How to Benchmark Solar Performance With PVWatts

Why benchmark solar system performance against PVWatts

Your solar installer gave you production estimates when you signed the contract, but those numbers may have been optimistic, rounded, or based on different assumptions than what actually got installed. PVWatts is a free calculator from the National Renewable Energy Laboratory (NREL), and it’s the most widely used tool for estimating what a solar system should produce. Its models are checked against real-world data. It’s the same tool installers use for preliminary design work, which makes it the right benchmark for checking whether your system is meeting expectations.

If your actual production consistently falls short of the PVWatts estimate, that’s a signal. It could mean shading you didn’t account for, a misconfigured inverter, dirty panels, or a system that wasn’t installed to the specifications you paid for. A performance ratio of roughly 75 to 80% generally means your system is turning sunlight into electricity about as well as it should. Running this comparison once or twice a year lets you catch problems early, before they cost you thousands in lost production.

Most importantly, PVWatts gives you an objective number to bring to your installer if you suspect underperformance. Instead of saying “I think my system isn’t working right,” you can say “PVWatts estimates 9,200 kWh annually and I’m tracking at 7,400 kWh—what’s going on?” That’s a conversation that gets results. If you’re dealing with a system that’s severely underperforming, check out our guide on what to do when your installer went out of business.

Gather your system data before you start

Before you can benchmark your solar system against PVWatts, you need accurate information about what’s actually installed on your roof. Start with your system’s DC nameplate capacity—this is the total wattage of all your panels added together. You’ll find this on your installation documents, your inverter monitoring app, or on the panels themselves if you can safely access your roof. If your contract says you have a “6 kW system,” that’s your DC size. Most home systems fall between 3 and 10 kW.

Next, confirm your array type, tilt angle, and azimuth (compass direction your panels face). Most residential systems are fixed roof-mount arrays. Tilt is the angle from horizontal—if your roof has a 5/12 pitch, that’s about 22 degrees. Azimuth is measured clockwise from true north: due south is 180 degrees, southwest is 225 degrees, southeast is 135 degrees. Your installer should have documented all of this in your final system design.

Finally, pull your actual production data from your monitoring app. Once the system is running, the app logs your kWh production along with usage and performance data. You’ll need at least a few months of data for a meaningful comparison, ideally a full year. Your app should let you export monthly or annual totals. If you’re using SolarEdge, Enphase Enlighten, or another major platform, look for an “Energy” or “Reports” tab where you can view cumulative production.

How to run a PVWatts estimate for your system

Open pvwatts.nrel.gov in any browser. The interface is straightforward. Start by entering your street address—this tells PVWatts which weather station to use for solar radiation and temperature data. The tool will drop a pin on a map and automatically select the nearest weather data file. Click through to the system information screen.

Now enter your system specs. DC System Size is the total kilowatts from step one. Module Type: choose “Standard” for most modern panels unless you have premium modules, in which case “Premium” accounts for slightly higher efficiency. Array Type will almost always be “Fixed (roof mount)” for residential systems. System Losses: leave this at the default 14% unless you have specific reasons to adjust it. PVWatts uses simplified assumptions, and engineers who need more precision use NREL’s System Advisor Model instead, but the defaults work well for benchmarking.

Enter your tilt angle and azimuth. Double-check these—getting the direction wrong by 45 degrees can throw off the estimate by 10-15%. Click “Go to PVWatts Results.” The results page will show you estimated monthly and annual AC energy output in kilowatt-hours. That annual number is your benchmark. Note the range shown—that range comes from 30 years of weather records for your location and shows how much output can swing from year to year. Your actual production should fall somewhere in that band if your system is working correctly.

Calculate your actual performance ratio

Now that you have both your PVWatts estimate and your actual production data, you can calculate your performance ratio. This is the metric solar professionals use to evaluate system health. Performance ratio compares what your system actually produced with what it could have produced in theory, given the sunlight it received.

The formula is simple: divide your actual annual production (in kWh) by your PVWatts estimated annual production (in kWh), then multiply by 100 to get a percentage. For example, if PVWatts estimates 9,200 kWh per year and your monitoring app shows 7,544 kWh over the past 12 months, your performance ratio is (7,544 ÷ 9,200) × 100 = 82%.

Healthy systems usually land between 75 and 85%, and where yours falls depends on sunlight, heat, shade, dirt and how the system was designed. If you’re consistently hitting 75-85%, your system is performing normally. Between 70-75%, something may be slightly off—check for shading changes, dirty panels, or minor inverter issues. Below 70%, you likely have a real problem that needs professional diagnosis. For help understanding what might be going wrong, read our article on how solar panels perform in less-than-ideal conditions.

Account for system age and degradation

If your system has been running for several years, you need to factor in panel degradation before deciding whether your performance ratio is acceptable. Solar panels gradually lose efficiency over time due to exposure to UV radiation, temperature cycling, and weather. Most panels lose less than 0.8% of their output per year, and most manufacturers guarantee at least 80% of original output at year 25.

At typical rates of 0.3 to 0.8% a year, panels usually still deliver 80 to 90% of their original output after 25 years. Premium brands like SunPower and Panasonic often spec 0.25-0.3% annual degradation, while mainstream brands range from 0.5-0.8%. Check your panel warranty documents for the guaranteed degradation rate.

To adjust your comparison: multiply your system’s age in years by the annual degradation rate, then reduce your PVWatts estimate by that percentage. For example, if your system is 5 years old with 0.5% annual degradation, total degradation is 2.5%. If PVWatts estimates 9,200 kWh, your age-adjusted estimate is 9,200 × 0.975 = 8,970 kWh. Now recalculate your performance ratio using this adjusted baseline. This prevents you from misdiagnosing normal aging as a system fault. For more on panel longevity, see our guide on how long solar panels actually last.

When to investigate further or call for service

Once you’ve calculated an age-adjusted performance ratio, you’ll know whether your system needs attention. If you’re above 75%, you’re in good shape—keep monitoring but no action needed. Between 70-75%, start investigating common issues you can check yourself: look for new shade sources (tree growth, new construction), inspect for visible dirt or debris on panels, and verify your inverter isn’t showing any error codes. Many underperformance issues at this level are environmental rather than equipment failures.

Below 70% performance ratio, or if you see a sudden drop of more than 10% compared to the prior year, call your installer or a qualified solar technician. This level of underperformance usually indicates a real equipment problem—failed inverter, damaged panels, incorrect inverter settings, or electrical issues. Don’t wait to address this. A system running at 65% efficiency is costing you 35% of your expected savings every single month. If it’s been underperforming for six months, you’ve already lost meaningful money.

Document everything before you call: your PVWatts estimate with all inputs shown, your actual production data exported from your monitoring app, your calculated performance ratio, and photos of any error messages or unusual conditions. This makes the diagnostic process faster and helps you hold your installer accountable if the system wasn’t installed correctly. If you’re past your installer’s workmanship warranty or they’re not responsive, you’ll need to find a qualified solar service provider in your area who can diagnose and repair the issue. Most problems are fixable, but they rarely resolve themselves.

For a full list of causes and fixes, see why a solar system underperforms.

Common reasons your actual production differs from PVWatts

Even a healthy system won’t match PVWatts exactly. The estimate is based on typical meteorological year data, but your actual weather might have been cloudier or sunnier than average. If you installed mid-year, make sure you’re comparing a full 12 months of production to the annual estimate—don’t compare 8 months of actual data to a 12-month projection.

PVWatts assumes 2% soiling loss by default, and real losses run higher where it’s dusty, polluted or rarely rains. If you live in a dusty area, near a farm, or in a region with low rainfall, your panels may need cleaning more often than the default assumes. Similarly, if your area had an unusually hot summer, panel efficiency drops in high heat—this is normal and doesn’t indicate a problem with your equipment.

Shading is the most common reason for lower-than-expected production, especially if trees have grown since installation or if a neighbor added a structure that casts afternoon shadow on your array. Shadows from nearby buildings or trees can cut production a lot, and PVWatts’ default of 1% shading assumes the array is almost entirely unshaded. Even partial shading on a few panels can disproportionately reduce whole-string output if you have a string inverter system. If shading has changed since install, you may need to trim trees or consider adding power optimizers.

Finally, inverter clipping can cause your actual production to fall slightly below estimates on very sunny days if your installer oversized your DC array relative to your inverter capacity. This is often intentional and economically optimal, but it does mean you’ll never quite hit the theoretical maximum. For more on system design decisions, read our explainer on how solar panel systems actually work.

See what reduces solar panel output for how shade, droppings and dust cut production.