Solar battery charging time

The equation that solves for charge time is as follows: charge time (hours) = (Amp-hours removed x 1.15)/charge rate; where Amp-hours removed equals the load being placed on the battery in amp-hours (think of this as how much battery capacity is being used by appliances), 1.15 is 115 percent of capacity, and the charge rate is equal to Amps = Watts/Volts. Keep in mind that this is a somewhat simplified equation because other components in the system may have to be taken into account, like the charge controller and inverter.

Now let’s plug in some numbers. We’ll be using a 12 Volt MK sealed gel battery and a 245 Watt Canadian Solar panel, with the battery at 50% capacity. This MK battery is a 245 Amp-hour battery, so if it’s at 50% capacity then amp-hours removed equals 122.5 Amp-hours. The product of 122.5 and 1.15 is 140.875. To find the charge rate, divide the wattage of the panel by the voltage of the battery (Amps = Watts/Volts), which is 245 Watts/12 Volts, which equals 20.42 Amps. So the equation now reads 140.875 Amp-hours/20.42 Amps, which equals about 6.9 hours.

So in this simple example, one 245 watt solar panel will take just under 7 hours to charge one 12 Volt battery with a load that is using half of the battery’s capacity. Keep in mind that if there are more batteries, the voltage number used in the charge rate (not charge time) equation will change. The same goes for adding on more solar panels.