How many solar panels do I need?
Drop in your power bill and get a panel count, system size, roof area and inverter size. The bill is read in your browser and never uploaded.
Drop your power bill PDF here
Read in your browser. The file is never uploaded.
The short answer
The number of panels you need is set by three things: how many kilowatt-hours you use in a year, how much energy a kilowatt of panels makes where you live, and how big each panel is. Most people guess from roof size or from a neighbor, and both guesses miss. Your bill already holds the answer. Upload it above, or type your kWh by hand, and the tool does the arithmetic below for you.
The formula this tool uses
System size in kilowatts of DC panels is your annual usage times your offset target, divided by your state's specific yield. Offset is the share of your bill you want solar to cover, and it defaults to 100 percent. Specific yield is the kilowatt-hours a 1 kW array makes in a year in your state.
- kWdc = annual kWh x offset / specific yield
- panels = round up (kWdc / 0.41 kW per panel), using a 410 W reference panel
- roof area = panels x 21 sq ft
- inverter AC size = kWdc / 1.2 (a DC to AC ratio of 1.2), rounded up to a common inverter size
A worked example
The average Florida home uses 1,104 kWh a month (EIA, 2024 residential average), which is 13,248 kWh a year. Florida's yield in our table is 1,455 kWh per kW per year. The result is 9.1 kWdc, which is 23 panels, about 483 sq ft of roof and a 7.6 kW inverter. Swap in your own state and usage above and the numbers change with them.
What the yield numbers are
State yields come from NREL PVWatts v8 runs for a south-facing array at a 20 degree tilt with default losses, compiled by TheGreenWatt and checked on 2026-11-03. Each state is represented by its largest city, so a real roof can differ, especially in large states where the west and east sun differ. Electricity use and prices are EIA 2024 residential averages. The full source table is in the project's data notes.
Why your number may differ from an installer's
An installer measures your roof direction, pitch and shade, and chooses a specific panel. A north-facing or shaded roof makes less energy, so it needs more panels for the same bill. A higher-wattage panel needs fewer. Use the count here as a starting point for quotes, not as a design.
Reduce first, then size
Every kilowatt-hour you stop using is a panel you do not buy. If your home uses far more than the national average, the results panel lists a few efficiency steps with their published savings before you size a system. Cheaper kilowatt-hours are the ones you never use, so compare efficiency quotes against solar quotes.
Planning estimate. Permits, interconnection approval and a licensed electrician are required for grid-tied systems.
Questions
How many solar panels do I need to run a whole house?
It depends on how much electricity you use and how much sun your state gets. The worked example below shows an average Florida home, and a high-usage home needs proportionally more. Your own bill is the only reliable input.
Does a bigger roof mean I need more panels?
No. Panel count follows your electricity use, not your roof. The roof only decides whether the panels fit. Allow about 21 square feet per 410 W panel and leave clear space at the edges for access.
Why does the same bill need fewer panels in Arizona than in Washington?
Each kilowatt of panels produces a different amount of energy per year depending on sun. The calculator divides your annual use by your state's yield per kilowatt, so sunnier states need fewer panels for the same bill.
Should I size for 100 percent of my bill?
Often that is the default, but it is a choice. Many utilities pay less for exported power than they charge for imported power, so some owners size smaller. Check your utility's net metering or net billing rules before you pick a target.
Is this an exact quote?
No. It is a planning estimate using state-average yield and a standard 410 W panel. A licensed installer will measure your roof, shading and tilt, and permits and interconnection approval are required for grid-tied systems.