Solar Panel Sizing Calculator Formula

Understand the math behind the solar panel sizing calculator. Each variable explained with a worked example.

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Formulas Used

Required Panel Wattage

panel_watts = daily_energy_wh / (sun_hours * eff_fraction)

Panel Current (12V system)

panel_current_12v = daily_energy_wh / (sun_hours * eff_fraction) / 12

Daily Generation

daily_generation_wh = daily_energy_wh / eff_fraction

Variables

VariableDescriptionDefault
daily_energy_whDaily Energy Needed(Wh)50
sun_hoursPeak Sun Hours(hours)5
system_efficiency_pctSystem Efficiency(%)75
eff_fractionDerived value= system_efficiency_pct / 100calculated

How It Works

Solar Panel Sizing

Formula

Panel Watts = Daily Energy / (Sun Hours x Efficiency)

Efficiency accounts for charge controller losses, wiring, temperature derating, and panel degradation. Typical system efficiency is 70-80%.

Worked Example

50 Wh daily energy need, 5 peak sun hours, 75% system efficiency.

daily_energy_wh = 50sun_hours = 5system_efficiency_pct = 75
  1. 01Required generation = 50 / 0.75 = 66.7 Wh
  2. 02Panel watts = 66.7 / 5 = 13.3 W
  3. 03At 12 V: current = 13.3 / 12 = 1.11 A

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