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.

Phoenix averages 6.5 peak sun hours. Seattle averages 3.5. London sits around 2.5-3. This single number is the biggest factor determining your system's actual output. You can look up your location on the NREL PVWatts tool or Global Solar Atlas for an accurate local figure.

Seasonal variation matters too. Phoenix drops from about 7.5 peak sun hours in June to 5.2 in December. 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 bell curve above shows why a panel's rated wattage is a ceiling it touches only briefly. A 300W panel sits near zero at dawn, climbs as the sun rises, and peaks close to its rating at solar noon 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. 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 × 0.85 for typical losses. 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 30; for a year, multiply by 365. A single 400W panel at 5 peak sun hours, for example, clears more than 600kWh 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 across twelve US cities, using each city's peak sun hours from NREL solar resource data at latitude tilt and the same 15% loss allowance. Look up your own location on NREL PVWatts for a local figure.

A panel in Las Vegas out-produces the same panel in Seattle by about 68% over a year, purely on available sunshine. That gap 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 monocrystalline panels. On a 40°C day, surface temperatures can reach 65°C, costing you roughly 14% from temperature alone.

Wiring resistance loses another 2-3%. Dust and dirt reduce output 2-5% depending on cleaning frequency. Inverter conversion adds 3-5% loss. Shading is the wild card — even partial shade on one cell can cut an entire string's output by 30-50% if your system lacks optimisers or microinverters.

The 15% default is conservative for a clean, well-installed system. Increase it to 20-25% if panels sit under trees, collect heavy dust, or face anything less than due south (in the northern hemisphere). Flat-mounted panels on commercial roofs often need 20% to account for suboptimal tilt.

Stacking those factors gives a typical derate from the lab rating down to delivered output:

These figures follow the NREL PVWatts loss model, which combines them multiplicatively rather than adding them up. A clean, well-sited array lands near 78-82% of nameplate; a hot, dusty, or partly shaded one can fall below 70%. The calculator's 15% default sits at the optimistic end of this stack, so dial it toward 20-25% for harsher sites.