How to size the right inverter

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Victron Energy 12V 180Va Phoenix Inverter
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Victron Energy 12V 180Va Phoenix Inverter
Inverters are electrical devices that convert DC battery power to 240V AC power to allow you to run appliances.

They come in different power capacities suitable to power small devices such as phone/tablet chargers, right up to refrigerators.

Define Your Load

Before purchasing an inverter, you need to know the power draw of your appliance in watts. You can find this information in the manufacturers specifications for the appliance shown in either watts or amps. 

If only amps are shown, you'll need to convert that to watts by multiplying the amps by voltage. For example, if the appliance draws 0.5 amps, multiply 0.5 amps by 240 volts to get 120 watts (A x V = W).

Some appliances also need a surge of power to start up, though the surge can last only a fraction of a second, it can be up to 2.5 times the rated running watts of the appliance.

If connecting multiple appliances, you'll need to add the total watts and allow for the total start up surge.

If applied to the example above, the surge could be up to 300 watts.

Inverter Power Ratings

The specifications on most inverters give 2 capacity values.
  • Continuous Power Watts - The AC power output available.
  • Peak Power Watts - The AC power output available for a split second.
The Peak Power Watts for most inverters is usually around double the capacity of the Continuous Power Watt rating.

If applied to the example above, you would need a 175W inverter with a peak of 350W

Battery Capacity

You'll need to determine how much DC battery capacity is required to run your appliance. To calculate this, divide the AC running watts by your nominal battery system voltage and multiply the result by 1.1. (WAC / VDC x 1.1 = ADC)

If applied to the example above, for a 12V system, 120WAC / 12VDC x 1.1 = 11ADC
The appliance will draw 11Amps DC per hour.
Victron Energy 12V 220Ah Deep Cycle AGM Battery
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Victron Energy 12V 220Ah Deep Cycle AGM Battery

Battery Ratings

Deep cycle AGM batteries are rated in ampere-hours (Ah) . A battery rated at 220Ah can deliver 220 DC amps for 1 hour before it's considered flat.

Typically, you can damage an AGM battery if you discharge it more then 50%, so you would consider a 220Ah AGM battery to only have 110Ah of usable DC power.

If applied to the example above, you could run your 11 amp appliance for 10 hours before you would need to recharge your battery.
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