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Calculator · updated October 7, 2026

How the solar quote checker works: $/W, PVWatts, payback

How the checker turns a price, a size and an installer's kWh estimate into dollars per watt, a PVWatts production ratio and a payback, and compares quotes.

A solar quote comes down to three numbers: the system size, the price, and the electricity the installer says it will make. The quote checker takes those three and runs the arithmetic below, with each benchmark and threshold written down.

Price per watt against NREL's cost benchmark

Price ÷ (kilowatts × 1,000) gives dollars per watt DC, the unit every installer and every study uses, because it makes a 6 kW quote comparable to a 12 kW one. The checker places that number against NREL's cost benchmark (Q1 2025), which models a turnkey 8 kW residential system from the bottom up: modules, inverters, racking, labor, permitting, sales and overhead. Its modeled market price is $2.95 per watt and its minimum sustainable price, the least a solvent installer can charge in a competitive market, is $2.78 (source). Both are cash prices before incentives.

The checker reads a quote's $/W in three bands: below NREL's floor (ask what is missing), at benchmark up to $3.25 (the market price plus 10% headroom for small systems, premium modules and hard roofs), and above benchmark, where it also shows how far above the market price the quote sits. A quote above the benchmark is not wrong on its own; it needs a reason, and a second quote for the same size is the way to test the reason.

Enter the cash price. If the quote is financed, ask for the cash price and the dealer fee separately. A financed price with the fee folded in will read "above benchmark" for reasons unrelated to the equipment.

Production estimate against PVWatts

The production estimate is the number your savings depend on, and it is the easiest one to overstate. The checker compares it with NREL's PVWatts for your ZIP: a physics model on typical-year weather, run for a south-facing roof at 25° tilt with a 14% system-loss allowance, then scaled by the orientation and shade you select. PVWatts already deducts soiling, wiring, mismatch, inverter and availability losses, so a competent design lands close to it and a very good one beats it by a few percent.

The bands follow from that. The installer's annual kWh divided by the roof-adjusted PVWatts figure is called conservative below 90% (fine if the roof is worse than you selected), plausible up to 108%, optimistic up to 120%, and very optimistic beyond that. An optimistic estimate can have a good reason, such as a better tilt, bifacial modules or a sunnier microclimate. It should come with a shade report and a production guarantee written for the same kWh.

Set orientation and shade as they are on your roof. If you select "no shade" for a roof under two oaks, the ratio will look poor for the wrong reason. The roof guide shows what each setting does.

Payback with PVWatts production

The checker runs the same formulas as the savings calculator, with two substitutions: the quoted size and price replace the bill-derived ones, and the production is PVWatts for your roof rather than the estimate on the quote. The line under the chart shows what break-even would be if the installer's number were right instead. When the two differ by years, the difference comes from the installer's estimate, not from the sun. Dollars are nominal and undiscounted, financing interest is ignored, and the federal residential credit is 0% for homeowner-owned systems installed in 2026 or later.

Worked example on the page: an 8 kW system (NREL's benchmark size) quoted at NREL's market price, $2.95 per watt, so $23,600; an installer estimate of 10,928 kWh a year, which equals PVWatts on a median Virginia roof (1,366 kWh per kW); and the calculator's default $180 bill. Each input comes from a published number.

Comparing two or three quotes

Compare mode runs the same checks on each quote and then looks across them. The rules are few, and each threshold is on the methodology page:

  • Price spread. Dearest $/W minus cheapest, as a share of the cheapest. Over 10% is more than roof and equipment differences normally explain, so the dearer installer gets the question.
  • Production on the same roof. Two quotes within 10% of each other in size are treated as designs for the same roof. If their kWh per kW differ by more than 8%, the headroom the checker allows above PVWatts, at least one estimate is off, and both installers get asked which tilt, orientation, shade and losses they assumed.
  • Mixed verdicts. One quote optimistic against PVWatts while another is plausible is a finding on its own: the optimistic one has to explain what its design assumes.
  • Different sizes. With no same-size pair, annual kWh stop being comparable, because a larger system may use worse roof planes. $/W and kWh per kW still compare.
  • Panel wattage. Modules 20 W or more apart (430 versus 450, say) usually mean a newer or more efficient panel, which can explain part of a price gap. The checker notes it and asks for the model; it does not score it.

The worked pair on the page: quote A is NREL's benchmark system at NREL's market price ($23,600 for 8 kW, 10,928 kWh a year); quote B is the same size priced 20% above NREL's market price ($28,320, twice the checker's headroom) with an estimate of 12,458 kWh, the middle of the checker's optimistic band. The checker reports a 20% price spread, a production spread for the same roof, and mixed verdicts. Every figure in the example is computed from the benchmarks.

Inverter and installation failures in NREL's reliability study

In the largest field study of U.S. residential systems, NREL and Lawrence Berkeley National Laboratory analyzed 100,000 systems installed 2009–2016 with 5 years of production and maintenance records each (Jordan et al., 2020). Inverters were the component that failed most often, on 4 to 6% of systems; module failures were rare, at 0.2%. Breakers, fuses, wiring, connectors and strings wired backwards showed up often in the first year, which the authors attribute to installation quality, since a properly installed system should need none of those replaced in year one. About 9% of systems had some hardware issue over the five years, while 80 to 90% produced within 10% of prediction or better.

That is why the questions list leads with the inverter and the electrical crew. A quote that is cheaper because of a shorter inverter warranty or an outsourced electrician is cheaper in the places most likely to cost you later, and none of that shows up in dollars per watt.

What the checker does not model

  • Batteries, panel upgrades, re-roofing. The benchmarks are for PV only. NREL's same benchmark puts a 13.5 kWh battery at about $1.64 more per watt of PV. Take those items out of the price before checking it.
  • Equipment and warranties. A quote above the benchmark with 25-year product warranties and a long inverter warranty may be the better buy. The checker reads price per watt, not equipment quality.
  • Financing. Interest, dealer fees and escalators in a lease or power-purchase agreement are outside the model.
  • The installer. Track record, licensing, who does the electrical work and who handles a service call years later. For that side of the comparison, this guide to comparing solar quotes has the checklist.

Every threshold above is listed on the methodology page, next to the dates the benchmarks were last reviewed.