Why Your Solar 'Savings' Calculator Was Wrong From Day One

By Olivier Beauchemin · Published 2026-05-15 · 6 min read

You worked out the payback the night you signed. The proposal had a savings number on it, that number is the reason you said yes, and three years later your electricity bill looks nothing like it. You're not imagining that, and you didn't misread the paperwork. The number was optimistic in three predictable ways, none of them malicious, all of them baked in before anyone climbed onto your roof. The good news is that you now have three years of your own data, and that data can settle every one of them.

What that number on your proposal actually was

You've probably still got the proposal in a drawer somewhere. There's a line on it that reads something like "Estimated 25-year savings: $43,200" or "You'll save $1,728 in year one." That line did the selling. It's the output of a calculator that took roughly these inputs:

• Your roof's square footage, tilt, and azimuth (often eyeballed from satellite imagery).
• A modeled production estimate, drawn from PVWatts or a vendor equivalent.
• Your current utility rate (averaged from a recent bill).
• An assumed annual rate increase (a figure that varies by proposal; 3–5% is a range that appeared commonly in the 2015–2022 boom era).
• An assumed annual degradation (often 0.5%, sometimes silently 0%).
• Your federal Investment Tax Credit, plus any state or utility incentives applicable at the time of signing.

Out of that comes a multi-decade projection. The math itself isn't wrong. It's just very sensitive to the assumptions you feed it, and the assumptions that got chosen are almost always the ones producing the bigger number. Solar isn't special in that respect, because any industry selling a multi-decade financial commitment does the same thing. What makes solar different is that the equipment is physically on your roof, so the gap between the promise and the reality is auditable. You can check it. Almost nobody does, because nobody hands you the tools to.

The three places it went optimistic

1. Weather wasn't modeled honestly

This is the least obvious of the three, and it's often the one costing you the most.

PVWatts can be run two ways. The honest way uses your specific location's TMY data. TMY stands for Typical Meteorological Year, a synthetic dataset per NREL derived from roughly 30 years of measured solar irradiance and cloud cover at the nearest weather station, smoothed into an "average year" you can expect over time. The other way uses a clear-sky model, which is what your system would make if every single day were cloudless.

If your installer's proposal showed numbers that match what clear-sky modeling predicts, it overstated your real production by 5–15% depending on your climate. Clear-sky is an optimistic upper bound. A system modeled against it will consistently beat the projection only in high-desert climates. Most US residential markets run 5–20% below clear-sky on an annual basis, depending on typical cloud cover and aerosol loading for that location.

In fairness, most reputable installers do use TMY data. But TMY is itself a 30-year average, and the last decade's weather hasn't been the 30-year average. Reports from the 2023 wildfire smoke season documented California production reductions of 7–11% in affected regions during smoke events. The 2024 polar vortex pattern increased Northeast winter cloud cover. Those aren't edge cases anymore. They happen every year now, and your proposal has no way to know about them.

2. Degradation was understated

This one creeps instead of jumping, which is why you never caught it on a monthly bill. Most calculators assume 0.5% per year of capacity loss. That's the industry-standard linear degradation rate, and it's roughly correct on average. The trouble is the word "linear," because real degradation isn't.

Most silicon panels lose 1–3% in their first year due to light-induced degradation (LID) burn-in, a range documented in NREL field studies, then settle into 0.5% per year for years 2–25, before accelerating again in the last decade as encapsulant materials break down. If the calculator showed you a flat 0.5% line, it quietly assumed your panels are immortal in year 1 and still youthful in year 25. Real-world cumulative loss at year 10 is closer to 7–10%, rather than 5%.

You can check this part yourself in about five minutes. Your panel's warranty document guarantees something like "at least 80% of original capacity after 25 years." Work backward from that and you land on roughly 0.9% per year, not the 0.5% in the sales math. The manufacturer and the calculator were never telling you the same story.

3. Tariff escalation was a guess that didn't age well

This is the one that looks like it went in your favour. The calculator multiplied your year-one savings by an assumed annual rate increase to project lifetime savings. Common assumption: 3% per year. The actual U.S. residential retail electricity price grew at 4.5% in 2023 and roughly 5.5% in 2024 (EIA EPM Table 5.6.A). For most of the country that means the savings estimate is now actually LOW, not high.

Which sounds like a win. In practice it isn't, because the same grid pressures pushing rates up are also pushing utilities to restructure net metering. California's NEM 3.0 cut export credits by roughly 75% for new systems. Massachusetts's SMART program ramped down. New Jersey transitioned from SREC II to SuSI with materially different economics. Every one of those changes invalidated a slice of the projection you were shown.

What to do with the numbers you actually have

Three years in, you don't have to estimate any of this. You have real data, and it settles the question. This is the order I'd work through:

1. Pull your actual production data from Enphase Enlighten or your SolarEdge portal. Sum the kWh for each calendar year since install.
2. Compare against modeled expected production for the same period, using pvlib's PVWatts with your system's actual specs, your latitude, your tilt and azimuth, and (importantly) measured weather for the year, not clear-sky. This is what OwlWatt does.
3. Compute the cumulative shortfall in kWh.
4. Multiply by your actual blended utility rate over the same period (your bills will show this; the EIA number is a fallback).
5. Compare the resulting number against the savings number on your original proposal. If the gap is more than 10–15%, you probably have a documentable underperformance.

What the gap is worth in real money

Step 4 is where this stops being an argument about modeling and turns into cash, so let's put numbers on it. Run this with your own figures: your own estimated annual production, and the rate straight off your own bill. I'm using a system expected to make 10,000 kWh in a year at $0.30/kWh purely as an illustration.

OwlWatt costs $9.99 a month, or $87.89 a year. Even the top row, a 5% shortfall no homeowner would ever spot by eye, works out to $150 a year on that example system, which is more than the subscription. At 10% you're looking at roughly three and a half times what the watching costs.

Now the part I want to be straight with you about. OwlWatt doesn't recover that money for you. It measures the gap and documents it, month after month, in a form you can actually hand to somebody. Whether a claim goes anywhere depends on what your contract says and, if it gets that far, on a licensed attorney in your state who can use the documentation. Nobody can promise you a dollar figure back. And your honest result might be that there's no meaningful gap at all, which is an answer worth having as well. Knowing your roof is fine beats spending another three years half-wondering.

What the calculator wasn't designed to tell you

The savings calculator was a sales document. Its job was to get your financing math to close, and it did that job. It wasn't built to be the basis for a warranty claim three years later, and it isn't the document to bring to a dispute with your installer or to an attorney.

What you need instead is a production audit: measured production, modeled expected production using the same parameters your installer used, cumulative shortfall in kWh, the dollar value at your tariff rate, and the contract clause that defines the guarantee. That's roughly what we build, and we build it independently. We don't install systems, we don't sell equipment, and we take no referral fees from installers, so the number lands wherever the data puts it. You can also assemble the same thing yourself in a spreadsheet if you've got a free weekend, the patience to read your contract closely, and a working install of pvlib.

Either way, the number on the original proposal isn't the number that decides whether your system was a good deal. Three years of your own production data is.

Further reading

• What a solar production guarantee actually obligates your installer to do
• How accelerated panel degradation looks in your production data
• Verifying your utility bill against your solar production
• OwlWatt's full measurement methodology