That is a Good Question!
It is very useful to be able to calculate watt hours to enable a good match for battery to your DC lighting project.
It doesn’t matter if your building a small solar PV lighting system for a 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?
There are TWO important facts that you need to be able to calculate watt hours. The first is the ‘Watt Hours’ you intend to use and the second is the ‘Watt hours’ that will be available from your battery pack. To give you an idea, I will give an illustration as follows…
You are using a 12V Lead Acid or Gel battery pack with a 10 Amp (Ah) rating. When fully charged (assuming as new condition) this little battery will have a capacity of around 120 watt hours. We calculate this by multiply the battery ‘Ah’ capacity by the nominal voltage of (in this case) 12V and we get the available watt hour or WH potential. To break this down into usable data we can see that a 7 Watt CCFL lamp would last for around 17 hours (120 watt hours divided by 7) before the battery will be exhausted. For a larger project, we can calculate as follows in the following example: A 2000 amp hour 12V battery bank. Fully charged will potentially give a watt hour ‘WH’ rating of 24,000 watts. I have to say, it is bad practice to completely discharge a battery in any application so we strongly advise not discharging your battery below 50% (at least not on a regular basis). If you need to discharge below this figure, you will need to increase the size of your battery bank 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 75-50% 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.