It takes a lot of energy to bring food to the boil, but once boiling the energy setting can be cut by two-thirds. Some vegetables cook quicker in the microwave in less water, with higher retention of water-soluble vitamins.
Place food in the oven immediately after the oven is turned on and you'll save energy.
Use the oven light to check on food in the oven rather than opening the door.
Use dividers and steamers to cook vegetables in the same pot.
It's best to thaw frozen food in the refrigerator, later assisted by microwave if necessary.