Solar Battery Charge Time Calculator
Work out how long a solar panel takes to charge a battery, with MPPT or PWM controllers and lead-acid or lithium batteries.
Your result
Enter your numbers and press the button. Your result appears here.
Formula
Charge time (h) = battery V × Ah × depth of discharge ÷ (panel W × controller efficiency × battery charging efficiency). Days = hours ÷ peak sun hours.
Example: 12 V × 100 Ah × 50% = 600 Wh. A 200 W panel through an MPPT controller into a lead-acid battery delivers 200 × 0.95 × 0.85 = 161.5 W, so 3.7 hours of full sun.
Why the controller matters
An MPPT controller converts the panel's higher voltage into extra charging current. A PWM controller connects the panel straight to the battery, so a panel rated at about 18 V works near the battery's 13–14 V and gives up roughly a quarter of its power.
Full-sun hours
The answer is in hours of full sun. Spread over a day, divide by your peak sun hours.
Charge time by panel size
12 V 100 Ah lead-acid, 50% to refill, MPPT controller.
| Panel | Full-sun hours | Days at 4.98 sun hours |
|---|---|---|
| 50 W | 14.9 | 3.0 |
| 100 W | 7.4 | 1.5 |
| 200 W | 3.7 | 0.7 |
| 300 W | 2.5 | 0.5 |
| 400 W | 1.9 | 0.4 |
Common questions
How long to charge a 100 Ah battery with a 200 W panel?
Refilling half of a 12 V 100 Ah lead-acid battery (600 Wh) through an MPPT controller takes about 3.7 hours of full sun.
Can I charge a battery straight from a panel?
Not safely. Use a charge controller to prevent overcharging.
Sources
- NREL PVWatts v8 state results compiled by TheGreenWatt: https://thegreenwatt.com/average-peak-sun-hours-by-state/