Solar & battery
Solar System Size Calculator
System size in kilowatts to cover a given share of a power bill, from usage and local sun.
Result
System derate covers inverter losses, wiring, heat, soiling and panel mismatch. Around 0.80 is the figure most design tools use; a hot climate, a dusty site or a shaded roof pushes it lower. It is why a system never produces the sum of its panel ratings. Peak sun hours are not daylight hours. They are the day's total solar energy expressed as hours at 1,000 W per square metre, so a long overcast day can be worth fewer peak sun hours than a short bright one. Use a figure for your own location and month rather than a national average. Roof area assumes about 21 square feet per panel with no allowance for setbacks, obstructions or fire access pathways, all of which local code requires and which reduce usable roof substantially.
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About this calculator
Why is the system bigger than my usage suggests?
Because of the derate and because sun hours are an average. A system sized exactly to the arithmetic underproduces in winter and on cloudy weeks.
Should I cover 100% of my bill?
Not always. Where net metering credits are worth less than retail, production beyond your own use earns less, so a smaller system sometimes pays back faster.
Does panel wattage change the system size?
No, it changes the panel count and the roof area. A 6 kW system is 6 kW whether it is fifteen 400 W panels or twenty 300 W ones.
The formula
kW = daily kWh needed / (peak sun hours x derate)
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