Solar Panel Charge Time Calculator
This calculator works out how long a solar panel takes to recharge a battery or portable power station. Enter the capacity, how much charge it has now, and the watts of your panels. The result is in peak sun hours, and in days once you add the peak sun hours your location gets.
Charge time from solar
A power station takes in only as many solar watts as its input limit allows, and charging slows near full. If the panel watts are above the input limit on the listing, enter the limit instead.
How the calculator works
1. Energy to put back. Capacity in Wh × (target charge − charge now). A 1,000 Wh power station going from 20% to 100% needs 800 Wh. If the battery lists amp-hours, Wh = Ah × volts.
2. Panel output. Rated panel watts × (1 − system losses). Panels reach their rated watts only in full, direct sun at the right angle, and wiring, dirt and heat take a share. With 14% losses, 200 watts of panels deliver about 172 watts.
3. Charge time. Energy to put back ÷ panel output gives hours of full sun, called peak sun hours. 800 ÷ 172 = 4.7 peak sun hours.
4. Days. Peak sun hours needed ÷ peak sun hours per day at your location. At 4 a day, 4.7 hours is a little over one day.
Where the starting values come from
| Input | Starting value | Where it comes from |
|---|---|---|
| Solar system losses | 14% | The default in NREL’s PVWatts calculator |
| Peak sun hours | 4 | A placeholder. Look up your location on NREL PVWatts |
Peak sun hours are much lower in winter than in summer in most of the US, and shade on even part of a panel cuts its output sharply, so use the figure for the season and spot you will charge in.
Reading the result
- Solar input limit. A power station accepts only so many watts of solar, within a stated voltage range. Panels above that limit do not charge it any faster.
- Using power while charging slows the refill. Subtract the watts your devices draw from the panel output, or size the whole setup with the solar generator and battery size calculator.
- The last few percent are slower. Chargers taper the current near full, so the top of the charge takes longer than the formula says.