Find out how much electricity your solar panels actually produce in a day. This calculator accounts for real-world losses that cut into the rated output printed on your panels.
What Are Peak Sun Hours?
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.
Where the 15% System Loss Comes From
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.
Frequently Asked Questions
Once you know your panel output, use our battery runtime calculator to see how long that stored energy will power your loads. Try it now →
Solar output varies daily and seasonally. The number this calculator gives you is a useful average — some days you will produce more, some less. For off-grid systems, size your panels and batteries for the worst month, not the average. For grid-tied setups, annual totals matter more than any single day.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.