How Water Heater Efficiency is Measured

You may know that, generally speaking, the more efficient your water heater, the lower your water heating bills. But do you know how water heater efficiency is measured? One method is Energy Factor (EF). This metric measures efficiency by taking into account how much hot water is produced per unit of fuel consumed in a day.


What affects a water heater’s energy factor?


  • How efficiently the energy source’s heat is transferred to the water (recovery efficiency)
  • What percentage of heat loss there is per hour from the stored water (standby losses)
  • Heat lost when water circulates through the pipes and/or tank (cycling losses)


There are specific formulas for calculating energy factor:


Gas and Oil Water Heaters

365 x 41045 ÷ EF X Fuel Cost (Btu) or 365 x 0.4105 ÷ EF X Fuel Cost (therm)


Electric Water Heaters

365 x 12.03 kWh/day ÷ EF x Fuel Cost ($/kWh)


The higher the energy factor in a water heater, the more efficient it is. However, as we alluded to earlier, high energy factor values don’t always mean lower costs. The size of the water heater will play a big role in determining how much your bill costs – a properly sized unit will be ultra efficient, whereas a unit that’s too big will result in higher bills, and a unit that’s too small won’t produce enough hot water. Fuel type and first hour rating (the amount of gallons of hot water it can supply in an hour) also play a role.


If you’re in the market for a higher-efficiency water heater, consult with us and we’ll help you weigh your options so you can maximize your comfort and savings. Contact us today.