About 90% of the energy consumed for washing clothes is used to heat the water. Unless your clothes have oily stains, washing with cold or warm water will clean your clothes just as effectively.
Step by step:
Run full loads. Wash full laundry loads whenever possible. Most load sizes use about the same amount of energy, so running fewer, larger loads helps avoid waste.
Use the coolest water possible. Switching your temperature setting from hot to warm can cut energy use in half for washing one load. Using the cold cycle—when you can—reduces energy use even more. Unless circumstances require, avoid the sanitary cycle, which uses extremely hot water and increases energy use.
Cold-water detergent. Though most detergents will work in cold water, some are specially made for cold-water washing.
Good for kids: Active kids mean lots of laundry. Running full loads becomes even more important when you need to wash clothes for several household members. Separate out the very oily, dirty clothes for a warm water cycle and leave the rest for cooler water cycles. A hamper or basket with multiple compartments that is easy to access in a laundry closet or hallway can make this task easy for everyone.