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How to calculate leisure battery size for your conversion

To calculate our battery size we will be filling out a load sheet that looks like the image above.

Column 1: In order to figure out the size of batteries required, we will first need to figure out all the appliances we will have in our van. On the load sheet, list out all the electrical appliances you want in your van.

Column 2: Enter the quantity you will have of each item.

Column 3: Enter the Voltage of the appliances; most appliances will be 12V DC, though some will be 230V AC (eg. the laptop is charged through the mains plug sockets).

Column 4: Enter the Amps of each individual appliance (you can find this on the spec sheet for the appliance).

Column 5: Total Amps = quantity × Amps.

Column 6: Watts = Total Amps × Volts.

Column 7: Enter the number of hours per day you estimate you will use the appliance (Note: fridges turn on and off throughout the day, so you don't need to enter 24 hours for your fridge).

Column 8: Total Watt hours = quantity × Watts × hours used.

The output at the very bottom of the worksheet is the total Watt hours (Wh) that will be used given all our appliances. It is the sum of all the total Watt hours.

Convert Wh to Ah: We will quickly convert the Watt hours (Wh) to Amp hours (Ah) as Ah is more commonly used for sizing 12V leisure batteries.

The formula to convert Wh to Ah is: Ah = Wh ÷ V

So given our estimated loads, Ah = 1185 Wh ÷ 12V. Which gives us ~98 Ah

In other words, we can expect to use 98 Ah of battery power per day in our campervan. However I want to be able to be off-grid for at least two days, so I'm going to multiply the Ah by two to give me 196 Ah.

One further caveat; remember we should only discharge lead-acid batteries to 50%, so we will actually need TWICE our calculated Ah. 2 × 196 Ah = 392 Ah. This gives us two full days off-grid, assuming no charging via solar or battery chargers during this time.

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