Using simple mathematical formulas, we set up a simple guide that will help you to calculate the charging time of your batteries using solar panels. In our example we consider the efficiency of an battery charger with MPPT controller which is more efficient compared to PWM chargers.

If you are using a MPPT controller the maximum efficiency you can achieve is 90%. So with 355 watt of solar panels and 5 sun hours the most energy you can generate =

Watts x Hours x .90 = 355 watts x 5 hours x .90 = 1597 watt hours

For the batteries to get a rough idea take the battery Voltage x Amp Hours = Watt Hours.

So 12 volts x 250 AH = 3000watt hours, and you have 2 batteries so 6000 watt hours of reserve capacity.

By now you have all you need to know. All you have to do is factor out the watts to be left with time in hours.

So if your batteries have 50% charge, you need to replace 3000 watt-hours. Your panels can generate 1597 wh with a 5 sun hour day or 1597 wh / 5 hours = 319 watts. So factor out the Sun Hours 3000 wh / 319 watts = 9 hours.

The formula is w = wh/h, which means h = wh/ w, and wh = w x h.

Let’s say you have the following solar power system:

• 48 volt,200Ah lead acid battery discharged to 50%
• 960 watt solar panel
• PWM charge controller

Here’s a simplified way to estimate how long it’d take for the solar panel to charge the battery:

1. Divide solar panel wattage by battery voltage to estimate maximum charge current output by solar charge controller:

960W / 48V = 20A

2. Multiply current by rule-of-thumb system losses (20%) and charge controller efficiency (PWM: 75%; MPPT: 95%):

20A * (1 - 15%) * 95% = 16A

3. Multiply battery capacity by 1 divided by rule-of-thumb battery charge efficiency (lead acid: 85%; lithium: 95%):

200Ah * (1 / 85%) = 235Ah

4. Divide battery capacity by current to estimate how long it’d take to charge the entire battery:

235Ah / 16A = 14 hrs

5. Multiply the charge time by the battery’s depth of discharge to estimate how long it’d take to charge the battery at its current level:

14 hrs * 50% DoD = 7 hrs

6. Add 2 hours to account for the absorption charging stage of most charge controllers:

7 hrs + 2 hrs = 9 hrs

So, in this example, it’d take about 9 hours to charge a 48 volt battery with a 960 watt solar panel.

A solar battery bank 24V, 250Ah is charged via an MPPT controller and solar panels.

1800Wp solar panels charge lead-acid batteries at 24v