Some Calculations : Battery Capacity

Ever wonder how long your battery will last using a number of light bulbs with a particular wattage?

One formula for power is P = V x I, where P is for power in Watts, V is for Volts and I is for the current in amperes (DC).  Not so simple for people like us, who do not have any electrical or engineering background.

Here is a simplified way on how to compute for it.

Best thing to do is to give an example.

So, how long will I be able to use my 12v, 70 Ah Battery if I am powering two (2) pieces of 5 Watts light bulbs?

First, we need to compute for the battery's capacity.

12v x 70 ah = 840W (V x I = P)

840W is the maximum amount of power stored in my battery, but you cannot use all of that, you should not fully drain your battery.  It is bad for the battery's health 😆.

They have what they call DOD or Depth Of Discharge.  If you want to use only 50% of the battery's capacity then you just have to multiply your total capacity to this percentage.  I am not sure what is the best DOD to use, but since the example given to me was 50%, I assume that this is a good enough DOD.

840 Watts x 50% DOD = 420 Watts, this is the available energy that you can use.

Now for our consumption, we have two 5W bulbs or a total of 10W (2 x 5).  We just have to divide our allowed capacity to this figure.

420W / 10W = 42 hours

This means that your fully charge 70Ah battery can power two (2) 5W bulbs for 42 hours,  and after 42 hours, your battery will still have a charge of 50%.

So, I hope this will help you in computing for the number of light bulbs that you can use, using a particular Ampere battery.  So that you would not over drain your battery, which in turn will enable you to use it for a longer time.

Many thanks to Sir Jun Bro Magno of Solar Ng Pilipino for his advice in this computation.