Solar Panel System Calculator
Estimate how many solar panels you need to cover your home's electricity usage based on your location's sun hours and panel specs. Ideal for planning a residential rooftop solar installation.
About this calculator
The number of solar panels required depends on your monthly energy consumption, the solar resource at your location, panel wattage, and real-world system losses. The formula used here is: panels = ⌈(monthlyUsage × 1000) / (30 × sunHours × panelWattage × systemEfficiency)⌉. Monthly usage in kWh is converted to watt-hours, then divided by the energy each panel produces per month — peak sun hours per day times panel wattage times 30 days — adjusted downward by system efficiency to account for inverter losses, wiring losses, soiling, and temperature derating. Peak sun hours represent the equivalent number of hours per day at 1,000 W/m² irradiance and vary from about 3 hours in cloudy northern climates to over 6 hours in desert regions. System efficiency typically ranges from 0.75 to 0.85 for modern grid-tied installations.
How to use
Suppose your home uses 900 kWh per month, you receive 5 peak sun hours per day, you choose 400 W panels, and your system efficiency is 0.80. Plug in: panels = ⌈(900 × 1000) / (30 × 5 × 400 × 0.80)⌉ = ⌈900,000 / 48,000⌉ = ⌈18.75⌉ = 19 panels. So you would need 19 × 400 W = 7,600 W (7.6 kW) of installed capacity. Adjusting sun hours or efficiency immediately shows how sensitive the result is to your local climate and equipment choice.
Frequently asked questions
How many solar panels does the average home need to go fully off-grid?
The average U.S. home consumes roughly 900 kWh per month. With 5 peak sun hours and 400 W panels at 80% system efficiency, you would need about 19 panels (7.6 kW). Homes in less sunny climates may need 25 or more. Off-grid systems also require battery storage sized for several days of autonomy, which significantly increases cost and complexity compared to grid-tied setups.
What are peak sun hours and how do they affect solar panel output?
Peak sun hours are the number of hours per day during which solar irradiance averages 1,000 W/m², the standard test condition for panel ratings. A location with 5 peak sun hours will produce 5 times the rated panel wattage in watt-hours each day. Phoenix, AZ averages about 6.5 peak sun hours; Seattle, WA averages closer to 3.5. Using accurate local peak sun hour data is the single biggest factor in sizing your system correctly; underestimating it leads to an oversized, more expensive array.
Why does system efficiency matter when calculating solar panel requirements?
No solar system converts 100% of harvested sunlight into usable AC electricity. Inverter conversion losses, DC wiring resistance, module temperature rise, soiling on panels, and mismatch between panels all reduce output. A typical modern grid-tied system achieves 77–85% overall efficiency. If you ignore these losses and design at 100% efficiency, your system will consistently underperform expectations. Entering a realistic efficiency value (e.g., 0.80) in the calculator ensures your panel count already accounts for these unavoidable real-world deductions.