About 90% of the energy used for washing clothes in a conventional top-load washer is for heating the water. There are two ways to reduce the amount of energy used for washing clothes — use less water and use cooler water. Unless you’re dealing with oily stains, the warmer cold water setting on your machine will generally do a good job of cleaning your clothes. Switching your temperature setting from hot to warm can cut a load’s energy use in half.
Some other ways to lessen your laundry's eco-impact include washing clothes in cold water whenever possible; using the appropriate water level setting that corresponds with the size of your load; drying towels and heavier cottons in a separate load from lighter-weight clothes; not over-drying your clothes (if your machine has a moisture sensor, use it!); and air-drying clothes on a clothesline or drying racks.
When buying a new washer, look for the Energy Star label. Energy Star washers clean clothes with 50% less energy than standard washers and use only 15 gallons of water per load, as compared to the 32.5 gallons used by standard washers.