Solar Panel Calculator

Please fill in the required fields with values greater than 0.
–system size (kW)
–number of panels
–roof area needed (sq ft)
–yearly production (kWh)

Estimate only. Includes 20% system losses. Shading, roof angle, local weather and equipment change the result. Get a quote from an installer.

How to use this calculator

Enter your monthly energy use from your bill, the peak sun hours for your area and the power of each panel. Press Calculate to see the system size, the number of panels and the roof space needed.

The formula

System size (kW) = daily kWh ÷ (peak sun hours × 0.8)
Panels = system size ÷ panel power
Yearly production = panels × panel kW × sun hours × 0.8 × 365

Example: A home using 900 kWh a month (30 kWh a day) with 4.5 sun hours needs about an 8.33 kW system. With 400 W panels that is 21 panels, using about 378 sq ft of roof and producing about 11,038 kWh a year.

Understanding your solar panel system size

The solar panel calculator estimates how large a system you need to cover your electricity use. The system size in kilowatts is the key number. Installers quote prices per watt, and your utility may limit system size based on past usage. The panel count and roof area tell you whether that system will fit.

What changes the result

  • Energy use. Use a full year of bills if you can. Summer and winter use can be very different.
  • Peak sun hours. Sunnier areas need fewer panels to make the same energy.
  • Panel power. Higher wattage panels mean fewer panels and less roof space.
  • Share to cover. Covering 70% of your use instead of 100% shrinks the system and the upfront cost.

Two examples

The page example uses 900 kWh a month, or about 30 kWh a day. With 4.5 sun hours and the 80% factor, the system is 30 ÷ 3.6, or about 8.33 kW. That is 21 panels at 400 watts, making about 11,038 kWh a year. At an installed cost of $3 per watt, that system costs roughly $25,000 before any incentives.

A home using 600 kWh a month in a sunnier area with 5.5 sun hours needs about 20 ÷ 4.4, or 4.55 kW. That is 12 panels at 400 watts, producing about 7,709 kWh a year.

Tips and common mistakes

  • Count only usable roof. Vents, chimneys, skylights and required fire setbacks reduce space.
  • South-facing roofs usually produce the most in the US. East and west roofs produce less.
  • Shade from trees or nearby buildings can cut output a lot. The tool does not account for shade.
  • Plan for future loads, such as an electric car or heat pump, before you size the system.

Cost and planning advice

Get at least three quotes and compare them by price per watt and by estimated yearly production. Ask about warranties on panels, inverters and labor. Check current federal, state and utility incentives, which change often. If your roof is near the end of its life, replace it first. Solar work involves high voltage, so use a licensed installer and get the required permits and utility approval.

Frequently asked questions

How many solar panels do I need?
Divide your daily energy use by the peak sun hours and the system efficiency to get the kW. Then divide by the panel power.
What are peak sun hours?
They are the hours per day when sunlight is strong enough to give full panel output. Most of the US gets about 3.5 to 6 hours.
How much roof space do I need?
A typical 400 W panel is about 18 square feet, so 20 panels need about 360 square feet.
Why is there a 20 percent loss?
Real systems lose some energy in the inverter, wiring, heat and dirt, so the output is lower than the panel rating.
Do solar panels work in winter or in cloudy weather?
Yes, but they make less power. Peak sun hours already average this over the year.
How long do solar panels last?
Most solar panels are built to last about 25 to 30 years, and many come with a performance warranty covering that period. Output slowly declines with age, usually by a small amount each year. Inverters typically need replacing sooner, often within 10 to 15 years for standard string inverters.
What is net metering?
Net metering is a billing setup where your utility credits you for extra solar power sent to the grid. You draw from those credits when your panels produce less, such as at night. Rules and credit rates vary widely by state and utility, so check your utility’s current policy before sizing your system.
Do I need a battery with solar panels?
Not usually. Most grid-tied systems work without a battery and shut off during outages for safety. A battery adds backup power and can help if your utility pays little for exported power. Batteries add significant cost, so decide based on outage risk and your utility’s rate structure.