Introduction
The Solar Panel Calculator helps you figure out how many solar panels you need for your home or building. Just enter a few details about your energy use and roof space to see your results. It tells you how many panels you need, how much power they can make, and how much money you could save. This tool makes it easy to plan your solar panel system. You get the answers you need to go solar with confidence.
How to Use Our Solar Panel Calculator
Enter details about your location, roof, energy use, and budget to find out how many solar panels you need, how much they will cost, and how much money you can save over time.
Address, ZIP Code, or City: Type in where you live, such as a ZIP code, city name, or state. The calculator uses your state or ZIP code to fill in a rough peak-sun-hours value. Click "Lookup" or press Enter to fill it in.
Peak Sun Hours per Day: This is the number of hours per day your area gets strong sunlight. It fills in automatically based on your location, but you can change it if you know your exact value.
Total Roof Area: Enter the size of your roof in square feet or square meters. A typical home has about 1,000 square feet of roof space. You can use our Square Footage Calculator if you need help measuring your roof area.
Usable Roof (%): Use the slider to set what portion of your roof can hold solar panels. This accounts for vents, chimneys, shade, and other obstacles. The default is 75%.
Roof Orientation: Click the direction your roof faces. For the highest yearly output, panels in the Northern Hemisphere face due south.4 Each direction shows a percentage that reflects how much solar energy it receives compared to south.
Roof Tilt / Pitch: Use the slider to set your roof angle in degrees. Solar panels typically perform best on south-facing roofs with a slope between 15 and 40 degrees.5 For the highest yearly output, panels are tilted at an angle equal to the local latitude.4 If you need to determine your roof angle, our Roof Pitch Calculator can help.
Average Monthly Electric Bill: Enter how much you pay for electricity each month in dollars. This helps estimate your energy usage if you do not know your exact kWh number.
Monthly Consumption (kWh): Enter how many kilowatt-hours you use each month. You can find this on your electric bill. If you enter your bill amount instead, this field will auto-calculate.
Electricity Rate ($/kWh): Enter the price you pay per kilowatt-hour of electricity. The U.S. average residential price was 18.19 cents per kWh for January through July 2026.2 Check your utility bill for your actual rate.
Solar Offset Target (%): Use the slider to choose how much of your electric bill you want solar to cover. Setting it to 100% means you want solar to handle all of your electricity needs.
Panel Wattage: Pick the power rating of each solar panel. Standard panels are 350–400 watts. Choose "Custom" if you have a specific panel in mind.
Panel Size (sq ft each): Enter the physical size of one solar panel. A standard residential panel is about 17.5 square feet.
Panel Efficiency (%): Enter the efficiency rating of the panels you plan to use. Most modern panels fall between 18% and 23%.
System Losses (%): Enter the percentage of energy lost due to wiring, the inverter, dirt, shade, and heat. A typical value is 14–20%.
Number of Panels (override): Leave this blank to let the calculator figure out how many panels you need. Enter a number here only if you want to set the panel count yourself.
Annual Degradation (%/yr): Solar panels slowly lose power over time. The typical rate is 0.5% per year. Premium panels may degrade more slowly.
Cost per Watt (installed): Enter the total installed cost per watt, including panels, inverter, labor, and permits. The US average is $2.50 to $3.50 per watt.
Federal Tax Credit %: The 30% federal Residential Clean Energy Credit covered qualified home solar installed from 2022 through December 31, 2025.1 It is not available for property placed in service after December 31, 2025.1 The default here is 0%. Enter a percentage only if another credit applies to you.
State/Local Incentive: Enter any extra rebates or incentives from your state or local government as a flat dollar amount.
Annual Electricity Rate Increase: Enter how much you expect electricity prices to go up each year. The US average is about 2.5% per year.
System Lifespan (years): Enter how many years you expect your solar system to last. Most systems are warrantied for 25 years.
Financing: Choose whether you plan to pay cash upfront or take out a solar loan. If you pick "Solar Loan," you will also enter the interest rate and loan term.
Once all fields are filled in, click Calculate to see your system size, number of panels, total cost, savings over time, payback period, return on investment, and environmental impact. Click Reset to return all inputs to their default values.
What Are Solar Panels and How Do They Work?
Solar panels are devices that turn sunlight into electricity. Each panel is made up of many small units called photovoltaic (PV) cells, usually made from silicon. When sunlight hits these cells, it knocks electrons loose from their atoms, creating a flow of electricity. This is called the photovoltaic effect. The electricity produced is direct current (DC), which an inverter converts into alternating current (AC), the type of electricity your home uses.4 An inverter is expected to need replacing at least once in the 25-year lifetime of a PV array.4
Key Factors That Affect Solar Panel Performance
Not every home gets the same results from solar panels. Several factors determine how much electricity your system will produce:
- Peak Sun Hours: This is the number of hours per day your location receives strong, direct sunlight equal to about 1,000 watts per square meter. The southwestern United States gets 6+ peak sun hours per day, while the Pacific Northwest may only get 3.5–4. More peak sun hours means more energy from the same number of panels.
- Roof Orientation: In the Northern Hemisphere, south-facing roofs capture the most sunlight throughout the day. East- and west-facing roofs still work well, producing about 85% as much energy. North-facing roofs produce the least, roughly 60% compared to south-facing.
- Roof Tilt: The angle of your roof matters. The ideal tilt is roughly equal to your latitude. For most of the United States, that means 25° to 35°. Flat roofs and very steep roofs produce less energy because sunlight doesn't hit the panels at the best angle.
- Shading and Usable Space: Chimneys, vents, skylights, nearby trees, and required setbacks from the roof edge all reduce the area available for panels. Most homes can use about 75% of their total roof area for solar.
- System Losses: No solar system is 100% efficient. Energy is lost through wiring, the inverter, dirt on panels, heat, and small amounts of shading. These losses typically add up to 14–20% of total production.
Understanding Solar Panel Specifications
Modern residential solar panels are rated between 300 and 450 watts each. A 400-watt panel is currently the most common premium option. Each panel is about 17.5 square feet in size (roughly 65 inches by 39 inches). Panel efficiency (the percentage of sunlight converted to electricity) ranges from 18% to 23% for most models available today. Panels also lose a small amount of efficiency each year, typically about 0.5% per year, which is why most manufacturers offer 25-year performance warranties.
Solar Costs and Financial Incentives
The average cost to install a residential solar system in the United States is $2.50 to $3.50 per watt before incentives. This price includes the panels, inverter, mounting hardware, wiring, labor, and permits. For a typical 7 kW system, that works out to roughly $17,500–$24,500 before any tax credits or rebates.
The federal Residential Clean Energy Credit equaled 30% of the cost of qualified home solar installed from 2022 through December 31, 2025, and it is not available for property placed in service after that date.1 Many states and local utilities offer additional rebates, tax credits, or Solar Renewable Energy Certificates (SRECs) that can lower your cost even further.
Payback Period and Long-Term Savings
The payback period is how long it takes for your electricity savings to equal the cost of the system. For most homeowners, this falls between 6 and 12 years, depending on local electricity rates, sunlight, and incentives. After the payback period, the electricity your panels produce is essentially free. Since electricity rates have historically risen 2–3% per year, the value of your solar savings actually increases over time. Over a 25-year lifespan, a well-sized solar system can save homeowners $20,000 to $50,000 or more.
Environmental Benefits of Solar Energy
Solar panels produce clean electricity with no greenhouse gas emissions during operation. This calculator counts 6.72 × 10−4 metric tons of CO₂ avoided for each kilowatt-hour your panels make, the EPA's U.S. national marginal emission rate for electricity.3 Over 25 years, the calculator's default system avoids about 164 metric tons of carbon dioxide. That is the same as planting thousands of trees or taking dozens of cars off the road for a year.