The formula
Daily Wh = Panel watts × Number of panels × Peak sun hours × Efficiency
Daily Ah = Daily Wh ÷ Battery voltage
Why panels produce less than their rating
- Heat: panels lose roughly 0.3–0.4% of output for every °C above 25 °C cell temperature, and cells run 20–30 °C hotter than the air in full sun.
- Angle and shading: flat or partly shaded panels collect far less. Even a roof rail's shadow can cut a panel's output sharply.
- Controller and wiring losses: MPPT controllers are about 95–98% efficient, PWM controllers waste more on most panels.
- Battery charging losses: lead-acid batteries need about 15–20% more energy in than you get out; lithium about 5%.
The "typical peak charge current" assumes about 80% of rated power at the battery's charging voltage. It's useful for sizing cables between controller and battery.
Frequently asked questions
How much power does a 100W solar panel produce per day?
With 4 peak sun hours and 75% efficiency, about 300 Wh a day, or roughly 25 Ah into a 12 V battery.
How many amps does a 200W solar panel produce?
Through an MPPT controller into a 12 V battery, a 200 W panel typically delivers 10–13 A in strong sun.