Water Heating Load Shift Calculator

Water Heating Load Shift Calculator — fast, accurate results online. Enter your values and get instant answers.

L
°C
%
$/kWh
$/kWh

Results

Calculated
Electricity per cycle
—
In kWh
Cost at peak price
—
Per cycle
Cost at off-peak price
—
Per cycle
Savings from shifting
—
Per year, one cycle per day

Why shift water heating

Heating water is one of the largest flexible electrical loads in a home. Because a tank stores heat, you can choose when to run the heater without changing the hot water you get. If your utility charges more during evening peak hours than overnight or at midday, moving the reheat to the cheaper window cuts the bill and eases demand on the grid at the times it is most strained.

This calculator estimates the electricity needed to raise a volume of water by a chosen temperature, then prices that energy at your peak and off-peak rates. Enter the litres heated per cycle, the temperature rise, the heater efficiency and the two prices. It reports kWh per cycle, cost at each rate, and the yearly saving if you shift one cycle every day. It suits time-of-use tariffs and is a planning estimate, not a bill.

Formula and variables

E = V × 4.186 × ΔT / 3600 / η

  • V is the water volume in litres; one litre is treated as one kilogram.
  • 4.186 is the specific heat of water in kJ per kg per °C.
  • ΔT is the temperature rise in °C (target minus incoming).
  • 3600 converts kilojoules to kilowatt-hours.
  • η is efficiency as a fraction. Resistance heaters are close to 1, while a heat pump water heater can be well above 1 because it moves heat.

Cost per cycle is E times the price per kWh. Annual saving is (peak cost − off-peak cost) × 365.

Worked example

A household reheats 150 L by 40 °C with a heater at 95% efficiency, peak price $0.30 and off-peak price $0.12 per kWh.

  • Thermal energy = 150 × 4.186 × 40 / 3600 = 6.977 kWh.
  • Electricity = 6.977 / 0.95 = 7.344 kWh.
  • Peak cost = 7.344 × 0.30 = $2.20.
  • Off-peak cost = 7.344 × 0.12 = $0.88.
  • Saving per day = $1.32; times 365 = $482.49 per year.

Those are the four values shown by the calculator. The energy used is identical either way; only the price changes.

Common mistakes and how to interpret the result

  • Using tank size as volume heated. Only the water actually reheated counts, so estimate daily hot water use.
  • Ignoring standby loss. A hotter tank held longer loses more heat through the walls, which the tool does not model.
  • Assuming the saving is guaranteed. If the tank cannot store enough hot water for the peak period, you may still need to heat at peak.
  • Mixing efficiency conventions. Enter 95 for 95%, not 0.95, and do not use a heat pump coefficient unless you intend it.

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Frequently Asked Questions

Does shifting use less energy?
No. The same energy is needed, so kWh stay the same. The gain comes from a lower price per kWh, or lower grid stress if your utility pays for shifting.
Can I use it with a heat pump water heater?
Yes. Enter the seasonal efficiency as a percentage above 100, for example 300 for a coefficient of performance near 3.
What temperature rise should I enter?
Use the difference between your setpoint and the incoming cold water temperature, for example 55 minus 15 = 40 degrees C.
Why is the annual saving assuming one cycle a day?
It keeps the estimate simple. Scale the result if you reheat more or less often, or if your tariff has a different peak-hours schedule on weekends.