Hot Water Energy Cost Calculator
Estimate a water-heating energy requirement from volume and temperature rise. Enter an efficiency appropriate to the heater and the energy price shown on your bill.
How to use this calculator
- Enter the water volume (litres) using the units shown beside the field.
- Enter the incoming water temperature (°c) using the units shown beside the field.
- Enter the target water temperature (°c) using the units shown beside the field.
- Review the calculated values and check that the assumptions match your scenario.
- Change one input at a time when comparing scenarios so you can see which assumption drives the result.
Formula and assumptions
Thermal kWh = litres × 4.186 kJ/(kg·K) × temperature rise / 3,600; input energy divides by heater efficiency.
- The estimate excludes tank losses, pipe losses, flow-rate effects and standing energy. Real efficiency depends on heater type and operating conditions.
- The result is computed from the values you provide; it does not fetch external rates, live account data or local rules.
Worked examples
Worked example
Heating 120 litres from 12°C to 50°C requires roughly 5.3 kWh of thermal energy before heater losses.
Example inputs: Water volume (litres) = 120, Incoming water temperature (°C) = 12, Target water temperature (°C) = 50, Heater efficiency (%) = 90.
Compare the displayed output with your own measurements before using it in a decision.
Understanding the result
The estimate excludes tank losses, pipe losses, flow-rate effects and standing energy. Real efficiency depends on heater type and operating conditions.
Results are estimates based on the stated model. If the situation depends on measured conditions or contractual specifications, use those source values rather than typical assumptions.
Important notes
The estimate excludes tank losses, pipe losses, flow-rate effects and standing energy. Real efficiency depends on heater type and operating conditions.
Frequently asked questions
What formula does this calculator use?
It uses the method described above: Thermal kWh = litres × 4.186 kJ/(kg·K) × temperature rise / 3,600; input energy divides by heater efficiency.
What should I verify before relying on the result?
The estimate excludes tank losses, pipe losses, flow-rate effects and standing energy. Real efficiency depends on heater type and operating conditions.
Can I compare different scenarios?
Yes. Change the inputs while keeping units and definitions consistent, and record the assumptions used for each scenario.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.