Van life · solar
Solar charge time calculator.
Work out how many full-sun hours, and how many days, a set of panels needs to bring a battery from one charge level to another.
Inputs
Charge time
Enter your numbers.
How long do solar panels take to charge a battery?
Energy to add = Ah x volts x (target % - start %). Charging hours at full sun = energy / (panel watts x (1 - losses)). Days = hours / peak sun hours per day.
Worked example. A 200 Ah 12 V battery is 2,400 Wh. Going from 20% to 100% adds 1,920 Wh. Three 100 W panels (300 W) at 25% losses deliver 225 W, so it takes 8.5 full-sun hours, which is 2.1 days at 4 peak sun hours.
Limits. Charging slows near full as the charger tapers, panels rarely sit at their rating, and your loads draw power while it charges. The 25% losses and 4 sun hours are placeholders to replace with your own figures.
Frequently asked questions
How long does it take a 100W solar panel to charge a 12V 100Ah battery?
A 100 Ah 12V battery is 1,200 Wh. Going from 0 to 100% at 25% losses a 100 W panel gives 75 W, so it needs about 16 full-sun hours, or 4 days at 4 sun hours. Use the form for your own numbers.
Why is the real charge time longer than the calculation?
Chargers taper near full, panels are shaded or angled badly, and your loads use some of the power as it arrives.
How do I convert Ah to Wh?
Multiply amp-hours by the battery voltage. 200 Ah at 12 V is 2,400 Wh.
What loss percentage should I use?
Start with the 25% planning assumption used here and adjust it to your setup. It is not a published standard.
Sources
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Planning estimate, not professional advice
Results are planning estimates from the numbers you enter. They are not professional, engineering, electrical or safety advice and are not a substitute for the manufacturer's instructions or local code. Battery banks, inverters and DC wiring can start fires or injure you if built wrongly: size cable and fuses to the equipment manual, fuse every positive conductor close to the battery, and have a qualified electrician check the build.