Blog ## HOW MUCH CURRENT IS DRAWN FROM THE 12V (OR 24V) BATTERY WHEN RUNNING AN INVERTER?

CHOOSING THE RIGHT SIZE INVERTER FOR YOUR BATTERY BANK.

One very common use of AMPTRON LiFePO4 batteries is to run an inverter. From wanting to run a kettle while out camping to whole house backup inverters. It is important that you match the size of the inverter to the output capacity of your batteries to ensure ease of operation and battery longevity.

How to calculate the maximum size inverter your battery bank can handle:

Max output watts=cut off voltage ×max continuous discharge
Max output watts=cut off voltage ×max⁡ continuous discharge

Start by finding the cut off voltage of your battery, 10v for 12v batteries, 20v for 24V batteries, 30v for 36v batteries and 40v for 48v batteries.
Then multiply that by the max continuous rating of your battery. If you have multiple batteries connected in parallel, it’s the combined maximum continuous discharge of all the batteries.
e.g:
10V ×100Amps=1000watts or 1kwatt
10V ×100Amps=1000watts or 1kwatt

Now you know the maximum output of your batteries. When looking at the power rating on inverters its crucial to look at the maximum input watts not output watts as they will consumer more watts than they output. If you can’t find the input watts multiply the output watts by 1.15 as its best practice to assume the inverter is 80% efficient. Now ensure your inverter is matched to your batteries voltage and has a maximum input wattage less than maximum output wattage of your batteries.

How to calculate how long can I run my inverter for:

Divide load watts by actual battery voltage, this will be 12V, 24V, 36V and 48V. Then to allow for inverter efficiency, typically 80%, divide the figure by 0.80.
Thus:
For a 300W load at 12V….300 ÷ 12 ÷ 0.80 = 31.25 Amps.
You can see the simple divide by 10 gives an easy “worst case” guide.
Similarly:
For a 300W load at 24V….300 ÷ 24 ÷ 0.80 = 15.625 Amps.

This is the consumption rating per hour. So, divide your remaining battery capacity until 80% depth of discharge by this number to find the amount of time you can run your inverter in hours:
80amp battery / 31.25-amp load = 2.56 hours.