Inverters are typically only about 85 per cent efficient . In the process of converting 12V to 240V , the inverter generates a lot of heat and many will need a cooling fan to dissipate this heat . That heat is the result of this inefficiency and it will mean even more amps are drawn from the battery .
And there ’ s another problem . Batteries are seldom ever exactly 12V . They range from between 12 to 14V when fully charged and , in the case of lead acid batteries , their voltage drops over time as their state of charge decreases . The lower the voltage of the battery , the higher the amps draw from the inverter .
As a rule of thumb , you are better off taking a conservative approach . That way , you give yourself a margin of error when working out how long you can run your appliances using an inverter . A very simple formula to determine the amps draw of a 240V appliance at the battery is to divide the watts rating of the appliance by 10 . Therefore , our 1000W appliance will draw 100A from the battery while in use through the inverter .
Interestingly , Apple rate this MacBook charger at 87-watts yet the label suggest it ’ s more like 150-watts . Always assume the higher rating when sizing an appropriate inverter
Now think about your caravan battery . If you have a typical 100Ah battery , that load of 100A will completely drain the battery in one hour , theoretically . Most caravans and campers will be equipped with lead acid batteries ( AGM , gel or wet cell ) and these should not be discharged below 50 per cent . As a consequence , you should not run that 1000W appliance for more than half an hour from a 100Ah battery , otherwise you risk damaging it .
Fortunately , most of these highwattage appliances do not need to be run for such a long period of time . You might typically use an electric kettle for about 10 minutes a day , in which case you will only take out 16.6Ah from the battery . That could easily be replaced during a day of good solar charging .
If you can turn off the heater and humidifier on your CPAP machine , it will consume a lot less power
If you ’ re running high power appliances from an inverter you will need a good battery management system
10 \