Probably the very best idea for saving energy when cooking was thought up by a troop of boyscouts.
What they did was get an oven hot and put meatloaf in the oven to bring it to the cooking temperature. Once at the cooking temperature, they took it out, stuffed a small electric coffee cup heater into it and wrapped it over and over again with aluminum foil and insulation.
The idea was that by providing a small heat source to replace the heat lost through the insulation, they could maintain the temperature of the meat loaf at it's normal 350 deg. F baking temperature, and it would cook the same as if it were in an oven the whole time.
Apparantly the meat loaf cooked properly by simply replacing the heat lost. I've never tried to do the same thing, but this is an idea that could really save the continent billions of kilowatts. What I'm thinking is that if we could create a large thermos bottle that would fit inside a microwave oven. You turn the microwave oven on to get the food inside the thermos bottle hot, and the food then stays hot (and cooks normally as a result) because there's very slow heat loss through the vaccuum.
A temperature sensor inside the oven could tell the microwave when the food is getting cold and when it's hot, so the microwave would come on and go off when told to by that sensor.
Microwaves will go through glass, (but not metal) and so there's no reason we couldn't build such an oven today. I think the idea has a great deal of potential. Up until now, no one's been concerned about the heat loss from an oven. But, if you can stop that heat loss, you can maintain food at an elevated temperature, and that's really what baking or roasting is really all about.