Calculators / Solar panel size calculator
What size solar system do I need?
Your bill tells the engine how much electricity you use; your state tells it how much a kilowatt of panels produces there. Those two numbers give the size.
How the size is worked out
Four steps, all of them visible above and all of them arithmetic:
- Your bill, less the fixed monthly charge the utility bills whether you generate or not, is divided by your state's average residential rate. That gives the kilowatt-hours you use in a year.
- That figure is divided by the specific yield — the kilowatt-hours a kilowatt of panels produces in a year where you live, from the National Renewable Energy Laboratory's PVWatts model.
- The result is the system size in kilowatts.
- Panels and roof area follow from the panel wattage and footprint listed under the results.
Specific yield is the reason two houses with identical bills need different systems. A kilowatt of panels in Arizona produces roughly half again what the same kilowatt produces in the north-east, so the same consumption needs a materially smaller array there. It is also the one input in this calculation that has nothing to do with money.
What this sizing does not know
The estimate assumes a roof that is unshaded, reasonably oriented and structurally sound, and it assumes your bill is typical of your year. Four things would change it and none are modelled here:
- Shade. A tree over the array in the afternoon can cost a fifth of the output. Only a site survey settles it.
- Orientation and pitch. The yield figure is for a well-oriented array. A north-facing roof in the northern hemisphere is a different system.
- A bill that is not typical. A summer month in Texas and a spring month in Massachusetts describe very different years. If you have twelve months of bills, use the average.
- What you are about to plug in. An electric car or a heat pump can raise consumption by half, and it is cheaper to size for it now than to add panels later.
Below about 5 kW you are outside what most installers routinely quote, and the price per watt rises because permitting, design and a day of labour do not scale down. The median system installed in the United States is 7.4 kW.
Once you have a size
A size on its own decides nothing. The two questions that follow it are what the system costs (the cost calculator) and how long it takes to repay (the payback calculator) — and the second depends far more on your state's export rules than on the size itself. Is solar worth it in 2026? ranks the states we publish by payback, computed on the same engine as this page.
Questions
- Why does the calculator ask for my bill instead of my roof?
- Because the bill is the constraint that decides the answer for almost every household. A roof large enough for twenty panels does not mean twenty panels are worth buying: the electricity a system produces beyond what you use is exported, and in most states an exported kilowatt-hour is worth less than one you avoid buying. So the model starts from what you consume and checks the roof afterwards.
- What if the roof area it reports is more than I have?
- Then the system does not fit, and the honest answer is a smaller one that covers less of your use. Enter a lower bill to see the size that would fit and what it would do; the ratio is close to linear. Shading, obstructions, chimneys and setback rules all reduce usable area further, and none of them are in this model.
- Is a bigger system always better?
- No, and in several states it is worse. Where exports are credited below retail — California and Arizona among the states here — every kilowatt-hour beyond your own consumption earns a fraction of what it would have saved. Some utilities also cap the size they will interconnect at your historic annual use, which makes an oversized system a permitting problem rather than an economic one.
- Why is the number not a round figure like 6 kW?
- Because it is computed, not chosen. It is your annual consumption divided by what a kilowatt of panels produces in your state. An installer will round it to whole panels and to the inverter sizes they stock, which is why the panel count here is the more useful number of the two.
The other calculators
All of them run the same engine on the same data, so two can never disagree. They differ in what they put first.
- Solar panel cost calculator
Installed price for your system on both price bases, and what is left after credits.
- Solar payback calculator
The year your savings repay the system, and the curve that gets you there.
- Solar savings calculator
What you keep over 25 years, and the bill you avoid each year on the way.
- Lease vs buy solar calculator
Cash, loan and lease on the same house, with who keeps the federal credit.
- Solar battery calculator
The same system with and without storage, side by side.
- Solar incentives calculator
Federal, state and performance payments for your system, each with its source.
A screening estimate from published data, not a quote and not tax or financial advice. Every input is listed under the results with the source it came from and the date it was read; the formulas are on the methodology page. Confirm against real quotes, and with a professional, before relying on any figure here.
Published 6 September 2026. Data last verified 6 September 2026. Rates: EIA 2026-07. Found a figure that looks wrong? Tell us — we publish corrections.