How does the physical size of a microwave oven affect its efficiency?

Factual Questions
1

It’s been a long time since I took physics, so I don’t really remember how to do these calculations:

Let’s say you have two 1000-watt microwaves (as in the cooking appliance). One is 1 cubic foot inside, and the other is 1.5 cubic feet.

[ol]
[li]Is the bigger one any more or less efficient at cooking, both mathematically and practically?[/li]
[li]On a similar note, would a 500-watt microwave take twice as long to cook the same food as a 1000-watt microwave but use the same amount of energy?[/li][/ol]

2

  1. To a first approximation, they are the same efficiency. The microwaves will bounce around the inside until they are absorbed by the food. In practice, there will be a (probably small) loss of efficiency as the oven gets larger, since more bounces will be required before the energy is absorbed, and on each bounce some of the energy is lost to the housing.

  2. Again, to a first approximation, yes. But there are again various inefficiencies, such as the fact that the food is constantly cooling, so halving the wattage requires over twice as long to cook (you can imagine the limiting case of a 1-watt microwave that never gets the food hot enough). Also, the electronics tend to get more efficient as you increase the wattage, so although the 1000 watt model may have twice the cooking power, it may well use less than twice the wall power.

3

The tag on microwave ovens shows the input and output. You need to convert the input amps to watts of course. The output is the rating of the MW. This output seems to vary from oven to oven even in the same product. All transformers and magnetrons are not created equal. This can be checked using a container of water and a thermometer. Put a pound or two in the container and cook it on high for two minutes and apply the BTU conversion rate.