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Example systems, calculated
With 4.5 average sun hours, 400 W panels, a 17.3¢ price and $2.50 per watt. The first three cover all of a household’s use, and the last two cover only a share of an average home.
| Home | System size | 400 W panels | Cost | Savings a year | Payback |
|---|---|---|---|---|---|
| 500 kWh a month, 100% of use | 4.8 kW | 12 | $12,000 | $1,038 | 11.6 years |
| 869 kWh a month, 100% of use | 8.0 kW | 20 | $20,000 | $1,804 | 11.1 years |
| 1,200 kWh a month, 100% of use | 11.2 kW | 28 | $28,000 | $2,491 | 11.2 years |
| 869 kWh a month, 25% of use | 2.0 kW | 5 | $5,000 | $455 | 11.0 years |
| 869 kWh a month, 50% of use | 4.0 kW | 10 | $10,000 | $909 | 11.0 years |
A smaller system pays back in the same time per watt, but it costs far less to start. That makes it an option for a household that cannot fund a full system at once: cover part of your use now, and add panels later.
How it is calculated
- Energy to cover (kWh a day) = monthly use × the share you want to cover ÷ 30.4.
- System size (kW) = energy to cover ÷ (average peak sun hours × 0.8). The 0.8 covers heat, dirt, wiring and inverter losses.
- Panels = system size ÷ panel watts, rounded up.
- Energy made per year = system kW × sun hours × 0.8 × 365.
- Cost = system watts × your cost per watt. Savings per year = energy used × your price per kWh.
- Simple payback = cost ÷ savings per year, with no financing, incentives or price changes.
It uses annual average sun hours, not the worst month, so it suits a grid-tied system and not an off-grid one. Use the solar and battery calculator for off-grid sizing.
Cut the bill before you buy solar
- Every kWh you save is a kWh you do not need to generate. Use the appliance energy cost calculator to find the biggest loads.
- Heating, cooling and electric water heating usually come first.
- Solar does not make sense for every roof or household. Compare more than one quote and check what your utility pays for exported power.
Incentives
This calculator includes no incentives. The 30% federal residential solar credit ended for systems placed in service after December 31, 2025. Some states and utilities still offer rebates, tax breaks or low-income programs, and rules change often, so check your own state’s energy office or utility before you decide.
Using it anywhere
- The state list is for the United States. Anywhere else, type your own monthly kWh and price per kWh.
- Choose the sun level that matches your region’s annual average, and use your own currency for the cost per watt.
Common questions
How many solar panels do I need to cover my electric bill?
For an average home using 869 kWh a month, with 4.5 sun hours and 400 W panels, you need about 7.9 kW of solar, or 20 panels. A home using 500 kWh needs about 12 panels, and one using 1,200 kWh needs about 28.
How much does it cost to cover an average home with solar?
At $2.50 per watt, the system above costs about $20,000, and saves about $1,804 a year at 17.3¢ per kWh, a simple payback of about 11 years. Your cost per watt, electricity price and sun hours change this a lot, so enter your own.
Can I start with a small solar system and add to it?
Often yes. Covering 25% of an average home’s use takes about 5 panels and costs about $5,000 at the same price per watt. Check that your inverter and wiring can be expanded before you start, and ask your utility about its rules for small systems.
Is the federal solar tax credit still available?
The 30% federal residential solar credit ended for systems placed in service after December 31, 2025. State and utility programs still exist in some places, so check them. This calculator leaves incentives out, and you can reduce the cost per watt to include one.
What do the sun hours mean?
Peak sun hours are the hours per day of full-strength sunlight, averaged over the year. Most of the United States gets about 4 to 5.5, with the desert Southwest at 6 and above and the Pacific Northwest and far north lower. Use a number for your own area.
Why is the payback so different from what installers quote?
Installers may include incentives, financing, rising electricity prices and utility credits for exported power, and they use your actual roof and shading. This calculator uses a plain average with no incentives, which usually gives a longer, more cautious payback.
Related calculators
Find your biggest loads with the appliance energy cost calculator. For off-grid sizing, use the solar and battery calculator, and to cover essentials in a blackout, the power outage calculator.