Inverter power usage (1 Viewer)

[Evildoody]

I have recently fitted a 1.5KW pure sine wave inverter to my motorhome, I thought I would do some maths for s***s and giggles (maybe have a laminated card with the info below fitted to the back of the door for future owners)

(if someone more knowledgeable wants to double check my maths I'd appreciate it) :-

Power Consumption on 1.5KW Inverter (based on 2 x 110AH batteries depleted to 50% = 110AH)

(calculation takes in to consideration inverter inefficiencies)

Microwave - AC 700W/4.79A (actual power requirement AC 1150 Watt) = DC 1150W/106AH

Fridge - AC 220W/0.91A = DC 218.4W/20.09A

Water Boiler - AC 850W/3.54A = DC 849.6W/78.16A

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Essentially :-

To deplete 220AH leisure batteries (110AH x 2 @50%) @ continuous runtime=

Microwave = 1hr constant

Fridge = 5.4hr constant

Water Boiler = 1.4hr constant

NB:-

Microwave will run on mains at site or inverter (IF needed) - however usage on inverter of a few mins here and there is negligible

Fridge will run on DC while engine is running, mains at site or gas (Inverter should never be used) - it can but why would you?

Water Boiler will run on mains hookup and/or gas (Gas and inverter is not advised), Gas OR mains is fine and Gas AND mains is fine

Essentially the 1.5KW pure sine wave inverter CAN power all of the above from leisure batteries alone, but is not advised as you can see from current draw outlined above.

NB - 110AH 12V battery = 1.3KW storage

If all above is run solely on leisure batteries, runtime to 50% battery depletion = 30mins max!

 

[DBK]

The problem with your calculations is the amp hour rating of a battery is based on a fairly low rate of current drain. At high current drains the estimated capacity is much lower.
For example, a 100Ah battery could deliver one amp for a hundred hours, and possibly even more as from memory (unreliable ) the rating is based on a discharge over twenty hours.
However, discharge the battery at 100 amps and sadly it won't last an hour. Ten or fifteen minutes might be more like it to get down to 50%.
Having just fitted a similar inverter I think the only reliable guide is to monitor the battery voltage and if it drops much below 12 volts then time to displug things - but we haven't given it a real life trial yet - that will come at an off grid CL this Sunday.

 

[funflair]

Well ye but, is the answer as for a start there is Peukerts law which I know nothing about other than it says the faster you draw from your batteries the less ah you will get. Obviously when the batteries are being worked hard the terminal voltage will drop considerably even if it recovers afterward and this in itself may trip out the inverter on low volts, forget heating water as that's just sillly which I am sure you know, I would not want to run the microwave for more than 5 minutes really but nothing wrong with your numbers.

Martin

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[autorouter]

Fridge will run on DC while engine is running, mains at site or gas (Inverter should never be used) - it can but why would you?

When the engine is stopped but gas can't be used. On a ferry crossing, or if the gas has run out. In many motorhomes you can't turn the fridge on DC when the engine is stopped.

The fridge consumption is a bit pessimistic because the 220 watt element is not on all the time. It switches on and of by the thermostat. If it's already cold, and the door stays closed, it's probably on for only 30 to 50% of the time, or even less.