System Size Calculator: how much solar do you actually need?

What this calculator is

The System Size Estimator converts your electricity consumption into a solar array size. Installers size systems in kilowatts (kW) of panel capacity; your utility bills you in kilowatt-hours (kWh) of energy. The bridge between the two is your state’s sunshine. Give the tool your bill (or your exact monthly kWh, printed on every statement) and it works out how many kilowatts of panels — and therefore how many panels — would generate the share of your usage you choose to offset.

How it works

First your bill is converted to energy: a $180 bill at 20¢/kWh means roughly 900 kWh a month. Then the tool divides the yearly energy you want to produce by your state’s specific yield — the kWh one kilowatt of panels realistically generates in a year where you live. That yield uses annual-average peak sun hours (5.0 in a typical mid-sun state) times a 0.80 performance factor that strips out inverter losses, heat, dust and non-ideal angles. Finally the kW figure is rounded up to whole panels at the wattage you select, because you cannot buy 0.4 of a panel.

Worked example (real numbers)

Sunshine by state: the numbers this calculator runs on

These are the exact state averages built into the tool above: annual-average peak sun hours, and the yearly production one kilowatt of panels delivers at those hours after the 0.80 performance factor (sun hours × 365 × 0.80). If a quote you are holding assumes noticeably more production per kW than your state’s row, ask the installer to show their working.

Limitations — read before trusting any size

• Bills hide fixed charges. A $15–$25 monthly connection fee never disappears, so a bill-derived kWh figure slightly overstates usage. Entering your true kWh from the bill is more accurate.
• Seasonality is averaged out. You’ll overproduce in June and underproduce in December. Whether that balances depends on your export rules — see our net metering guide.
• Future usage isn’t included. Planning an EV, a heat pump or a hot tub? Add 2,000–3,500 kWh/yr for an EV and re-run at a higher offset.
• Roof reality check still required. Shading, orientation and roof age can shrink the practical system — the panel count tool and a site survey settle that.

Common mistakes

• Oversizing because “panels are cheap.” Exported surplus is often paid far below retail (or nothing). Size to your usage, not your roof.
• Using the summer bill. One peak month can double the “needed” size. Always average 12 months.
• Chasing 100% offset in a net-billing state. Where exports earn little, a 70–85% offset system frequently pays back sooner.
• Ignoring panel wattage games. A quote with “more panels” may simply use weaker panels. Compare total kW and $/W, not panel count.

Questions about sizing

Should I size for 100% of my usage?

Only if your utility credits exports at or near the retail rate (true net metering) or your usage is growing. Under net billing, the last 20% of a big system earns the least and pays back slowest.

Why does the same house need more panels in Seattle than Phoenix?

Sunshine. Phoenix averages about 6.5 peak sun hours a day; Seattle about 3.9. A kilowatt of panels in Phoenix produces roughly two-thirds more energy per year.

Can I add panels later?

Usually yes, but a second installation repeats fixed costs and may need a new inverter. If an EV is definitely coming within 2–3 years, sizing slightly larger now is often cheaper than expanding later.

Rates, prices and incentive rules on this page checked November 2026.

Disclaimer: results are planning estimates based on state-average sunshine, rates and 2025 pricing. They are not a quote, an engineering assessment or financial advice. Only a site survey by a licensed installer can confirm the right size for your roof.