Calculating Current

Sep 16, 2010

How much current will you use?

You need to have a clear idea of the total current usage on your boat, mainly so that you can determine the capacity of the batteries you need. Battery capacity is measured in amp/hours (Ah). The results will also be useful when selecting cable.

Start by making a list of all the electrical equipment on your boat. The engine starter motor will normally have its own dedicated battery, if so don’t include it on this list. Firstly, for each item on the list allocate its current requirement in Amps. To find the Amps you can divide the consumption (Watts) by the system voltage, i.e. 12v or 24v.

Amps   =   Watts
                    Volts

Then, secondly you should multiply the current requirements of each item by its total hours of use between one full battery recharge and the next. This will give you the amp/hours (Ah) needed for each item. Add all the Ah figures together for all the items on the list and you will have the total Ah requirement for your system.

Ideally batteries should not be discharged by more than 50%, so you should install sufficient battery capacity to meet at least double your calculated requirement.