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12V Battery sizing for the off-grid in your RV

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To start from the end. I did calculations to cover our family’s RV needs in an off-grid situation and decided to buy a AGM 140ah deep cycle battery. We also had two solar panels rated at 260 watts installed on the roof of the RV to support our off grid life.

What devices do you use?

So how did I do my calculations for right capacity battery? I counted different devices in the RV:

  • Total of 15 led lights
  • Truma gas boiler/heater
  • Diesel heater
  • 1500w 12V – > 240V inverter
  • 12V TV
  • Gas heated fridge
  • Water pump
  • Some other devices like battery monitors, power panels, etc.

Also took into account the charging need for phones and tables that we use.

After that I calculated a rough power consumption of these devices for the period of no charge from solar panels. Basically the period that we would totally run on a battery, this is evening/night time.

What is the power consumption of those devices?

First more calculations when there is more active use during the evening probably from 8PM until 10-11pm:

  • Lights on – approximately 2 amps per hour;
  • Water boiler and heater – at maximum heating cycle and water heating it takes around 6 amps per hour, but on average heating cycle it consumes 1.2 amps;
  • Inverter – very much depends what we use there. But usually we have a laptop, router and maybe some other devices connected to it and it consumes around 2-6 amps per hour through an inverter;
  • 12 volt TV’s power consumption according to manufacture is 4.5 amps per hour (but we barely use it);
  • Water pump is rated at 80watts, which is 6.66 amps (so if you use it for dish washing for 5-10minutes or toilet then it is around 1 amp per hour.
  • Diesel heater at low to mid level operation it consumes approximately 4 amps an hour.
  • + charging of phones and battery banks probably also up to 1 amp at maximum.

For that 2-3 hour period we are consuming at maximum 15 amps per hour, so total of 45 amps. After that, during the night time for about 8 hours when it is cold outside and diesel heater is on, with about 32 amps of consumption in total for that period. So absolute maximum would be around 77 hours at cold evenings (below freezing degrees outside) with everyone sitting in the RV and doing their stuff.

Choice of the battery

I figured that we mostly use our RV when it is actually warmer outside and not so much need for heating. So our power need would remain around 40-45 amps. But if to do a backwards calculation then considering potential 40-80amps discharge based on my calculations, and considering that AGM battery could be safely discharged to 50% (but should not exceed 70% discharge) it meant that I had to have a battery double of that 40-80 amps, i.e. I needed to aim between 80-160ah AGM battery. In case of lithium ion battery the targeted capacity would have been 70 amps. But considering that 70amp LiFePo4 battery cost is about 3x the 140ah AGM/gel battery, then the choice was easy.

So do your calculations and you can find some of the battery suggestions for deep cycle batteries here: 12V Deep cycle battery