Amps/Watt draw for a microwave and any heat element

rjsurfer wrote:
From what I understand I'm probably lucky im getting the 15 minutes.

I would say so. You should check your batteries and wiring for heating while doing this.

15 minutes is a VERY long time on 4 batteries.

I'm running four Trojan GC battery's (T-105's) and a 2000 inverter and I can get between 13 to 15 minutes of operation on my 1200 watt microwave running on high.

I would like more run time so I'm checking on an possible issue with cable thicknesses.

From what I understand I'm probably lucky im getting the 15 minutes.

I might also look into the Panasonic inverter style microwave supposedly more efficient.

Good luck.

Ron W.

Hi,

Nothing wrong with your math, but you have assumed a pure sine wave inverter is 100% efficient. They are not. The best seem to be about 88%, but that figure is not under full load. Often (but not always) peak efficiency is at about 60% of full load.

So the 10 amps @ 120 volts becomes 100 amps @ 12 volts.

100 /.88 = 113 amps @ 12 volts. However under a 100 amp load there will be voltage drop. It is often 0.5 volts.

So now the 113 amps is going to be about 4% larger or about 117 amps.

But that doesn't allow for line losses due to resistance.

By the time all those factors are taken into account, dividing the original 1200 watts by 10 is a good ball park for the amps of demand at 12 volts.

avvidclif1 wrote:
Ohms Law.

I = E/P where I = Current, E = Voltage (120), and P = Watts.

A 1200w Microwave would be 120/1200 = 10A

Microwaves do not have a heating element. The cooking energy is generated by a magnetron. Convection ovens do have a heating element. If you have a microwave/convection oven combo then it has both.
A heating element is a resistance heating device. It uses exactly the same amount of electricity at start up as it does any other time. Most of the larger heating element type appliances will use at most 1,500 watts on a high setting. Some smaller toasters and coffee makers will run about 800 watts.
1,500 watts from an electric skillet for instance is about 150 amps at 12 volt DC. You would want to have around 500 Ah of battery capacity as an absolute minimum to try and run a load like that. Otherwise you're really not going to get much life from your batteries.
A microwave using about 800 to 1,000 watts would still be best served by that size of bank as a minimum.

Go with as large of and as many batteries as you can hold and afford. The axiom "You can never have too much power." holds true for off-grid RV'ing :)

To your questions: HEATING elements are resistive loads. They consume their rated wattage from power on to power off, there is no surge or startup current.

Anything with a motor is an inductive load. That's your rooftop AC, your microwave (it has a motor and huge transformer), the bedside fan, etc. They have startup current and run current. The startup current for something like a A/C unit is huge, a microwave's not so bad. Figure 50% for a microwave and 100% for an A/C (1200w microwave = 1800 watt surge, 1800 watt A/C = 3600 watt surge)

If you have a big budget, consider a 48 volt system (8x 12v batteries in two banks for example). 48 volts will let you use smaller wire (maybe 00 gauge instead of doubled up 0000 gauge). you can run dual high-wattage inverters, so you can power up both legs of your RV's 50 amp service.

Ohms Law.

I = E/P where I = Current, E = Voltage (120), and P = Watts.

A 1200w Microwave would be 120/1200 = 10A

I know we all hate to look in the owners manual for information, it is a male thing, but for an idea of what something uses check the value of the circuit breaker that it is on.

I think convection microwaves have a heating element but I am pretty sure they switch back and forth between microwave and element when cooking convection mode.

Hi ekyyc7,

If you want help then it is best to be polite.

Microwaves do not have a 'heating element' in the sense of a resistance heater.

The accepted way to convert from watts to amps with an inverter in use is to divide the watts by 10. For example my microwave is 1591 watts so it draws about 160 amps.

ekyyc7 wrote:

2oldman wrote:
And I don't think Mw's have a heat element.

They do have a heat element.

I know I can look stuff up genius...pointless answer.

Microwave ovens use radio waves at a specifically set frequency to agitate water molecules in food. As these water molecules get increasingly agitated they begin to vibrate at the atomic level and generate heat. This heat is what actually cooks food in the oven.

Need a decent battery bank and good quality inverter if you want to use a microwave.