Solar Panel Count Calculator
Estimate how many solar panels you need to meet your daily energy usage based on panel wattage and peak sun hours.
How to use this tool
- Find your daily kWh use on your electricity bill and enter it.
- Enter the wattage of the solar panels you plan to use.
- Enter peak sun hours for your location (find these on solar resource maps).
- Set system efficiency at 80% as a conservative starting estimate.
Calculate the number of solar panels needed to power your home from your electricity usage and location.
Formula
Panel daily output (kWh) = (panel_watts ÷ 1000) × sun_hours × (efficiency% ÷ 100)
Panels needed = ⌈daily_energy_use ÷ panel_daily_output⌉
System size (kW) = panels × panel_watts ÷ 1000
How it works
The calculator converts panel wattage to kilowatts, multiplies by the number of peak sun hours per day, and then applies the system efficiency factor (which captures inverter losses, wiring losses, soiling, and temperature derating) to arrive at a realistic daily energy output per panel. Dividing your daily consumption by this figure gives the minimum whole-panel count needed.
Peak sun hours represent equivalent hours of full 1,000 W/m² irradiance and vary significantly by location and season; using a site-specific value from a solar atlas will improve accuracy. The system efficiency default of 80% is a common industry estimate — newer inverters and well-maintained panels may achieve 82–85%.
Worked example
Worked example
- Panel output = (400 W ÷ 1,000) × 5 h × (80 ÷ 100) = 0.4 × 5 × 0.8 = 1.6 kWh/day per panel.
- Panels needed = ⌈30 ÷ 1.6⌉ = ⌈18.75⌉ = 19 panels.
- System size = 19 × 400 W ÷ 1,000 = 7.6 kW.
Daily output per panel: 1.6 kWh; Panels needed: 19; System size: 7.6 kW
Key terms
- Peak sun hours
- The number of hours per day during which solar irradiance averages 1,000 W/m². A location receiving 5 peak sun hours accumulates the same energy as 5 hours of full midday sun, regardless of actual daylight hours.
- System efficiency
- A combined derating factor accounting for inverter conversion losses (~4%), cable losses (~2%), soiling (~2%), and temperature losses (~5–8%). Typical values range from 75% to 85%.
- Watt-peak (Wp)
- The rated output of a solar panel under Standard Test Conditions (1,000 W/m² irradiance, 25°C cell temperature, AM 1.5 spectrum). Real-world output is usually lower.
- Inverter
- A device that converts the DC electricity generated by solar panels into AC electricity compatible with home appliances and the grid. Its efficiency is the main component of system losses.
- Energy offset
- The fraction of a household's annual electricity consumption provided by the solar system. A properly sized system typically targets 80–100% offset.
Frequently asked questions
- How many solar panels do I need?
- Divide your daily kWh consumption by the energy produced per panel per day (panel kW × peak sun hours × efficiency). Round up to the nearest whole panel.
- How many peak sun hours does my location get?
- The US averages 4–6 peak sun hours per day. Arizona and Nevada can exceed 6.5. The Pacific Northwest and Northeast average 3.5–4.5 hours.