The higher the SEF, the more energy efficient the water heater. SEF ratings run from 1.0-11.
Solar Energy Factor
Higher SEF systems are more expensive initially, so the money that you may save depends on several factors, i.e. how much hot water you use, the cost of conventional fuels in your area, available solar energy to be converted into solar power, etc.
Costs vary greatly by region (and even by zip code). To get free estimates from local contractors, please indicate yours.