Solar system size calculator

Calculator

Solar system size calculator

How this is calculated

How the number is derived

1. Step 1

   Convert monthly usage to annual kWh

   Monthly kWh x 12. Most US homes use 800 to 1,400 kWh per month depending on size, climate and electric heating. Your utility bill shows this directly.

2. Step 2

   Divide by annual production per kW

   Peak sun hours per day x 365 x system efficiency = annual kWh produced per kW of panels. For the US, this ranges from about 1,100 kWh per kW per year in the Pacific Northwest to 1,700+ kWh per kW per year in the Southwest.

3. Step 3

   Compute panel count

   Modern residential panels are 400 to 450 watts each. System kW / 0.4 (or 0.45) = panel count. A 7 kW system is 16 to 18 panels.

4. Step 4

   Compute roof area

   Each panel is roughly 20 square feet. Panel count x 20 = required roof area. Add 15 to 25 percent for setbacks, obstructions, and spacing.

What this is not

What this is not

This is a planning number. A real installer quote accounts for roof pitch, azimuth, shading (trees, chimneys, dormers), structural capacity and local permit limits. Two homes on the same street with the same electric bill can need different-sized systems because one has a south-facing roof and the other has an east-west split. Use this calculator to sanity-check a quote, not to replace one.

Questions people ask

Solar system size calculator: FAQ

What size solar system do I need?

For an average US home using 900 to 1,200 kWh per month, a 7 to 10 kW system (16 to 24 panels) typically covers 90 to 100 percent of annual electric use. The specific size depends on your region's peak sun hours: a 7 kW system in Arizona produces as much as a 10 kW system in Oregon.

How many solar panels do I need?

For an average US home, 16 to 24 panels is typical. Modern residential panels are 400 to 450 watts each, so a 7 kW system needs about 16 to 18 panels and a 10 kW system needs about 22 to 25 panels. The panel count scales linearly with system size.

How much roof space do I need for solar?

About 20 square feet per panel. A typical 7 kW residential system needs roughly 320 to 400 square feet of unshaded, structurally sound roof. Setbacks required by local code (usually 18 inches from the roof edge for fire access) and obstructions reduce usable area; most homes with a dedicated south, east or west roof plane have enough.

Should I oversize my solar system?

In most US states with full net metering, sizing to 100 percent of annual usage is the right target: oversizing produces excess that gets credited at retail or near-retail. In states with capped export (California NEM 3.0, Hawaii), oversizing beyond self-consumption plus battery capacity usually produces electricity that is credited at wholesale rates and does not pay for itself.