Updated March 2016
Caravan and motor home microwave oven power draw cannot be sustained for long by deep cycle lead acid batteries. AGMs and lithium-ion are far better suited. Microwave oven power draw is also increased by losses within the inverter used to drive it.
A typical caravan or motor home microwave oven is rated at 800 watts (in terms of heat produced). As most are about 50% efficient, that 800 watt microwave oven power draw is about 1200 watts. Inverters vary from 75% efficient (for really old ones best taken to the tip) to a now 85%-90% efficient. The total microwave oven power draw may thus be anywhere from 1330-1600 watts. If run from 12 volts that’s 110-133 amps.
This 850 watt Dometic microwave oven draws about 1350 watts. Pic: Dometic
The amount of energy used by a microwave oven is proportional to time. If run for 15 minutes a 12 volt (800 watt) such oven will draw 25-33 amp hours. This may not seem much if you have (say) a 150 amp hour battery but it cannot do so for long.
When high current is drawn from a small capacity battery its voltage will drop to the level where the battery can no longer sustain that load. That may happen when the battery is still well charged. The battery may partially recover after resting, but cannot sustain that high microwave oven power draw for long. The battery’s remaining capacity however is still there. It can readily be accessed used – but at lower current draw.
In practice, unless you have at least 350 amp hour (of 12 volt) preferably AGM batteries, or about 100 amp hour of lithium-ion batteries, caravan and motor home microwave power usage is best reserved to where you have grid or generator 230 volts. It can be done from lower capacity conventional deep cycle lead acid batteries – but not for long at a time. Such high draw also shorten such batteries’ life.
See also: What Peukert really meant
Every aspect of electrical stuff in RVs is covered in depth in my totally up-to-date book Caravan & Motorhome Electrics, that on solar in RVs is in my book Solar That Really Works. Solar Success covers bigger homes and property systems. The all-new Caravan & Motorhome Book covers every aspect of RV usage. The more specialised The Camper Trailer Book provides in depth cover of that area.