Free Solar Panel Calculator (+ Guide)

What is a solar panel calculator?

A solar panel calculator estimates how much electricity panels make at your home and what that saves you.

This one works for any city the national lab's model has weather data for, in the US and abroad.

How it works

Pick your city and your roof, and the calculator reads the sunlight there.

Where the sunlight numbers come from

PVWatts is the US national lab's model for solar output, run by the National Laboratory of the Rockies, formerly NREL.

It uses typical meteorological year data from the National Solar Radiation Database. Its answer is a typical year, so any one year can come in above or below it.

The calculator asks for 1 kW of panels at your city, roof direction and pitch, and multiplies by your size. The output grows in step with the size, so changing the size needs nothing new.

What happened to the solar tax credit?

The IRS says its 30% federal tax credit for home solar covered panels installed from 2022 through December 31, 2025. It isn't available for systems placed in service after that date.

So for a US install in 2026, type only the rebates and incentives you've been offered, such as your state's or your utility's.

A worked example

Here's the example the calculator loads: a Phoenix home that uses 900 kWh a month, or 10,800 kWh a year, with a roof facing south at a 20° pitch.

PVWatts says 1 kW of panels there makes 1,755 kWh a year, so the calculator sizes the system at 6.2 kW. That makes 10,881 kWh, 81 kWh more than the home uses.

At $0.18 a kWh, the 10,800 kWh the home would otherwise buy are worth $1,944 a year. An $18,000 quote pays back in 9.3 years, and after 25 years the home is $30,600 ahead.

Mistakes to avoid

These are the slips that come up most.

• Counting every extra kWh at your full price, when your utility may credit it for less
• Using a national average price instead of the price on your own bill
• Counting the federal 30% credit on a system installed after 2025

Who is the solar panel calculator for?

It's for homeowners weighing solar, from a first quote to a second opinion, and these tools help around it.

• Electricity cost calculator – for what your appliances use
• Home equity loan calculator – if you borrow for the panels
• Roofing calculator – if the roof needs work first

Run your own numbers before the installer runs theirs, and you'll know what to ask.