The median installer year-1 production estimate runs 8.4% high.

The first-year kWh estimate anchors everything downstream: the bill-savings calculation, the payback math, and, crucially, the financed amount on the loan. So when we tell homeowners the median estimate in our 312-proposal audit set ran 8.4% high, the first question is always the same: are installers cheating?

Mostly, no. They're using defaults.

What the number models.

The kWh estimate is the output of a physics simulation. PVWatts is the most common — an NREL-built model that takes your system's vitals (size, tilt, compass direction, panel type) and a file of "typical" local weather, and returns expected hourly production. The simulation is good. The defaults are bad.

Three places where the defaults run high:

Soiling. The PVWatts default is 2%. Real soiling losses in dusty climates — Arizona, Texas, the Central Valley — are 4–7%. Coastal Florida, with regular rainfall, is closer to the default. Almost no proposal we've audited used a region-specific soiling figure.

Snow loss. Defaults to zero in many tools. New Jersey and Massachusetts roofs lose 3–6% annually to snow cover. Installers in Boston are running the zero-snow default.

TMY weather files. A "typical meteorological year" is a composite stitched from three decades of records — a greatest-hits album of ordinary weather. Slow trends in cloud cover and atmospheric haze never chart: recent years run consistently dimmer than the "typical" year in some regions and brighter in others. Recent California summers, for example, are 4–8% sunnier than the TMY would predict.

"The estimate is a default that nobody updated, multiplied by an installer with no incentive to update it."

What we found.

For each of the 312 proposals, we had monitored year-one production from the customer's inverter. We computed proposal-vs-actual for first-year kWh and recorded the system's location, tilt, azimuth, soiling region, and snow zone.

Median overstatement: 8.4%. One proposal in ten ran conservative, understating by 1.6% or more (P10: −1.6%). One in ten overstated by 14.7% or more (P90: +14.7%). The misses pile up on the high side; random measurement noise would land evenly around zero. The lopsidedness points at something structural.

The skew comes mostly from the structural defaults. Snow-zone systems missed by an average of 4.8% on the snow line alone. Dusty-climate systems missed by 3.1% on soiling. Together those two factors account for about 60% of the spread.

What an independent estimate looks like.

We re-run every proposal through our own model — the same physics family as PVWatts, but with fresher weather data (NSRDB in the US, PVGIS in Europe) and region-specific loss defaults. When the proposal's stated first-year kWh materially exceeds our forecast, something is wrong with its loss stack — usually the soiling, snow, or shading assumption — and we flag it for a manual review.

The fix is mundane: regional defaults plus an independent re-forecast as a sanity check. That's table stakes for any planning tool that wants to call its number a forecast rather than a quote. Most installer software is still running national defaults from 2014. The number on your proposal is exactly as accurate as the lowest-effort version of that math allows.

If you have a quote, ask which soiling factor, which snow factor, and which TMY vintage. A salesperson who can't answer is telling you the number came straight from the software defaults.