Water Heater Energy Calculator
The water-heater energy calculator estimates the heat needed to raise a water volume to a target temperature, adjusts for heater efficiency and estimates energy cost.
How to use this calculator
- Enter water volume, starting and target temperature, heater efficiency and energy price. The target temperature must be higher than the starting temperature.
- Check the units and definitions before comparing results with another source.
- Change one input at a time when comparing scenarios, and keep the reporting period and assumptions consistent.
Formula used by the water heater energy calculator
Q(kWh) ≈ litres × 4.186 × ΔT ÷ 3,600; input energy = Q ÷ efficiency
- Q(kWh) ≈ litres × 4.186 × ΔT ÷ 3,600; input energy = Q ÷ efficiency
- It assumes water density near 1 kg/L and constant specific heat. Tank standing losses, pipe losses, heating controls and changing tariffs are not included.
- Inputs are checked for required values and valid ranges; displayed results are rounded for readability.
Worked examples
Worked example
Heat 100 litres from 15°C to 55°C with a 90% efficient heater at 0.30 currency units/kWh.
Thermal demand is about 4.651 kWh; input energy is about 5.168 kWh and estimated cost is about 1.55.
Thermal demand is about 4.651 kWh; input energy is about 5.168 kWh and estimated cost is about 1.55.
Understanding the result
The water-heater energy calculator estimates the heat needed to raise a water volume to a target temperature, adjusts for heater efficiency and estimates energy cost.
It assumes water density near 1 kg/L and constant specific heat. Tank standing losses, pipe losses, heating controls and changing tariffs are not included.
Important notes
It assumes water density near 1 kg/L and constant specific heat. Tank standing losses, pipe losses, heating controls and changing tariffs are not included.
The result applies to the numbers and assumptions entered here; verify specifications for safety-critical or regulated uses.
Frequently asked questions
Does this include keeping water hot overnight?
No. It estimates one temperature rise only; standing losses and repeated reheating require additional modelling.
Why is supplied energy higher than thermal demand?
A heater with less than 100% efficiency must draw more input energy than the ideal heat transferred to the water.
How do I use this calculator?
Enter water volume, starting and target temperature, heater efficiency and energy price. The target temperature must be higher than the starting temperature.
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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.