Cost of Owning

Solar System Size Calculator

Enter your monthly kWh, peak sun hours and panel wattage. See the system size in kW, how many panels it takes, the yearly production it should make and an installed cost range.

Updated October 2026 · Free · No sign-up

How big a solar system do you need?

Estimated system size
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Installed cost atEstimate

Estimates only. Peak sun hours and losses are planning assumptions, not measurements of your roof; NREL's PVWatts tool gives a location-specific figure. Cost range uses $2.50 to $3.50 per watt as quoted in our solar installation cost guide, before incentives. The 30% federal tax credit expired December 31, 2025. A real quote depends on roof, equipment and installer.

How the math works

  1. Annual kWh to cover = monthly kWh × 12 × the share of usage you want to offset.
  2. Yield per installed kW = peak sun hours × (1 − losses) × 365.
  3. System size in kW = annual kWh ÷ yield per kW.
  4. Panels = system size × 1,000 ÷ panel watts, rounded up to a whole panel; production and cost are then recalculated from the whole-panel size.
  5. Cost = system watts × price per watt. The cost table shows $2.50, your price and $3.50 per watt.

With the defaults (899 kWh a month, 4.5 sun hours, 14% losses, 400 W panels) this gives about 20 panels and 8 kW, which lines up with EnergySage's figure of 11,614 kWh a year for an 8 kW system quoted in our how many solar panels guide.

What this calculator assumes

  • Peak sun hours are an estimate. The default of 4.5 is the middle of a rough 3.5 to 5.5 range for much of the US. Your roof's tilt, direction and shade matter, and a tool such as NREL's PVWatts gives a location-specific number.
  • Losses are an estimate. We use 14% as a round planning figure, not a measurement of your system. Panels also degrade slowly over time, which this one-year estimate ignores.
  • Price per watt. $2.60 is EnergySage's national average (June 30, 2026), before incentives. State averages in the same data run from $2.12 to $2.90, and any real quote will differ.
  • Usage. The 899 kWh default is the EIA's 2022 average per residential customer. Use 12 months of your own bills, and add load if you plan an EV or heat pump.
  • Not modeled. Net metering rules, export rates, batteries, roof replacement, panel upgrades, financing, and the payback period.

Keep going

Read how many solar panels to power a house, solar panel installation cost and solar system cost by size for sources and price context. When you have a size and a quote, the solar panel payback calculator estimates how long it takes to pay for itself, and payback by state shows the local picture. To see what your usage costs today, use the electric bill estimator.

Frequently asked questions

How do I calculate what size solar system I need?
Divide your annual kWh by the kWh one installed kilowatt makes in a year. That yield is peak sun hours times (1 minus system losses) times 365. Divide the resulting kW by your panel wattage and round up to get the panel count. The calculator does all of this from your monthly kWh.
What are peak sun hours?
Peak sun hours are the equivalent number of hours per day at full sun intensity (1,000 W per square meter). Our default range of 3.5 to 5.5 is an estimate for much of the US, with cloudier regions near the low end and sunny regions near the high end. NREL's PVWatts tool gives a figure for your exact address, which is better than our estimate.
What are system losses?
The share of panel output lost before it reaches your meter: wiring, inverter conversion, heat, dirt and shading. We default to 14% as a round planning estimate, not a measured figure. Heavy shade or a poor roof angle can push it much higher.
How many solar panels do I need for an average house?
At the average household use of about 899 kWh a month, 4.5 peak sun hours, 14% losses and 400 W panels, the calculator gives about 20 panels (8 kW), our estimate. More sun means fewer panels, and our how-many-panels guide shows EnergySage's figures for different locations.
How much does that system cost?
The calculator multiplies the system size by a price per watt. EnergySage's national average was $2.60 per watt on June 30, 2026, and our installation cost guide puts typical installs at $2.50 to $3.50 per watt before incentives. The 30% federal tax credit expired on December 31, 2025. Get written quotes for a real number.