How solar system size is calculated
The required size comes from one formula: kW = daily kWh ÷ (sun-hours × performance ratio). Daily kWh is the monthly kWh on your bill divided by 30. The performance ratio (we use 0.8) accounts for inverter efficiency, temperature, dust and wiring losses — a system never delivers its full nameplate rating.
Why location changes everything
The same 900 kWh-a-month household needs about 7 kW in Phoenix but 10 kW in Boston, purely because of sun-hours. This calculator uses curated city data where available and a latitude-based model everywhere else, so it works worldwide.
Roof area rule of thumb
Traditional mounting needs roughly 60–70 square feet per kW, including maintenance walkways. Tight roofs can use higher-efficiency panels to fit more power in less space.
How many kW of solar do I need per 100 kWh on my bill?
As a rule of thumb, roughly 0.8–1 kW per 100 kWh of monthly usage in sunny regions, and more in cloudier climates. This calculator uses your local sun-hours for an exact figure.
How much electricity does the average US home use?
About 900 kWh a month (10,791 kWh a year) per the U.S. Energy Information Administration, 2022 data. It varies widely by state — from roughly 515 kWh a month in Hawaii to about 1,230 in Louisiana — so always size from the kWh on your own bill.
Why is the recommended size bigger than the calculated size?
A 10% headroom covers future load growth and the slow, normal degradation of panels over their life.
What are peak sun hours?
The equivalent number of hours per day at full-strength (1000 W/m²) sunshine. Phoenix averages about 6.2, Los Angeles about 5.6 and Boston about 4.2.