Don't confuse voltage and wattage of an appliance. There will probably be a range of wattages. What you need to worry about is the current that it will draw.
Please don't even think of using a microwave from a liesure battery or through an inverter. You will knacker the battery very quickly. Additionally a low voltage microwave would need heavy duty wiring, switches etc..
A 500 Watt unit after allowing for inefficiencies will probably draw 70-80 Amps.
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Low voltage microwaves seem in many cases to be simply 'mains' units with a built in inverter, so generally they come in the same sort of wattage range - 500 to 750 ish.
These will only draw 3 - 4 amps from the EHU, so depending on the site supply, there should be a couple of amps spare. I've never used a microwave in a van - I don't believe in carrying round anything more than I need, but I'd check up if any 'switch-on surge' would trip the EHU. I've seen some units described as 'soft start' which should eliminate this.
------------- Two drifters off to see the world.
I'm tired of reality, so I'm off to look for a good fantasy.
We have a Daewoo Compact QT1 it's a 14ltr 600W Microwave.
Never had any problems on site EHU and when we are off grid it runs off a 1000W/2400W Peak Invertor supplied by 2x 110Ah leisure Battery's in parallel.
Just to add to your confusion! The 600W (or whatever rating it claims) is the Microwave OUTPUT power (the cooking power if you like), the input power (the actual power drawn from the supply) can be a third or more higher (600W output, 850W input!).
No hard and fast 'rules' as to the difference between input and output power, generally you have to find the rating plate on the actual appliance to find it's power consumption as it's rarely stated in any of the 'figures' on the box or adverts etc.
600/850W (around 4 Amp load)is not usually a problem with 240v EHU, but pushing the boundaries for powering from a modest leisure battery/inverter set up (around 80 Amp load on 12v battery), you could run the batteries stone dead very quickly (a good inverter should stop you actually damaging the batteries by over discharge).
hi I also have the Daewoo compact qt microwave never had any problem using this with on site electrics. also use halogen oven as don't like the gas fitted oven don't use both at same time tho.