How to work out the time it takes a solar panel to recharge a battery

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

25 February 2024

A reader asked me this question recently: how long would it take a 200W solar panel to charge two 75 amp-hour (Ah) batteries on a clear sunny day? To do the calculation, we need to know the voltage rating of the battery; he said it is 12V.

The first step is to find out how much electrical energy one battery can hold. We get that by multiply the voltage by the current rating. This comes to 12V x 75Ah = 900 watt-hours (Wh). But there two batteries, so combined value is 1,800Wh.

Next, we divide this storage capacity by the panel’s power out put to get the time. Thus; 1,800Wh divided by 200W = 9h. But, when doing that calculation, there some assumptions that we have made that might not be valid.

First of all, we have assumed that the solar panel will produce the 200W throughout the day. But we know that the intensity of sunlight is very low at sunrise, reaches its maximum at noon and then drops to a very low value at sunset. The 200W rating is only produced at peak intensity.

Consequently, the average output during a clear, sunny day is only a half of the rated value; in this case, it is 100W. With that adjustment, it turns out that we need 18 hours to generate the 1,800Wh needed to fill up the two batteries.

We have also assumed that the entire 1,800Wh of energy stored in the battery is available for usage. This is not correct. In solar batteries, the available charge is about 50% of the rated value. If it is discharged below half capacity, it loses its ability to recharge. This is the reason there is a charge controller in the system to cut off further draining of power once the 50% capacity is reached.

Therefore, we should only work with half of the 1800Wh; that is, 900Wh. Now when we divide this 900Wh by the real 100W of available power from the panel, we still get the same 9h to fill the battery.

That outcome is just coincidence. If these were normal 75Wh car batteries, the available capacity would be 30%; that is 540Wh. In that case, the re-charge time comes to 5.4h. But we must, not celebrate! There is no free lunch: the car battery will discharge equally fast!