What it costs to heat water
Heating water requires both water and energy. The inlet-to-outlet temperature rise matters more than the outlet temperature alone. Raising 50 L from 15°C to 55°C means a 40°C rise.
Delivered heat and input energy
Heat delivered to the water in kWh is approximately liters × temperature rise in °C × 0.001163. For 50 L and 40°C, that is 2.326 kWh of heat. With 90% efficiency, input energy is 2.5844 kWh. At an illustrative USD 0.25/kWh, energy costs about USD 0.65.
Efficiency and COP
Add the water charge using the appropriate volume unit. Heat pumps use a coefficient of performance (COP), which can exceed one. A COP of 3 means about three units of delivered heat per unit of electricity under those operating conditions.
Losses beyond the draw
The calculation excludes tank standby losses, distribution losses and fixed fees. Enter a fuel price per input kWh; a price per therm or cubic meter must be converted first. Do not change heater safety settings based on this estimate.
Match heated volume to its temperature rise
A mixed shower stream includes cold water. Use the full mixed volume with its rise above the inlet, or the hot-supply volume with its larger rise. Do not use the full mixed volume with the tank temperature rise. Water charges still apply to all metered volume.
Worked example: Compare two heat-pump operating conditions
Related reading: Hot water from the tank is not the same as mixed shower volume
Reference: ASHRAE Handbook: service water heating.