What's Watt?

Ongoing correspondence from members and various magazine articles show continuing confusion about the energy drawn by microwave ovens (and also electric motors). This matters when attempting to run them from an inverter - and finding sometimes that they don't. The confusion is that the same unit of measurement, i.e., the 'watt', is used both for work performed and for energy consumed in performing work. It is easy to mistake one for the other, and ongoing articles are doing just this.

Microwave Ovens

A small microwave oven is typically rated as '800 watts'. This rating refers to the heat energy that it produces, not the amount of energy that it consumes. That is typically 1600-1700 watts and thus requires an inverter rated at about 1200 watts (drawing on inverters' ability to produce 40-50% above rated output for short periods). Allowing for an inverter loss of 10%, the '800 watt' microwave thus draws about 150 amps at 12 volts. Such current necessitates starter motor-sized cable from the battery to the inverter to avoid further losses through voltage drop. Because of this all-but universal misunderstanding, most campervans and motorhomes have inadequate battery/inverter cabling. This includes many presently-produced units. To check, feel the cable after the microwave has been running for about five minutes. If it's uncomfortable to hold, it's inadequate and should be upgraded.

Further, it is generally assumed that because a microwave is generally used for only 10 minutes or so, the energy drain from the battery is negligible. This is far from the case. The amount of stored energy that a battery can hand back depends substantially on the magnitude of the current that is drawn. A 100 amp/hour 12 volt battery discharged at 5 amps/hour should last for 20 hours until it falls to 10.5 volts. But if discharged at 150 amps, it will run out of puff long before the arithmetic 100 amp/hrs/150 amps (about 40 minutes). At best it will go for 15-20 minutes, half of the energy being lost as heat as the battery warms up due to internal inefficiencies. The percentage of energy lost as heat depends also on the capacity of the battery bank.

The smaller the battery bank, the greater the amount of energy by which it is depleted when high currents are drawn. We have a situation that is quite different from 'it only draws 800 watts for a few minutes, so we can forget the battery drain'. Microwave oven reality is this: Current draw is typically 150 amps for ten minutes. This is 25 amp/hours - and already twice that which most people wrongly assume. Depending on battery capacity, the depletion effect is a further absolute minimum of 20% (with say 450 amp/hr available) and a probable maximum of 100% (with 100 amp/hour available). The battery bank is thus discharged not by the probably presumed 12 amp/hr (for ten minutes running) but somewhere between 30 amp/hours and 50 amp/hours - or 5 - 8 amp/hr per minute. For a typical small motorhome's battery bank of 250 amp/hours (typically charged to 70% and discharged to 40%) running an 800 watt microwave for ten minutes sucks out about 50% of the available energy.

Electric Motors

A motor's wattage is not a measure of the energy drawn. It relates to the maximum power that the motor can develop (replacing the earlier 'horse-power' -one horsepower equals 746 watts). The energy drawn depends on the motor's efficiency (or lack of it) and the work that it is doing. It is complicated slightly by an effect known as 'power factor' that causes some electrical devices working on alternating current to require 20% more power to be available than they actually use. Power factor is indicated as a decimal - in the above example the power factor is 0.8. As a result, manufacturers rate their motors in watts to indicate the maximum power output, and also in volt/amps (volts x amps x power factor) to indicate the energy they draw whilst working at maximum power. This latter figure can be several times the rated wattage, partly because of power factor, but mostly because some motors are massively inefficient.

Most power tools are an exception. Their so-called commutator motors (also known as 'universal' motors) typically draw 30%-50% more than their rated wattage, and up to twice that whilst starting. The real villains are the small induction motors used in 240-volt pumps, water fountains, some 240-volt fans, domestic 240-volt refrigerators etc. A nominally rated 125 watt (1/6th HP) 240 volt induction motor typically draws over three times that whilst running, and up to six times that whilst starting. A small fountain motor may draw up to ten times its rated wattage (I found that out the hard way!). The worst case is an induction motor driving an air-compressor.

These machines start up on virtually full-load and may require five or six times their running current to begin running. Bigger induction motors are a bit better but still typically draw twice their rated wattage and three/four times for starting. (This is mostly why 12/24 volt dc fridges are so much more efficient - they use commutator motors). Any half-decent inverter can put out about double its continuous rated power for a few seconds and will usually start a motor rated at a third to half its output, so a 2000-watt inverter should comfortably start a one horsepower (750 watt) motor, but not an air compressor.