Jennifer Asks:

Hi Solar Ron, I want to run 4 LED lamps in my horses stable. The lamps are 7 Watts each. I want to run them for 5 hours a day. What size of Battery will I need for this application, I would like the battery to have 3 days of autonomy to counteract cloudy days where the solar panels won’t produce much. Thanks in advance. Jen.

Hi Jennifer. Thank you for your question.

Firstly we need to calculate the total load of the lamps, which will be 4 X 7 Watts = 28 Watts. The next this we have to calculate is the required energy per day that will be required, this is the total load X run hours per day which is 28 Watts X 5 Hours = 140 Watt Hours (Wh). Now that we know the energy we need per day, we now need to multiply this figure by the days of autonomy. Which will be 140Wh X 3 Days = 420Wh.

So now we know that you are going to need 420 Wh of energy to run the 4 lamps for 5 hours a day for 3 days.

Now we can calculate the battery size we are going to need,this is done by dividing the energy required (420Wh) by the battery voltage (12VDC) Which works out at 35Ah. It is not possible to run an AGM battery completely flat as it will kill the battery. So we now need to look at what the maximum Depth of Discharge of the battery will be. Most of the time we base this on 50% DOD. So we calculate it like this: 35 / 0.50 = 70Ah.

If you cannot find a suitable battery available in the size you need, then just round up to what is available.

Presto, you will need a 70Ah 12 Volt Battery to power your lamps.

Solar Ron.