Solar System Sizing Calculator
Calculate the solar array size in kilowatts needed to cover a chosen percentage of your electricity usage. Use this before contacting installers to arrive with an informed system size estimate.
About this calculator
The required solar system size is determined by dividing annual energy demand by the annual energy output per kilowatt of installed capacity: System Size (kW) = (monthlyKwh × 12 × offsetPercentage) / (sunHours × 365 × systemEfficiency). Monthly kWh scaled to a year gives annual consumption. Multiplying by the offset percentage (e.g., 0.80 for 80% solar coverage) sets the target generation. The denominator represents how many kWh one kilowatt of panels produces annually: peak sun hours per day × 365 days × system efficiency (typically 0.75–0.85, accounting for inverter losses, wiring losses, and shading). Peak sun hours are a location-specific value representing equivalent full-sun irradiance and typically range from 3.5 to 6.5 hours/day in the contiguous US.
How to use
Monthly usage: 900 kWh. Peak sun hours: 5.0 hr/day. System efficiency: 0.80. Desired offset: 100% (1.0). System Size = (900 × 12 × 1.0) / (5.0 × 365 × 0.80) = 10,800 / 1,460 ≈ 7.4 kW. At a standard 400 W panel size, you would need approximately 19 panels (7,400 W ÷ 400 W). Reducing the desired offset to 80% lowers the required system to about 5.9 kW — 15 panels — which may better fit a smaller roof or tighter budget.
Frequently asked questions
How do I find the peak sun hours for my location?
Peak sun hours (PSH) represent the number of hours per day during which solar irradiance averages 1,000 W/m², the standard test condition for panel ratings. You can find your local PSH using the NREL PVWatts Calculator, the Global Solar Atlas, or the EU's PVGIS tool. In the contiguous US, PSH ranges from about 3.5 in the Pacific Northwest to 6.5 in the desert Southwest. PSH is not the same as daylight hours — a location with 14 daylight hours might only have 5 PSH due to cloud cover and low-angle morning and evening sun.
What system efficiency factor should I use for a residential solar installation?
System efficiency (also called performance ratio or derate factor) accounts for real-world energy losses between the rated panel output and what actually reaches your meter. Typical losses include inverter efficiency (96–98%), wiring resistance (1–2%), temperature derating (5–8% in hot climates), and shading (variable). NREL's PVWatts uses a default derate factor of 0.86 for a well-designed residential system with minimal shading. If your roof has partial shading or uses older string inverters, use 0.75–0.80. Microinverters and power optimizers can raise effective efficiency closer to 0.85.
Should I size my solar system to cover 100% of my electricity usage?
Sizing for 100% offset is not always the best financial decision. Generating more electricity than you consume sends surplus to the grid, which many utilities compensate at a lower credit rate than retail — reducing the value of oversizing. Most installers recommend targeting 80–100% offset based on your utility's net metering policy and roof space. If you plan to add an EV or heat pump in the near future, oversizing slightly now avoids a costly second installation later. A battery storage system paired with a slightly oversized array can further maximize self-consumption.