Shaun wrote:
When I did similar tests, I found the order of magnitude was right,
but the result depended on the shape and volume of the mass of water
and position in the oven. [...]

Wonder what the "official" water configuration is when they determine
the spec?

I don't know about a US Federal Trade Commission (or other agency, or
equivalent in other countries) test procedure; there probably is one but
I don't know it.

I know that older GE microwave ovens, in the little service information
sheet that was folded up inside the oven, gave directions on the test, a
GE part number for a beaker you were supposed to use, and I think a
third-party part number for the thermometer you were supposed to use to
measure the water temperature before and after. I think the water level
was marked on the beaker, and the service sheet said to put it right in
the middle of the oven. You didn't boil it, just heated it for a fixed
amount of time.

If I remember right, the criteria was something like "if the oven
produced between X and Y degrees temperature rise in the water, it's
within spec" - it didn't give you an answer in watts, just an acceptable
temperature range for that particular model oven.

Matt Roberds