Once space heating and cooling has been addressed, water heating methods –  the second  largest opportunity for efficiency improvement – must be considered. To optimize energy efficiency in a zero energy home, builders, and designers should look at the type and location of the water heating system they install as well as strategies for minimizing the use of hot water by residents. Water heater efficiency is expressed as the uniform energy factor (UEF), which is essentially the percentage of fuel converted to useful heat. UEF is expressed as a decimal, where .90 UEF is about 90% efficient. In addition to selecting a low UEF water heater, the type of water heater chosen will depend on the amount of hot water to be used, the climate, and the building design.