If you have spent even an afternoon researching solar, you have probably been pointed to PVWatts, the free calculator from the National Renewable Energy Laboratory that estimates how much electricity a rooftop system will produce. It is fast, it is free, and most installers quietly run the same engine behind their own proposals.
Here is the problem. Most homeowners use PVWatts to answer a question it was never built to answer. PVWatts tells you how many kilowatt-hours your panels will make. It does not tell you how many dollars you will save. Those are two different numbers, and the gap between them is where a lot of solar disappointment starts.
Quick answer: PVWatts is genuinely accurate for annual production, typically within 5 to 10 percent when the inputs are honest. It is not built to calculate savings, because it does not know your rate plan, your NEM 3.0 export credit, or when you actually use power. This guide breaks down both halves so you know exactly how much to trust the number PVWatts gives you.
What PVWatts Actually Is
PVWatts is a production estimator built and maintained by NREL, a federal research lab, so it carries more credibility than a random calculator on an installer’s homepage. You enter an address and a handful of system details, and it returns an estimate of how much energy a photovoltaic array will generate over a typical year, broken down month by month.
What it asks for
| Input | What it controls |
|---|---|
| Location | Which weather station and solar radiation dataset the model pulls from |
| DC system size (kW) | Total panel nameplate capacity before losses |
| Module type | Standard, Premium, or Thin Film; affects temperature sensitivity |
| Array type | Fixed roof mount, fixed open rack, or tracking |
| Tilt (degrees) | Panel angle from horizontal |
| Azimuth (degrees) | Compass direction the panels face; 180° equals true south |
| System losses (%) | A bundled deduction for wiring, soiling, mismatch, and age, defaults to about 14% |
What it returns
One headline number: estimated annual kilowatt-hours, plus a monthly breakdown and the modeled solar radiation for your site. That production figure is the foundation every other solar number is built on, which is exactly why understanding how solar estimates work starts with understanding where the kilowatt-hour figure comes from.
What it deliberately leaves out
Older versions of PVWatts offered a rough bill-savings field using a flat average electricity rate. That field was never meant to model a modern residential tariff. The tool does not know your rate plan, it does not model net metering or net billing, and it has no idea when during the day you actually use power. It answers a physics question, not a billing question.
How PVWatts Estimates Your Production
PVWatts runs a simplified but well-validated model. It pulls decades of typical weather data for your location, including solar radiation, temperature, and cloud patterns, then simulates how a panel array of your size and orientation would perform hour by hour across a representative year.
The weather and system model
The single biggest driver of the estimate is the weather file for your site, pulled from the National Solar Radiation Database. That data captures how much sunlight actually reaches your latitude, adjusted for typical cloud cover and haze. On top of that, the model applies the physics of your specific setup. A steeper tilt or a west-facing roof shifts the shape of the daily curve, and higher temperatures shave a little output because panels lose efficiency as they heat up. For the fuller picture of how orientation and sizing translate into generation, see our guide to how to calculate solar panel output.
The system loss percentage, broken down
This is the number that trips up most first-time users. It defaults to roughly 14 percent and bundles together every small real-world drag on output:
| Loss category | Typical default | What it represents |
|---|---|---|
| Soiling | 2.0% | Dust, pollen, and debris on the panel surface |
| Shading | 3.0% | Only applies if you manually add it, PVWatts assumes a clear roof by default |
| Wiring | 2.0% | DC and AC conductor resistance |
| Mismatch | 2.0% | Small output differences between panels in a string |
| Light-induced degradation | 1.5% | First-year efficiency drop common in crystalline silicon |
| Nameplate rating | 1.0% | Manufacturer’s stated output tolerance |
| Age or availability | 3.0% combined | Long-term degradation and downtime |
Leave it at the default, and you get a reasonable all-purpose estimate. Set it to zero because you assume your brand-new panels are perfect, and PVWatts will hand you a production figure your roof will never hit.
How to Run a PVWatts Estimate Step by Step
- Enter your address. Confirm the weather station PVWatts assigns is reasonably close to your actual roof. A station 30 to 40 miles away in a different microclimate can shift your estimate a few percent in either direction.
- Enter your DC system size in kW. Multiply the number of panels by each panel’s wattage and divide by 1,000. Twenty 400-watt panels is an 8.0 kW system.
- Choose your module type. Standard covers most crystalline silicon panels. Only choose Premium if your installer has specified a high-efficiency panel.
- Set tilt and azimuth to match your actual roof. Do not default to 180° azimuth unless your roof genuinely faces true south.
- Leave system losses at the default (about 14%) unless a professional installer gives you a site-specific reason to change it.
- Read the annual total, not any single month. Treat the twelve monthly numbers as a shape, not a promise.
- Ignore the built-in savings box. It uses a flat average rate and will not resemble a real NEM 3.0 or time-of-use bill.
How Accurate Is PVWatts, Really?
For what it is designed to do, PVWatts is genuinely good.
| Time frame | Typical accuracy | Why |
|---|---|---|
| Annual total | Within 5 to 10 percent | Long-run weather averages smooth out short-term swings |
| Single month | Can vary considerably wider | A cloudy winter or a smoky wildfire summer can push one month well off the typical-year model |
| Multi-year average | Tightest of all | Individual year-to-year weather noise cancels out over time |
A sample estimate
Here is what a well-inputted estimate might look like for a 6 kW south-facing system in the Los Angeles area:
| Month | Estimated production (kWh) | Notes |
|---|---|---|
| January | 620 | Shorter days, lower sun angle |
| April | 890 | Peak spring irradiance |
| July | 940 | Long days, some heat-related loss |
| October | 780 | Solid shoulder-season output |
| December | 580 | Lowest output of the year |
| Annual total | ~9,300 | Trust this number far more than any single month |
What throws the estimate off
A few inputs do most of the damage:
- Shading. PVWatts assumes an unshaded array unless you manually add losses. A chimney, a tall tree, or a neighbor’s roofline can pull real output well under the estimate.
- An optimistic loss percentage. Dropping it toward zero manufactures a best-case number your roof will not reach.
- A wrong tilt or azimuth. Even a 10-degree error can shift the annual estimate by several percent.
- A system size that ignores real roof space. The number is only as honest as the inputs behind it.
If any of these apply to your roof, a full custom solar design that models each obstruction directly will get you closer to reality than adjusting the flat loss percentage.
Why Production Accuracy Is Not Savings Accuracy
This is the part that catches most homeowners off guard. Even a perfect production estimate, the exact kilowatt-hours your roof will make, does not tell you what those kilowatt-hours are worth. Value depends entirely on your utility’s rate structure, and that is the one thing PVWatts does not touch.
Under California net billing, commonly called NEM 3.0, the surplus power your array exports at midday is credited at low avoided-cost values, often a small fraction of the retail rate, while the power you buy back in the evening costs full price. Seeing how NEM 3.0 reshaped solar savings makes the point concrete: two homes with identical PVWatts production can end up with very different savings depending on how much of their own solar they use versus export.
Most utilities also bill on time-of-use plans, where a kilowatt-hour at 6 p.m. can cost two or three times one at noon. PVWatts is blind to all of it. It reports the sunlight, not the tariff, which is why our take on whether solar is still worth it in California after NEM 3.0 leans on rate structure and self-consumption rather than raw production.
Common Mistakes That Skew a PVWatts Estimate
- Setting system losses near zero to make the number look better
- Leaving azimuth at the default 180° when your roof actually faces east or west
- Ignoring shading from a chimney, tree, or neighboring structure
- Treating the built-in savings box as a real bill projection
- Comparing one cloudy or unusually sunny month against the annual average and assuming something is broken
- Sizing the system to the array you wish you could fit instead of the roof space you actually have
Turning PVWatts Production Into Real Dollars
To get from a PVWatts kilowatt-hour figure to a believable savings number, layer on everything the tool leaves out.
- Start with the annual production estimate.
- Split it into the share you consume on-site as it is generated and the share you export.
- Value the self-consumed portion at your actual retail rate.
- Value the exported portion at your utility’s export credit, which under NEM 3.0 is much lower than retail.
That two-bucket split is the whole game, and it is why what those savings-calculator numbers actually mean matters more than the headline kilowatt-hour count. Utilities also keep raising retail rates over time, so the dollars a given kilowatt-hour offsets tend to grow, and rising utility rates keep pushing solar ROI higher even when production stays flat. Only once production has been translated into rate-adjusted dollars can you honestly estimate how long solar takes to pay for itself.
How to Use PVWatts Without Fooling Yourself
- Keep the default 14 percent loss figure unless a professional gives you a site-specific reason to change it
- Add shading losses manually if anything shades your roof
- Enter your real tilt and azimuth rather than guessing
- Match the system size to the roof area you actually have
- Treat the annual production figure as reliable and ignore the built-in bill-savings box entirely
- Compare any installer proposal against your own careful PVWatts run, and treat a big gap as a flag worth questioning before you sign
Lining a proposal up against a couple of solar quotes from other installers is a quick way to see whether a production number is an outlier.
Getting a Savings Estimate That Goes Beyond PVWatts
PVWatts answers the production half of the question well, which makes it a great first stop. The savings half needs a tool that knows your rate plan, your export credit, and your usage shape, one that starts from your real bills and models self-consumption against export rather than multiplying kilowatt-hours by one average price.
When you run your own numbers through a savings-focused tool like the California solar savings calculator, check that it separates the power you use on-site from the power you export and that it reflects your actual time-of-use windows. Those are the levers PVWatts cannot see and the ones that decide your real bill.
The Tool Is Only As Honest As Its Inputs
PVWatts earns its reputation as a fast, credible way to estimate how much energy a solar array will make, and its annual production numbers hold up well when the inputs are realistic. What it cannot do is tell you what those kilowatt-hours are worth, because value lives in your rate plan, your export credit, and when you use power, none of which the tool models.
Treat a PVWatts figure as the reliable production foundation and build the dollars on top of it with a rate-aware calculator. When you are ready to see what your own roof and rate plan would really earn, you can get a free, no-pressure solar estimate built around your actual bills instead of a national average.
Frequently Asked Questions
Is PVWatts accurate for estimating solar production?
For annual production, yes, it is typically accurate within about 5 to 10 percent when the inputs are realistic. Accuracy is tighter over a full year than in any single month, since the tool uses long-run typical weather rather than a forecast. The most common cause of a bad estimate is user error, especially ignoring shading or dropping the default loss percentage too low.
Does PVWatts calculate my solar savings or just production?
PVWatts is fundamentally a production estimator. It tells you kilowatt-hours, not dollars. Older versions include a rough bill-savings field based on a flat average rate, but it does not model net metering, NEM 3.0 export credits, or time-of-use pricing. To turn its production figure into real savings, apply your actual rate plan and self-consumption yourself.
How accurate is PVWatts compared to actual system performance?
Independent validation studies generally place PVWatts within 5 to 10 percent of real annual output for a correctly modeled, unshaded system. Some installations land even closer, within 2 to 5 percent. Wider gaps almost always trace back to an input problem, most often unaccounted-for shading or an unrealistic loss percentage, rather than a flaw in the model itself.
Why is my real solar bill different from what PVWatts showed?
Usually because PVWatts reported production while your bill reflects economics. If you export midday power at low NEM 3.0 credit values and buy evening power at full time-of-use rates, your dollar savings can trail a production-based expectation even when the panels made exactly the energy PVWatts predicted. The tool was accurate about kilowatt-hours. The rate structure decided the dollars.
Does PVWatts account for shading from trees or chimneys?
No, not automatically. PVWatts assumes an unshaded array unless you manually raise the system loss percentage or model shading separately. A roof with real obstructions can produce meaningfully less than the default estimate suggests, so any shaded site needs that adjustment before the number means anything.
What loss percentage should I use in PVWatts?
Leave it at the roughly 14 percent default unless an installer gives you a site-specific reason to change it. That figure bundles wiring, inverter, soiling, mismatch, and degradation losses that every real system experiences. Setting it near zero produces a best-case number your roof will not actually reach and quietly inflates every downstream savings estimate.
Is PVWatts free to use, and do I need an account?
Yes, PVWatts is a free public tool from NREL and does not require an account for the standard web calculator. An API key is only needed if you or a developer want to pull results programmatically into another application.