Solar Panel System Sizing Calculator
Find out exactly how many solar panels your home needs to cover its electricity usage. Enter your monthly consumption, local sun hours, panel wattage, and system efficiency to get a panel count.
About this calculator
Sizing a solar array correctly ensures you generate enough energy annually without over-investing in panels. The core formula is: Number of Panels = ⌈(monthlyUsage × 12) / (365 × sunHours × systemEfficiency × panelWattage / 1000)⌉. The denominator calculates how many kWh a single panel produces per year: multiply daily peak sun hours by system efficiency (typically 0.75–0.85 to account for inverter losses, wiring, and temperature derating) and by panel output in kilowatts. Dividing annual consumption by single-panel annual output gives the exact count, which is then rounded up to the nearest whole panel using the ceiling function. System efficiency is the most important variable to get right — a 10-percentage-point difference can change the panel count by one or two panels.
How to use
Your home uses 900 kWh/month, your location receives 5 peak sun hours per day, your chosen 400 W panels have an efficiency factor of 0.80. Annual usage = 900 × 12 = 10,800 kWh. Single-panel annual output = 365 × 5 × 0.80 × (400 / 1000) = 365 × 5 × 0.80 × 0.40 = 584 kWh/year. Panels needed = ⌈10,800 / 584⌉ = ⌈18.49⌉ = 19 panels. So you would need 19 × 400 W = 7.6 kW system. Check that your roof has enough usable area to accommodate 19 panels before finalizing the design.
Frequently asked questions
How do I find peak sun hours for my location when sizing a solar system?
Peak sun hours (PSH) represent the equivalent number of hours per day when solar irradiance averages 1,000 W/m², the standard test condition used to rate panel output. They are not the same as daylight hours. The National Renewable Energy Laboratory's PVWatts tool and the Global Solar Atlas both provide free PSH data by ZIP code or GPS coordinates. In the US, PSH ranges from about 3.5 hours in the Pacific Northwest to over 6 hours in the Desert Southwest. Using your location's annual average PSH gives the most representative panel-count estimate, though monthly variation means winter may require grid backup even with an adequately sized system.
What system efficiency factor should I use when calculating how many solar panels I need?
System efficiency (also called the performance ratio or derate factor) accounts for all real-world losses between the rated panel output and actual AC electricity delivered to your home. These losses include inverter conversion losses (4–8%), wiring resistance (1–2%), soiling and dust (2–5%), temperature derating (5–10%), and shading (variable). A commonly used default is 0.80, meaning 80% of rated panel capacity reaches your panel as usable electricity. High-quality microinverter systems in clean, cool climates may achieve 0.85, while older string-inverter systems in hot, dusty environments may be closer to 0.75. Erring slightly low (e.g., 0.78 instead of 0.82) adds a small safety margin to ensure your system meets your usage target.
Why does the solar panel sizing formula round up instead of rounding to the nearest whole number?
Solar panels are discrete units — you cannot install a fraction of a panel — and the goal of system sizing is to ensure you produce at least enough electricity to meet your annual usage target. Rounding down would leave you short of your energy goal, forcing you to buy more grid electricity than planned. The ceiling function (⌈x⌉) always rounds up to the next whole panel, guaranteeing that your system meets or slightly exceeds your consumption. The cost of one extra panel is typically $200–$400, a small premium for the assurance that your system fully covers your usage, and any slight over-production is simply exported to the grid as a net-metering credit.