Solar Panel Calculator

1. Enter your monthly electricity use in kWh from your utility bill.
2. Enter the average peak sun hours per day for your area.
3. Adjust panel wattage, electricity rate and installed cost if you have quotes.
4. Read the number of panels, system size, net cost and payback period.

A solar panel calculator estimates how big a home solar system you need and whether it is worth it. It works backwards from your electricity use and local sun hours to a system size, then a panel count, cost and payback period. It turns a rough idea into concrete numbers before you gather installer quotes.

The result is a planning estimate. Your real system depends on roof space and shading, your exact tariff, net metering rules and the quotes you receive, so treat it as a starting point.

1. Convert monthly use to a daily figure by dividing by 30.44.
2. Divide by peak sun hours and an 80 percent efficiency factor to size the system.
3. Divide the system watts by your panel wattage and round up for the panel count.
4. Multiply size by cost per watt, take 70 percent for the tax credit, then divide by yearly savings for payback.

The 80 percent factor covers real-world losses from wiring, heat, dust and inverter efficiency, so the array is sized a little larger than the raw numbers suggest.

A home using 900 kWh a month with 4.5 peak sun hours needs about a 8.4 kW system, or 21 panels at 400 watts. At 3 dollars per watt that is 25,200 dollars, or about 17,600 dollars after the 30 percent federal credit. Producing roughly 11,000 kWh a year at 0.17 dollars saves about 1,836 dollars annually, so it pays back in around 10 years and keeps saving for the panels 25-year life.

Your Electricity Use

More kWh means more panels. Cutting waste first, with efficient appliances and habits, can shrink the system you need and its cost.

Peak Sun Hours

Sunnier regions need fewer panels for the same output. Peak sun hours range from about 3 in the north to 6 in the southwest.

Panel Wattage and Shading

Higher-wattage panels cut the count, while shade, roof angle and direction reduce output and can raise the number you need.

How you pay changes the payback and who gets the tax credit.

Owning, by cash or loan, almost always beats leasing over the systems life because you keep the credit and full savings.

Before Getting Quotes

Size the system yourself so you can judge whether an installers proposal is reasonable.

Comparing Homes or Regions

See how sun hours and rates change the payback in different places.

Deciding If Solar Is Worth It

A payback well inside the 25-year panel life usually means solar saves money over time.

1. Ignoring Efficiency Losses

Sizing on raw sun hours without a derate factor undersizes the system, so it never fully offsets your bill.

2. Forgetting the Tax Credit

The 30 percent federal credit dramatically shortens payback, so always use the net cost.

3. Overlooking Roof Space and Shade

The panels must physically fit an unshaded roof, which can cap the system regardless of your use.

4. Using the Wrong Electricity Rate

Savings depend on your real tariff and net metering, not a national average.

The calculator uses standard sizing and payback maths with an 80 percent efficiency factor. It is a planning estimate, not an installer design.

It depends on your electricity use and sun hours. A typical US home using 900 kWh a month needs about 20 to 22 panels, roughly an 8 kW system.

Divide your daily kWh use by your peak sun hours and an 80 percent efficiency factor to get the system size, then divide by your panel wattage and round up.

For most US homes the payback is about 8 to 12 years after the 30 percent federal tax credit, then the panels keep saving for their 25-year life.

The federal residential solar tax credit has been 30 percent, which sharply cuts payback. Rates change, so confirm the current credit before you buy.

Most of the US gets 3 to 6 peak sun hours a day, higher in the southwest and lower in the north. Local solar maps give an exact figure.

If the payback is well within the panels 25-year life, solar usually saves money overall, on top of energy independence and lower bills.

Each panel is about 18 square feet, so a 21-panel system needs roughly 380 square feet of unshaded, well-angled roof.

Yes, but at reduced output. Peak sun hours already average this out across the year, which is why they are used for sizing.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• How solar panels work (Wikipedia).
• Home photovoltaic systems (Wikipedia).
• Federal solar investment tax credit (Wikipedia).