Battery bank size?

Question

How do I determine the amp size of my battery bank? Do I add all the amps being used or do I add up all the watts being used? Or am I completely off track?

Answer 1

Count up the watts of all the devices in your RV.  You also need to determine how long each device will be operated.

Divide the watts by the battery bank voltage to determine the amps.  There will also be losses for any inverter system used to convert from DC back to AC.

Battery banks are typically rated in amp-hours.

Answer 2

It is very common to confuse these units.

Amps - rate of current used. How much current is being used.

Amp Hours - a unit of charge. Used to define battery capacity.

Watts - rate of power consumption.

If you want to know what the capacity of a battery bank is then just add the amp hours together.

The Amps is a different matter and is mainly used to determine what sort of fusing and cables are required. It depends on the cells used and their configuration, ie. parallel or serial.

Answer 3

to answer your question, either add up the amps and multiply by 12 volts (assuming this is a 12 v system) to get watts or add up the watts directly, either one works.

That will get you the number of watts used, or amps used, which you can multiply by 12 to get the watts also.

If you have an inverter in the system and generate 120 VAC for some of the appliances, this complicates things.

Calculate the items on the AC line via wattage or current (multiply by 120), then multiply by 1.2 to allow for inefficiencies in the inverter.

But I suspect you want the energy capability of the batteries. This you can only get by looking at the specs on each. Usually they are rated in amp-hours, and 100 amp-hrs is typical.  You can convert this number to energy stored by multiplying by 12, so a 100 amp-hr battery would have a capacity of 1200 watt hrs or 1.2 kw-hrs.

Once you have the energy rating, you can calculate how long your batteries will last before being discharged. If your number is 3000 watt-hrs and you have loads of 300 watts, then you have 10 hours of run time.

Don't forget items like stoves and refrigerators only are on part of the time.

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