That is a Good Question!

This is a very important factor when calculating what size of battery to use for your project.

It doesn’t matter if your building a small solar PV system to enable you to have some light in your shed or workshop or designing your first electric car, it is going to be Very valuable to know how to calculate your battery size correctly.  So, how can I do this then?

You need to be able to calculate the Watt Hours you intend to use and the power available from your chosen battery pack. To give you an idea I will illustrate this for you…

You are using a 12V Lead Acid or Gel battery pack with a 10 Amp Hour rating. Now then; a fully charged Lead Acid battery (in as new condition) will have a cell voltage of 2.2V. When we multiply this by the number of cells in a 12V battery (which is 6) we calculate that the voltage will be 13.2V. If we take the battery voltage and multiply it by the battery capacity (which in this case is 10Ah), we get the available watt hour or WH potential of 132. To break this down into usable date we can see that a 7 Watt CCFL lamp would last for around 18 hours (132 divide by 7) before the battery was almost completely exhausted. For a larger project, we can calculate as follows: 2000 amp hour 12V battery bank. Fully charges to 13.2V gives a watt hour ‘WH’ rating of 26,400 watts. I have to say, it would be a bit reckless to completely discharge ones battery in whatever application you have in mind so we strongly advise using or drawing power down to around 50% OR doubling the size of your battery to give extra reserve. It is also worth bearing in mind that a regular grade lead acid / gel battery of say 3-5 years of age may well be working at around 50% capacity and may also be less efficient to charge (takes longer and or absorbs less of the charge power).

Ok, so lets look at installing a simple shed lighting installation then. Ok, so we want to use a 12V battery and we need to use a single 11Watt CFL lamp to provide strong illumination. If using the lamps for say 3 hours a day (night), we would use 33 watt hours a day. Over 7 days we would consume over 230 watt hours of available power. The nearest size battery to provide this level of power would probably be around 20 Ah (amp hours), but because we don’t want to end up completely discharging the battery we suggest doubling this to 40Ah. In this case a regular sized Car Battery may suffice. However, using a modest solar PV cell and regulator would ensure the battery was kept in peak condition at all times.