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Calculator · Updated August 2026
Solar system size calculator
Enter annual usage and your state. Get kilowatts, panel count and roof area.
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The calculator
Size from your usage
Roof area assumes about 18 square feet per 400 W panel plus setbacks. Fire code setbacks and obstructions usually mean you need more usable roof than the raw panel area suggests.
This is an estimate, not a savings guarantee.
It uses published utility rates, NREL regional sun data and a standard system derate. It does not know your roof orientation, your shading, your usage pattern or what your utility will charge next year. No solar company can honestly guarantee savings, and any that does is describing a model rather than making a promise.
Where to find your annual usage
Your utility's online account has twelve months of kilowatt-hours. Use that rather than estimating from a bill amount — dollars move with rates, kilowatt-hours are the real quantity. If you have only a recent bill, multiply a typical month by twelve and accept that seasonal homes and electric-heat homes will be badly served by that shortcut.
Why 100% offset is usually the wrong target
It is the default people reach for and it is frequently a mistake. Where your state credits exports at less than the retail rate — California under the Net Billing Tariff, Ohio at the generation rate, Illinois on supply-only net metering since January 2025 — the surplus you build to hit 100% is worth much less than the power you use directly. In those states an array sized to your daytime load, sometimes with storage, beats an array sized to your annual total.
Where net metering is one-for-one at retail — Pennsylvania, New Jersey, Virginia, Maine — sizing to 100% of annual usage is defensible, because an exported kilowatt-hour and a consumed one are worth the same.
What limits system size in practice
Usable roof area
Not roof area. Fire code setbacks around ridges and edges, plumbing vents, chimneys, skylights and dormers all remove usable space. A roof that looks large often supports a much smaller array than its square footage suggests.
Orientation
South-facing planes produce most. East and west lose meaningful output. North-facing is usually not worth the hardware. A house with one good plane may be capped well below its usage.
Electrical service
Your main panel has to accept the interconnection. Older 100-amp services frequently require a panel upgrade, which is a real cost line that appears late in some proposals.
Utility interconnection limits
Many utilities cap residential systems, commonly at 25 kW but sometimes lower, and some cap at a percentage of historical usage so you cannot build a small power plant.
About the panel wattage assumption
This uses 400 W panels, which is a reasonable mid-range residential figure in 2026. Higher-wattage panels reduce panel count and roof area for the same system size but do not change the kilowatts you need. System size in kilowatts is the number that matters; panel count is an implementation detail.
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Who you would be working with
SolarAmerica
We design your system and manage the project; installation is performed by vetted licensed partners in your area. SolarAmerica is not itself the installing contractor.
Questions
The things people actually ask
What size solar system do I need?
Divide your annual kilowatt-hours by what one kilowatt of panels produces in your area, then multiply by your offset target. In most of the Northeast a kilowatt produces roughly 1,100 to 1,200 kWh a year; in Arizona closer to 1,750.
How many solar panels do I need?
System size in kilowatts divided by panel wattage. A 9 kW system at 400 W per panel is about 23 panels, needing roughly 400 square feet of usable roof plus setbacks.
Should I size for 100% of my usage?
Only where exports are credited at the retail rate. Where export credit is well below retail, sizing to your daytime load, sometimes with storage, produces a better result than sizing to your annual total.
Does a bigger system always save more?
No. Beyond the point where you are exporting at a low credit rate, extra capacity earns very little while still costing full price.
Get the number for your actual roof
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