When using a power inverter, it is always a good idea to turn on the car’s engine every 30 minutes to help maintain your battery's charge. Assuming your battery is fully charged and in good condition, it is possible to use a 400W power inverter for about an hour (not starting you car) without fully discharging your battery.
To find how long your battery’s charge will last with any given inverter, use this formula:
Take the wattage being used (400W) divided by the battery voltage (12 Volts) to see how many amps are being drawn from the inverter : 400 Watts/12 Volts = 33.33 Amps. Inverters only convert power at around 90% efficiency, so that means the inverter is really drawing around 37 amps (33.33 Amps / .90 = 37 Amps). A typical car battery has a reserve capacity of about 80 minutes (80 minutes @ 25 Amps). So if you are drawing 37 Amps with the 400W inverter it will completely discharge the battery in 54 minutes ((80 minutes * 25 Amps) / 37 amps = 54 minutes).
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