How many solar panels do I need to charge a 100Ah battery?

One 200 watt panel can refill a 12 volt 100 Ah battery from half full in a single sunny day. At 75% system efficiency it delivers 150 Wh per peak sun hour, or 750 Wh in a 5 sun hour day. The battery needs 600 Wh to go from 50% to full.

From empty the battery needs 1,200 Wh, which one 200 watt panel covers in 1.6 days at 5 sun hours a day. To charge from empty in one day, you would need about 240 watts of usable output per sun hour, which is 1,200 Wh divided by 5.

Work backward from your daily use. Divide the watt-hours you need by your peak sun hours and system efficiency to get panel watts. An MPPT charge controller can help when the panel voltage is much higher than the battery's. The solar charging time calculator checks any combination.