Why a water heater uses energy between hot-water draws
An estimate based only on hot water used at the taps can be lower than measured heater consumption because the system also maintains stored heat and replaces distribution losses. The difference does not automatically indicate a failed heater.
Follow the heat beyond the tap
Draw energy raises incoming water to the required temperature. A tank loses heat to its surroundings between draws. Pipes lose heat during and after delivery, while a recirculation loop may lose heat to keep water available. Pumps and controls also consume electricity. There is no universal extra percentage for these effects.
Treat a measurement gap as a question
Suppose a fictional electric resistance system supplies 100 L/day at a 35°C rise. At 100% conversion, the draw model gives 100 × 35 × 0.001163 = 4.0705 kWh/day. A representative heater-only measurement of 5.0 kWh/day leaves a gap of 0.9295 kWh. That gap is not a direct measurement of tank loss: volume error, changing temperatures and measurement boundaries also need checking.
Avoid counting an allowance twice
A burner’s conversion efficiency describes how much fuel energy reaches the water. A whole-system seasonal performance factor can account for a broader set of operating losses. Rated performance figures also depend on specified test conditions. If the performance input already includes standby effects, adding a separate standby allowance may duplicate them. Heat-pump operating COP can change with air temperature and resistance backup use.
Record heater-only energy across representative days alongside draw estimates and conditions. Whole-home consumption cannot isolate the heater without separating other loads. The Hot Water Cost Calculator models draw energy; keep any independently justified loss allowance outside it. Compare similar occupied days, arrange assessment of operating faults, and follow manufacturer and local health requirements for settings.
Worked example: Convert two shower routines into monthly heated volume
Related reading: Hot water from the tank is not the same as mixed shower volume
Reference: ASHRAE Handbook: service water heating.