Hong Life CalculatorsUnderstanding the water heater size calculator — concept and principle
This calculator estimates how much hot water a household uses per day, suggests a storage tank size and tells you how long an electric heater of a given power needs to heat that tank. It helps when replacing a water heater, comparing element sizes or checking whether a tank will recover in time between morning and evening showers.
How it is calculated
- Daily hot water = people × litres per person per day (measured at 60 °C).
- Suggested tank = daily use × 0.7, on the assumption that the tank does not need to hold the whole day's demand because it reheats during the day.
- Heat-up time t = V × 4.186 × ΔT ÷ 3,600 ÷ P, where V is the tank volume in litres, 4.186 kJ/(kg·K) is the specific heat of water, ΔT is the temperature rise in kelvin and P is the heater power in kW.
Dividing by 3,600 converts kilojoules to kilowatt-hours, so the result is in hours.
The principle
Heating water is a direct energy balance. One litre of water has a mass of about 1 kg, and raising 1 kg by 1 K takes about 4.186 kJ. Multiply by the litres and by the temperature rise and you get the energy the tank needs; divide that energy by the heater's power and you get the time. An electric resistance element turns almost all of its input into heat, so the formula is a good first estimate, but it ignores heat lost through the tank walls and pipes during heating, which makes real times a little longer.
Worked example
- Inputs: 4 people, 50 L each per day at 60 °C, 3 kW heater, temperature rise 50 K (for example 10 °C mains water heated to 60 °C).
- Daily hot water: 4 × 50 = 200 L.
- Suggested tank: 200 × 0.7 = 140 L.
- Energy: 140 × 4.186 × 50 = 29,302 kJ, which is 29,302 ÷ 3,600 ≈ 8.14 kWh.
- Heat-up time: 8.14 ÷ 3 ≈ 2.71 h, about 2 hours 43 minutes.
| People | Daily hot water (L) |
|---|---|
| 1 | 50 |
| 2 | 100 |
| 3 | 150 |
| 4 | 200 |
| 5 | 250 |
| 6 | 300 |
| 8 | 400 |
How to read the result
Daily use of 30–60 L of 60 °C water per person is a common planning range; long showers or a bathtub push it higher. If the heat-up time is long compared with the gap between peak uses, pick a larger tank, a stronger element or schedule heating during off-peak hours. In winter, colder mains water raises ΔT, so recovery takes longer than in summer.
Common mistakes
- Counting mixed shower water at 40 °C as 60 °C water; 60 °C water is blended with cold, so you need less of it.
- Forgetting the seasonal change in inlet temperature.
- Assuming a heat pump water heater follows the same kW figure; its electrical input and heat output differ.
Frequently asked questions
Why store hot water at 60 °C?
Keeping stored water at about 60 °C helps control Legionella bacteria, which multiply in lukewarm water. Use a mixing valve at taps to reduce scald risk.
How much electricity does heating the tank use?
About V × 4.186 × ΔT ÷ 3,600 kWh plus losses; in the example around 8.1 kWh for a full reheat.
Sources: World Health Organization, Legionella and the prevention of legionellosis (2007) · Halliday, Resnick and Walker, Fundamentals of Physics (specific heat of water)