Solar Panel Calculator — How Many Panels Do I Need?
Find out how many solar panels your home needs, how much energy they will produce, how much CO₂ they offset, and when the system pays for itself. Enter your monthly electricity usage, the peak sun hours for your location (available from PVGIS or similar solar maps), and the panel specifications.
kWh
h/day
W
%
$/kWh
$
Number of panels to cover your monthly electricity usage
- 1
Daily electricity usage
900 ÷ 30.44 = 29.566 - 2
Panel daily output (kWh)
0.4 × 4.5 × 0.8 = 1.44Panel kW × peak sun hours × efficiency fraction. - 3
Raw panels required
29.566 ÷ 1.44 = 20.532 - 4
Panels needed (ceiling)
⌈20.532⌉ = 21
How does this calculator work?
Panels needed = ⌈(monthly kWh ÷ 30.44) ÷ (panel kW × peak sun hours × efficiency)⌉. Annual production = panels × panel kW × peak sun hours × 365 × efficiency. Payback = system cost ÷ annual savings. CO₂ offset uses 0.386 kg/kWh (US average — adjust for your grid). Peak sun hours and electricity price are the two most location-sensitive inputs.
Formula
How this is calculated
Solar panel sizing starts with your daily electricity demand: monthly usage divided by 30.44 days. A single panel produces energy equal to its rated wattage (in kilowatts) multiplied by the peak sun hours at your location — the number of hours per day the sun shines at 1 000 W/m² equivalent intensity. This figure varies from about 2.5 h/day in northern Europe to over 6 h/day in the Middle East and desert south-west USA; check a solar irradiance map (PVGIS, PVWatts, or GlobalSolarAtlas) for your exact location.
The system efficiency factor (typically 75–85%) accounts for inverter conversion losses, wiring resistance, shading, soiling, and temperature de-rating. A 400 W panel running at 80% efficiency in 4.5 hours of peak sun produces 400 × 0.001 × 4.5 × 0.80 = 1.44 kWh per day. The number of panels is then the daily demand divided by single-panel daily output, rounded up to whole panels.
The CO₂ offset uses the US EPA average grid emission factor of 0.386 kg CO₂ per kWh (2022 figure — your grid factor will differ; adjust if needed). The payback period assumes you sell savings, not the panels — divide the total installed cost by the annual electricity bill reduction. Actual paybacks are affected by feed-in tariffs, battery storage, panel degradation (~0.5% per year), and future electricity price changes. All cost figures are editable estimates; consult a local installer for precise quotes.
Frequently asked questions
Peak sun hours is the equivalent number of hours per day your location receives 1 000 W/m² of solar irradiance — the standard test condition intensity for panel ratings. It is not the same as daylight hours. Look up your value on PVGIS (EU), PVWatts (US), or GlobalSolarAtlas: most of southern Europe and the US Sun Belt averages 4.5–6 h/day; northern Europe and the UK average 2.5–3.5 h/day.
Yes, significantly. Real-world efficiency of a PV system is typically 75–85% of the rated panel wattage, due to inverter losses (95%+), wiring resistance, temperature effects on output, and soiling. For example, moving from 85% to 75% efficiency increases the panels needed by about 13%. Always use a realistic efficiency factor, not the nameplate rating.
The default CO₂ factor (0.386 kg/kWh) is the US EPA average grid emission intensity for 2022. It varies widely: the UK grid is around 0.22 kg/kWh, France (nuclear-heavy) is around 0.06 kg/kWh, and coal-heavy grids can exceed 0.8 kg/kWh. The avoided CO₂ per kWh equals the displaced grid generation intensity — scale accordingly for your country.
Also known as
TG we-Calculate Editorial Team. (2026). Solar Panel Calculator — How Many Panels Do I Need? [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/solar-panel-calculator
TG we-Calculate Editorial Team. "Solar Panel Calculator — How Many Panels Do I Need?." TG we-Calculate. 2026. https://we-calculate.com/calculator/solar-panel-calculator.
TG we-Calculate Editorial Team, "Solar Panel Calculator — How Many Panels Do I Need?," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/solar-panel-calculator
@misc{wecalculate_solar_panel_calculator, title = {Solar Panel Calculator — How Many Panels Do I Need?}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/solar-panel-calculator}}, year = {2026}, note = {TG we-Calculate} }
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