Some 50-gallon water heaters are energy-efficient, including the M150L6DS and the M250L6DS models of electric water heaters from Bradford White, but the overall level of energy efficiency typically depends on the brand and model. The two listed models feature energy factors of 0.90 and 0.91, respectively.
Some 50-gallon water heaters that run on natural gas, such as the ND50T122-403 model from Whirlpool, feature a slightly lower energy factor in comparison to energy-efficient electric models. They typically use an automatic flue damper system to ensure an energy factor ranging from 0.67 to 0.7, which is enough for the Energy Star specification. Despite the lower energy efficiency, water heaters that utilize natural gas are more cost-effective than electric models due to the lower price of gas in comparison to electricity, as of 2015.