Panel sizing starts with honest daily watt-hours and realistic sun data—not nameplate sticker watts alone. This guide walks through the solar panel size calculator logic so your array matches load, not guesswork.
Benefits
- Clear formula: minimum panel W ≈ daily Wh ÷ (peak sun hours × system efficiency).
- Uses location-based peak sun hours instead of assuming full noon all day.
- Shows intermediate values so you can cross-check against solar daily yield and battery bank planners.
How it works
- Enter total daily energy need in watt-hours (sum loads × hours or use a load audit).
- Set peak sun hours for your site and season—typical values range from 3–6 hours.
- Apply system efficiency (default ~80%) for wiring, controller, and inverter losses, then review minimum panel watts.
FAQ
How do I calculate solar panel size in watts?
Divide daily watt-hours by the product of peak sun hours and system efficiency (as a decimal). Example: 2,400 Wh ÷ (5 h × 0.80) ≈ 600 W minimum array rating before safety margin.
What are peak sun hours?
Peak sun hours are the equivalent full-sun hours per day for your location—one peak hour equals about 1,000 W/m² irradiance. Winter, tilt, and shading lower this number; use seasonal data when sizing off-grid systems.
Should I add extra panel capacity beyond the calculator result?
Yes. Real installs need headroom for cloudy days, battery round-trip loss, and inverter clipping. Many designers add 20–30% on top of the minimum watts unless a detailed yield model is used.
Technical specifications
- Formula: panel_W ≈ daily_Wh ÷ (peak_sun_h × efficiency_fraction).
- Inputs: positive Wh/day, sun hours, and efficiency (1–100%).
- Output: minimum panel wattage at STC nameplate—not guaranteed daily harvest.
- Planning margin: +20–30% recommended for weather and storage losses.
Daily Wh drives the array
Nameplate panel watts are laboratory figures. Your load budget in watt-hours per day sets how much harvest you must collect. Start with an appliance and runtime list, convert to Wh, then work backward to required array watts using sun hours at your site.
Efficiency is not optional
PWM controllers, long DC runs, hot modules, and partial shade all reduce delivered energy. The efficiency field accounts for system-level loss between the module face and usable Wh at the battery or load. Underestimating efficiency oversizes expectations and undersizes the array.