Solar Panel Calculator

How to use the calculator

1. Monthly electricity use: the average kWh on your bills. Averaging twelve months smooths out summer air conditioning and winter heating.
2. Location: the reference city whose climate is closest to yours, or choose “Other” and enter your own yield, for example from NREL's PVWatts.
3. Share of use to cover: 100% aims to produce as much as you use over a year; lower it for a smaller system.
4. Panel power: the wattage on the module's spec sheet.
5. Electricity price and, under advanced options, installed cost per kW and panel area in square feet (21.5 sq ft by default).
6. Self-consumption and export price (advanced): the share of the solar power you use directly instead of buying from the grid (100% by default), and what your utility pays or credits for each kWh you send back (empty counts as $0).

How the calculator works

The key input is the specific yield: the kWh a system produces per year for each kW of panels. The city values come from a 2020 study by Home Innovation Research Labs for the National Association of Home Builders, which modeled south-facing roofs with NREL's System Advisor Model (PVWatts) and 14.08% total system losses:

• Size needed (kW) = monthly kWh × 12 × coverage ÷ yield
• Panels = size × 1,000 ÷ panel watts, rounded up; the system size shown is panels × watts.
• Production = system size × yield
• Used = (the smaller of production and annual use) × self-consumption; Exported = production − used
• Savings = used × price + exported × export price
• Payback = system cost ÷ yearly savings

East- or west-facing roofs, shade, panel degradation, rising rates and financing are not modeled.

Worked examples

1. Same home, five climates

A home using 900 kWh a month, with 450 W panels and 100% coverage:

Seattle needs six more panels than Phoenix for the same bill.

2. Savings and payback

In Kansas City, at an electricity price of $0.17 per kWh and a quote of $3,000 per kW, the 7.65 kW system costs $22,950 and saves $1,836 a year, a simple payback of 12.5 years. With 100% self-consumption and no export price, savings count only the 10,800 kWh you use; the 132 kWh surplus shows as exported and earns nothing.

3. Using only half of the power directly

Same system, but you use 50% of the solar power as it's produced and your utility credits $0.05 for each exported kWh. Then 5,532 kWh a year go to the grid, savings fall to $1,194.59 and the payback stretches to 19.2 years. With no export credit, savings would be $918 and the payback 25 years.

Before you get quotes

Treat the result as a first estimate. An installer will look at your roof's direction, pitch, shading and space, which can change the size a lot. The self-consumption share and a single export price are a simplification: the calculator doesn't model net metering rules, such as how credits carry over between months, time-of-use rates or fixed monthly charges. Ask your utility how it credits exports, and compare quotes on cost per watt and on the production estimate each one gives you.