What changes the cost of a water heater?
The quick answer is power multiplied by time and your electricity rate—but the right input depends on the appliance. Wattwise starts this calculator with a representative estimate, not a manufacturer-specific claim. Replace defaults with measured, label, or bill values wherever possible.
- Daily hot-water volume
- Starting and target temperatures
- Tank or instant-heater efficiency
- Standing heat loss and schedule
- Showers and occupants
Use water volume and temperature rise
For a tank or immersion heater, the energy needed for hot water depends on how many litres are heated and how far the water temperature rises. The electrical input rating tells you how quickly it can heat; it does not by itself tell you how much hot water your household used.
Heating 50 litres by 30°C
Water needs about 4.186 kJ per kilogram per °C. Approximating 50 litres as 50 kg gives 50 × 30 × 4.186 ÷ 3,600 = 1.74 kWh of heat. At 90% heater efficiency, the electrical input is about 1.94 kWh before tank standing losses. Enter your own volume, temperatures and efficiency where known.
A stored tank can lose heat between draws, and mixing with cold water changes delivered volume. Treat the worked example as a single heating event, not a whole-day bill.
Example water heater running costs
These examples use the selected market’s clearly labeled planning estimate and the typical schedule shown in the calculator. Electricity tariffs vary by provider, tier, time, tax, and subsidy, so replace the rate with the value on your bill.
| Example | Energy input | Indicative cost |
|---|---|---|
| 15 L storage geyser | 2,000 W | $0.44 / tank |
| 25 L storage geyser | 2,000 W | $0.44 / tank |
| Instant shower heater | 3,500 W | $0.44 / tank |
Ways to reduce this appliance’s electricity cost
Savings depend on comfort, safety, usage, and the equipment you already own. Model a change before buying a replacement, and verify high-cost appliances with a meter or label value.
- Heat only the volume you expect to use.
- Compare a 5°C lower set point while maintaining safe hygiene guidance.
- Use a timer to avoid reheating an unused storage tank all day.
How Wattwise calculated this estimate
For the default inputs, Wattwise estimates $13.44 per month and 67.2 kWh at the global reference rate. The active formula is: litres × 4.186 kJ/kg°C × temperature rise ÷ 3,600 ÷ efficiency.
The displayed range widens when power, duty cycle, weather, or tariff data are estimates. It narrows when you provide measured energy, an official annual kWh value, or a per-cycle energy label. Fixed charges are not allocated to one appliance unless you explicitly create a household bill allocation.
Review formulas, rounding, and confidence