PVWatts calculator: what to enter and how to read the result

→Not sure where to start? Take the Solar Check.About 2 minutes. Rough answers are fine.

The PVWatts calculator, at pvwatts.nlr.gov, is a tool from the National Laboratory of the Rockies (NLR, formerly NREL). It estimates how many kilowatt-hours a grid-tied solar system at your address would make in a typical year, month by month. Besides your address, it needs six inputs: the system size in DC kilowatts, a module type, an array type, a system-loss percentage, the tilt and the azimuth (the direction the panels face). The result is a production estimate. It doesn't tell you what the system is worth on your utility bill.

What the PVWatts calculator does

The lab's Get Started help says PVWatts "estimates the monthly and annual electricity production of a photovoltaic system using an hour-by-hour simulation over a period of one year." It runs that simulation on weather data for your location, then adds up the hours.

The lab is upfront about the tool's limits. The same page calls it "suitable for very preliminary studies" and says that if you're designing a system with it, "you should work with a qualified professional to make final design decisions." The Department of Energy's homeowner guide says much the same thing: tools like PVWatts are "great for getting started," but you should "work with a solar installer for a custom estimate."

That makes it a good second opinion on production, which is the number every solar quote is built on. It is not a design.

Step 1: location and weather data

Type a street address, a ZIP code or a latitude and longitude, then click GO. PVWatts matches your location to typical-year weather data from the lab's National Solar Radiation Database, one year of hourly data meant to stand for a longer period, according to its solar resource help. PVWatts shows which data cell it used and how far it is from your address. You can't upload your own weather file.

Step 2: the six system inputs, box by box

DC system size. This is the panels' rating at standard test conditions, in kilowatts. The system size help gives the formula: module nameplate watts × number of modules ÷ 1,000. Twenty 400-watt panels come to 8 kW (our arithmetic). Use the DC figure, not the inverter's AC rating. For a rough size based on your usage, see how many solar panels you need.

Module type. PVWatts gives three choices: Standard (about 19% efficient), Premium (about 21%) and Thin film (about 18%), per its module type help. The lab says to "use the default Standard module type" if you don't know the panels; if you do, match the efficiency on the panel's spec sheet to the closest type.

Array type. Pick "Fixed (roof mount)." The array type help says roof mount "is typical of residential installations." It runs the panels a little hotter than open rack, which is meant for ground-mounted systems. Leave the tracking options alone unless the panels are on a ground tracker.

System losses. Leave it at 14% unless you know your roof is shaded. The losses help builds that number from categories like soiling, shading, mismatch, wiring, light-induced degradation and availability. It warns against adding inverter or temperature losses, because PVWatts models those separately.

Tilt. For a roof array, the tilt help says you "may want to choose a tilt angle equal to the roof pitch", and converts pitch to degrees: 4/12 is 18.4°, 6/12 is 26.6°, 8/12 is 33.7° and 12/12 is 45°. The default for a fixed array is 20°. Our guide to the best angle for solar panels covers what tilt changes.

Azimuth. This is the compass direction the panels face, measured clockwise from true north: 90° east, 180° south, 270° west, per the azimuth help. South is the default in the northern hemisphere because it "typically" gives the most output over the year.

Step 3: the advanced inputs (usually leave them)

The lab's System Info help says that "for most analyses, you can ignore the advanced parameters and use their default values."

• DC to AC size ratio, default 1.2. That's the panels' DC rating divided by the inverter's AC rating. The ratio help calls 1.20 "reasonable for most systems" and gives a typical range of 1.10 to 1.25. If the quote lists both numbers, divide one by the other: 8 kW of panels on a 6.4 kW inverter is 1.25 (our arithmetic).
• Inverter efficiency, default 96%. The PVWatts V8 API documentation allows values from 90 to 99.5. The inverter's spec sheet lists its rated efficiency.
• Ground coverage ratio, albedo, bifacial and soiling. The lab's system design help says the ground coverage ratio "does not apply to the fixed roof mount" array type. Bifaciality matters only for two-sided (bifacial) panels, and the monthly soiling input is for losses that change with the seasons. A standard roof system can leave them blank.

How to read the PVWatts results

The big number at the top, in kWh per year, is AC energy, the kind your house actually uses. The results help says these figures "do not represent the quantity of electricity that the system generates in a particular year." They represent "the typical electric production that you can expect" over the years the weather data covers.

Here's a worked example. On 2026-11-06 we ran the lab's PVWatts V8 API for central Denver (39.74, -104.99): 7 kW DC, standard modules, fixed roof mount, 14% losses, 25° tilt, south-facing (180°), DC to AC ratio 1.2 and inverter efficiency 96%. The weather file it used was the NSRDB 2020 typical year.

The ±10% annual swing the lab warns about puts a normal year for this system somewhere around 10,245 to 12,522 kWh (our arithmetic). Once you add the model's own uncertainty, the cautions page says individual years can differ by "up to 20%." Roof direction matters too. A quote that puts panels on a west face should show a lower estimate than a south face would. For what the same model shows across U.S. cities, see how much energy a solar panel produces.

The capacity factor is annual output divided by what the system would make running at full DC rating every hour of the year. The API documentation defines it the same way.

What the PVWatts calculator leaves out

The cautions page lists the gaps itself.

• Detailed shading. With the default losses, the estimate "may be overly optimistic" if "nearby buildings, trees, objects" shade the panels. PVWatts doesn't model shade from objects around your house. An installer's shade analysis does.
• Aging. The cautions page notes that a system's output "may degrade by as much as 1% per year." PVWatts gives you year one.
• Your actual rate and export pay. PVWatts values the output with "an annual average retail electricity rate" (Get Started). The cautions page adds that "because PVWatts effectively assumes net metering, the energy value it reports is higher than it would be without net metering." Time-of-use rates, fixed charges and a lower export credit are all missing. Our guide to what your utility pays for exported solar shows how to find yours.
• Incentives and financing. For "more detailed financial modeling," the cautions page points to the lab's System Advisor Model. The IRS says the Residential Clean Energy Credit, section 25D, "is not available for any property placed in service after December 31, 2025" (IRS), so a 2026 payback built on that credit is wrong.
• Weather swings. Real months and years differ from the typical year, as described above.

How to compare PVWatts with an installer quote

Run PVWatts with the quote's own numbers: total DC kW, the panel type, your roof's real pitch and direction, and default losses. Then put your result next to the installer's year-one production estimate.

• Close to yours: the installer's assumptions are probably in line with the lab's.
• Well above yours: ask what loss, shading and weather assumptions they used, and whether their figure is DC or AC.
• Well below yours: they may have found shading you didn't account for. Ask to see the shade report.

Our checklist of what a solar quote must show lists the other lines worth checking. Once production makes sense, weigh the price against what solar costs to install. A licensed installer, your utility and your local permitting office have the final say on what can go on your roof.

Bottom line

The PVWatts calculator gives a homeowner an independent production estimate from six inputs, and the lab is clear about where it falls short. Enter the quote's DC size and your roof's real tilt and direction, keep the 14% losses unless there's shade, and read the result as typical, not exact. It won't tell you what you'll save, since it skips your rate plan, your export pay and detailed shading. For that you need your utility's rules and a real quote.

Common questions

Is the PVWatts calculator accurate?

For a typical year, yes, within limits the lab spells out. Its help says a single year's output can differ from the estimate by up to ±10%, and a single month by up to ±30%, because the weather data is a typical year and real years vary. It also says the estimate may be too optimistic if your roof is shaded and you keep the default losses.

What system losses should I use in PVWatts?

The default is 14%, built from categories such as soiling, shading, wiring, mismatch and availability. Don't add inverter or temperature losses, because PVWatts models those separately. If trees or buildings shade your roof, the default is too low, and a shade report from an installer gives a better number.

What does the kWh per year number in PVWatts mean?

It's the estimated AC electricity, the kind your house uses, that the system would make in a typical year, summed from an hour-by-hour simulation. It's not a forecast for any particular year, and it doesn't account for the panels slowly losing output as they age.

Does PVWatts include the federal solar credit or my utility's export rate?

No. Its dollar figure multiplies output by one average retail rate and effectively assumes net metering, so it ignores lower export pay and incentives. The federal Residential Clean Energy Credit (section 25D) isn't available for property placed in service after December 31, 2025, so a 2026 estimate shouldn't count it anyway.

How do I use PVWatts to check a solar quote?

Enter the quote's system size in DC kW, your roof's real tilt and direction, and the default losses, then compare the annual kWh with the installer's year-one production estimate. If the installer's number is well above yours, ask what loss, shading and weather assumptions they used.

Sources

1. National Laboratory of the Rockies: PVWatts calculator
2. National Laboratory of the Rockies: PVWatts help, Get Started
3. National Laboratory of the Rockies: PVWatts help, solar resource data
4. National Laboratory of the Rockies: PVWatts help, System Info
5. National Laboratory of the Rockies: PVWatts help, DC system size
6. National Laboratory of the Rockies: PVWatts help, module type
7. National Laboratory of the Rockies: PVWatts help, array type
8. National Laboratory of the Rockies: PVWatts help, system losses
9. National Laboratory of the Rockies: PVWatts help, tilt
10. National Laboratory of the Rockies: PVWatts help, azimuth
11. National Laboratory of the Rockies: PVWatts help, DC to AC size ratio
12. National Laboratory of the Rockies: PVWatts help, results
13. National Laboratory of the Rockies: PVWatts help, cautions for interpreting the results
14. National Laboratory of the Rockies: PVWatts system design help (System Advisor Model)
15. National Laboratory of the Rockies: PVWatts V8 API documentation
16. IRS: Residential Clean Energy Credit
17. U.S. Department of Energy: Homeowner's guide to going solar

By SolarRealCost Editorial Team. First published November 7, 2026. Advertiser disclosure