Solar Charge Time

Formula

Description

This estimates the time needed to fully charge a battery from a solar panel, accounting for system efficiency losses. The numerator is the total energy needed in watt-hours, and the denominator is the effective charging power. Real-world solar charging is complicated by variable sunlight, panel angle, temperature derating, and charge controller behavior. The panel wattage is its rated peak power under standard test conditions (1000 W/m² irradiance, 25°C cell temperature), which is rarely achieved in practice. Actual average power is typically 4-6 peak sun hours per day depending on location.

Variables

• t — Charge time in hours (h)
• Capacity — Battery capacity (Ah)
• V — Battery nominal voltage (V)
• Ppanel — Solar panel rated power (W)
• η — System efficiency including charge controller losses (0-1)

Practical Notes

For a realistic estimate, divide the panel rating by 2-3 to account for non-ideal conditions. MPPT charge controllers are 95-98% efficient, while PWM controllers are 70-80% efficient. Battery charge acceptance also decreases above 80% state of charge, extending total charge time.