail-rate NEM states.

Hawaii (Smart Export): Export rate of $0.14–$0.20/kWh is well below retail ($0.40–$0.46/kWh). An estimate that doesn't distinguish self-consumption from exported production will significantly overstate financial value. See the Hawaii solar guide.

Texas (Utility-Dependent): Austin Energy's PVFIT pays per kWh produced; CPS's Value of Solar pays much less. Estimates for Austin Energy customers should show PVFIT income separately from bill savings. For CPS customers, self-consumption optimization matters as much as total production. See the Texas solar guide.

What to Ask Your Installer (Script)

Print these questions and use them in every sales appointment:

1. "What software did you use to generate this production estimate?" (Aurora, PVWatts, HelioScope, or proprietary)
2. "What derate/system loss factor did you apply?" (Acceptable: 14–20%; anything below 14% is optimistic)
3. "What was the TSRF percentage from your shade analysis?" (Ask for the shading report)
4. "Can you show me the year-by-year production table from Year 1 to Year 25?" (Should account for 0.5%/year degradation)
5. "What electricity rate escalation did you assume?" (3–4% annually is reasonable)
6. "What is your production guarantee? If the system underperforms by 10% after weather adjustment, what is your remedy?"
7. "Can I see your PVWatts output for this address and compare it to your estimate?"

If an installer refuses to answer any of these questions or becomes defensive, treat that as information about how they'll handle underperformance concerns after installation.

Quick Sanity Check: The 5-Minute Estimate Verification

If you don't have time for a full analysis:

1. Divide the estimated annual kWh by the system size in kW to get specific yield
2. Compare to the specific yield table for your city above
3. If the specific yield is more than 10% above the upper end of the benchmark range, ask for an explanation
4. Run PVWatts at pvwatts.nrel.gov with 14% losses, your roof tilt and azimuth, and system size
5. If the installer's estimate exceeds PVWatts by more than 10%, request the specific inputs that explain the difference

A legitimate installer should welcome this review. If they're defensive about showing their work, that tells you something important about how they'll treat you as a customer over the next 25 years.

Next Steps

Use these tools together for a complete pre-purchase evaluation:

• Solar ROI Calculator — Get a baseline expected savings estimate using our system size and state inputs
• Solar System Designer — Size your system correctly before comparing installer proposals
• How to Compare Solar Quotes — Normalize competing proposals across more dimensions than production alone
• Solar Performance Testing Guide — After installation, use this guide to verify your system is delivering as promised
• Solar Panel Monitoring Guide — Set up production tracking from Day 1 to catch underperformance early
• State Incentives Hub — Verify that state-specific incentive calculations (SMART, Illinois Shines, SREC) in your proposal are correct