How does the power setting on a microwave work?
The majority of microwaves cannot modulate their power output. The power setting in most microwaves simply turns the magnetron (microwave generator) off and in in cycles. So a power setting of .5 for 10 minutes would simply cycle the magnetron on and off every few seconds, with a total on time of 5 minutes. You can actually hear this occurring.

According to wikipedia, some newer microwaves can actually achieve a more or less constant level of reduced power using a technique called pulse-width modulation. I have never seen or used one of these though.

Microwave cooking is never very exact, so cooking something at 700 W vs 1000 W at .7 power would yield very different results. The only reliable way I've found in cooking things properly in different wattage microwave ovens is simple observation.