Solar Panel System Size Calculator
Determine how many kilowatts of solar panels you need to meet a chosen percentage of your electricity bill. Ideal for homeowners and installers sizing a new rooftop or ground-mount system.
About this calculator
The required solar system size depends on your annual energy demand, how many hours of strong sunlight your location receives each day, and how efficiently the system converts sunlight to usable power. The formula is: System Size (kW) = (monthlyUsage × 12 × offset) / (sunHours × 365 × systemEfficiency). Monthly usage multiplied by 12 gives annual demand in kWh. Multiplying by the offset (e.g., 0.8 for 80% coverage) sets your target. Dividing by peak sun hours × 365 gives the daily energy per installed kW, and system efficiency (typically 0.75–0.85) accounts for inverter losses, wiring losses, and temperature derating. The result is the DC nameplate capacity you need to purchase.
How to use
A household uses 900 kWh/month and wants to offset 100% of usage. Their location has 5 peak sun hours/day and a system efficiency of 0.80. Calculate: (900 × 12 × 1.0) / (5 × 365 × 0.80) = 10,800 / 1,460 ≈ 7.4 kW. This means the homeowner needs approximately a 7.4 kW system. At a typical panel rating of 400 W, that equals about 19 panels. Reducing the offset goal to 80% would lower the required size to roughly 5.9 kW.
Frequently asked questions
What are peak sun hours and how do they differ from daylight hours?
Peak sun hours measure the equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter — the standard test condition for panel ratings. A sunny city like Phoenix averages 5.5–6.5 peak sun hours, while Seattle averages 3.5–4. Daylight hours can be 14+ in summer but include morning and evening periods of weak sunlight that contribute little energy. Using peak sun hours gives a realistic estimate of actual energy harvest.
How does system efficiency affect the solar panel size I need?
System efficiency captures all energy losses between the panels and your outlet, including inverter conversion losses (~4–6%), wiring resistance, soiling, and temperature derating (panels produce less in extreme heat). A typical overall derate factor is 0.75–0.85. A lower efficiency means more panels are needed to deliver the same usable energy. Choosing a high-quality inverter and keeping panels clean can push efficiency toward the upper end of that range.
What energy offset goal should I aim for when sizing my solar system?
Most homeowners target 80–100% offset, but the optimal choice depends on your utility's net metering policy. If your utility pays retail rate for exported power, a 100% offset maximizes savings. If export compensation is low, oversizing can be wasteful and an 80–90% offset may offer better return on investment. Battery storage changes the equation by letting you use surplus power at night rather than exporting it.