The heat in the water is only the starting point; storage losses, recovery rate and safe temperature control determine the real system.
1. Describe the hot-water demand
Estimate both daily hot-water volume and the largest short period of use. Storage systems must recover after clustered draws, while instantaneous heaters must meet the peak flow and temperature rise at the same time.
Use measured habits when possible. Shower flow, inlet-water temperature and seasonal changes can alter the result more than a small improvement in arithmetic.
2. Calculate useful heat first
For a first estimate, one litre of water is approximately one kilogram. Multiply water mass by its specific heat capacity and temperature rise. The calculator converts that useful heat to kWh and divides by a stated heating efficiency.
This simplified number excludes heat lost from the tank, distribution pipes and recirculation. Long pipe runs can waste both energy and water, so layout and insulation belong in the design.
3. Match storage and recovery to the household
Compare tank capacity, heater input, recovery time and the sequence of expected draws. A larger tank is not automatically better if standby loss, available power or slow recovery creates another constraint.
DOE advises considering size, energy source, efficiency and cost when selecting a residential water heater. Product ratings and regional energy prices belong in the final comparison.
DOE · Residential water-heater purchasing guidance ↗4. Separate energy estimates from safety decisions
Do not select storage temperature solely from an energy calculator. Scald protection, microbial control, thermostatic mixing and local plumbing requirements need an installation-specific decision.
Confirm electrical or fuel supply, relief discharge, drainage, ventilation and service access with qualified trades. Record the settings and maintenance instructions for future occupants.
Hot-water questions to answer
| Question | Quantity | Why it changes the design |
|---|---|---|
| How much is used? | L/day | Daily energy |
| How quickly is it used? | L/min and draw duration | Peak output or storage |
| How cold is inlet water? | °C | Required temperature rise |
| How fast can the heater recover? | kW or L/h | Time between draws |
| Where is heat lost? | Tank and pipe route | Actual input energy |
Worked example
Heating a 100-litre draw
Water is heated by 40°C and the assumed efficiency is 90%.
Useful heat = 100 × 4.186 × 40 / 3600 = 4.65 kWh Input = 4.65 / 0.90 = 5.17 kWh
This is the energy for the water only. It does not include standby or distribution losses and does not size a heater by itself.
What to have ready
- Measured or estimated draw pattern
- Seasonal inlet temperature
- Available electrical or fuel input
- Storage, recovery and pipe-loss data
Common mistakes
- Sizing from daily volume alone.
- Calling useful heat the metered energy.
- Choosing a storage temperature from energy use alone.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.