Solar Panel Output Calculator

Peak sun hours are not the same as daylight hours. One peak sun hour equals one hour of sunlight at 1,000 watts per square meter (the intensity used to rate solar panels). A location with 5 peak sun hours might have 10 hours of daylight, but only 5 hours' worth of strong enough sun to match rated output.

PVWatts prints an annual average of 6.66 peak sun hours for Phoenix (tilt 33.45, roof mount) and 3.98 for Seattle (tilt 47.61, roof mount), its annual prints at latitude tilt, rounded to two decimals. This single number is the biggest factor determining your system's actual output. You can look up your location on the NLR PVWatts tool or Global Solar Atlas for an accurate local figure.

Seasonal variation matters too. At the same Phoenix point, PVWatts' monthly prints are 7.44 peak sun hours in June and 5.52 in December, its lowest month (rounded to two decimals). If you are sizing an off-grid system, use the worst month (not the annual average), or you will run short in winter. Our solar panel angle calculator shows how tilt adjustments can help maximise winter capture.

The chart above shows why a panel's rated wattage is a ceiling it seldom reaches. A 300W panel sits near zero at dawn, climbs as the sun rises, and peaks at solar noon, approaching its rating only on a clear day when sunlight strikes the surface most directly. By mid-afternoon it tapers off again, reaching zero at dusk. The total energy you harvest is the area under that whole curve, not the noon peak alone.

This is why peak sun hours work as a shortcut: they compress the entire curve into an equivalent number of hours at full rated output. A panel that touches 285W for a moment at noon and produces far less morning and evening might deliver the same daily total as five hours at its full rating, which is what "5 peak sun hours" means. PVWatts' hourly figure for a 0.3 kW array at Kansas City on June 19 of its typical year peaks lower, at 211.8 W (the chart above). Cloud cover flattens the curve; a clear day in June stretches it far wider than a short day in December.

The math behind every result is the same: rated watts × peak sun hours × 0.86 (14% losses, the PVWatts default) × 0.96 (the PVWatts default inverter efficiency) × 302 / 320 (cell temperature: Trina prints 302W at NOCT for its 400W TSM-400DE09.05, against 400 × 800 / 1,000 = 320W scaled to the 800 W/m² NOCT irradiance, and 302 / 320 = 0.944). The three multiply to 0.779. Your own panel's datasheet prints its own NOCT power, and PVWatts models temperature hour by hour. This table runs that calculation for common panel sizes at three sun-hour levels, so you can estimate daily output at a glance before reaching for the calculator.

To turn any daily figure into a monthly estimate, multiply by 365 / 12 = 30.4; for a year, multiply by 365. A single 400W panel at 5 peak sun hours, for example, makes about 569kWh a year on its own.

Location is the single biggest variable in solar output. The table below lists the energy a single south-facing 400W panel produces in a year at the points where PVWatts runs were captured, using each run's printed annual plane-of-array sun hours (its solrad_annual figure, rounded to two decimals, with the tilt and mounting that run set) and this page's factors (14% losses, 96% inverter, 302 / 320 temperature). Chicago's figure is NASA POWER's latitude-tilt annual climatology, also rounded to two decimals, not a PVWatts run. Your own address is on pvwatts.nlr.gov.

A panel in Phoenix out-produces the same panel in Seattle by about 67% over a year (6.66 / 3.98 = 1.67), purely on available sunshine. The two Phoenix rows and the two Seattle rows are separate runs at the same tilt with different mounting settings. At the Single Rooftop Panel preset (Kansas City, 5.18 peak sun hours) one 400W panel makes 1,614Wh a day; the Desert Ground Mount preset (Flagstaff, 6.30) makes 21,598Wh a day from eight 550W panels. The Cloudy Climate preset takes PVWatts' 4.03 for a point in Oregon (45.5N, 122.7W, tilt 20, roof mount), its annual print rounded to two decimals. PVWatts itself prints 589.97kWh a year for one 400W panel at Kansas City, within 0.2% of this page's 589kWh. That gap in sunshine is why off-grid systems in cloudy regions need more panels and larger battery banks than identical loads would require in the Southwest.

Solar panels never hit their rated output in real conditions. Temperature alone costs 5-10%: panels are rated at 25°C (77°F) but rooftop panels regularly reach 50-60°C in summer. Every degree above 25°C reduces output by about 0.35% for premium modules (standard modules about 0.47% per °C, PVWatts Table 3). On a 40°C day, surface temperatures can reach 65°C, costing you roughly 14% from temperature alone. This calculator carries temperature as its own factor, 302 / 320, from Trina's printed 302W at NOCT against 320W.

PVWatts' 14% default system loss covers the losses it does not model separately. Its printed default inverter efficiency is 96%, applied as a separate factor. Shading is the wild card: partial shade on one cell can cut a whole string's output if your system lacks optimisers or microinverters.

The 14% default suits a clean, well-installed system. Raise it if panels sit under trees, collect heavy dust or snow, or face away from due south (in the northern hemisphere); PVWatts at pvwatts.nlr.gov models your own tilt and azimuth.

PVWatts' printed default losses (the PVWatts V5 manual, NREL/TP-6A20-62641, Table 6) and the two factors this page applies beside them:

PVWatts multiplies its loss rows rather than adding them, and runs the inverter and cell temperature hour by hour. The calculator's flat 0.779 is a derivation from those printed defaults; at Kansas City it lands within 0.2% of PVWatts' own annual figure. A hot, dusty, or partly shaded array can fall well below it.