The majority of energy used when washing clothes goes into heating the water.  Up to 90% of the energy needed to run a washing machine on the hot water cycle setting is used to heat the water.  Why do you need hot water to wash clothes?  You really don't.  Hot water can make colors fade quicker and can aid in overall wear and tear of your clothing.  Having said that, sometimes cold water even with adequate detergent might not be able to handle oil or greasy stains effectively and you will most likely still need to use a warm water cycle once in awhile.