Home & Construction📅 8 April 2025⏱ 4 min read
How to Calculate the Right Water Heater Size for Your Home
A water heater that is too small runs cold mid-shower. One too large wastes energy heating water you never use. Here is the calculation for cylinder size, immersion heater power, and heat pump hot water.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
Most homeowners inherit whatever water heater was installed by the previous owner. Understanding the sizing calculation ensures you replace it correctly — and potentially save money on energy bills.
Daily Hot Water Demand
Calculate daily hot water usage:
Average hot water consumption per person per day:
Bath (full): 80-120 litres at 40°C
Shower (8 min, standard): 40-60 litres at 40°C
Shower (8 min, power): 80-120 litres at 40°C
Washing up (by hand): 6-8 litres
Washing machine (if hot fill): 20-40 litres
Household of 4 (2 showers, 2 baths per day):
2 showers: 2 x 50 = 100 litres
2 baths: 2 x 100 = 200 litres
Washing up: 3 x 7 = 21 litres
Miscellaneous: 20 litres
Total: 341 litres per day at mixed temperatures
Hot Water Cylinder Sizing
Hot water cylinder stores water at 60-65°C.
When mixed to 40°C for use, you get more hot water volume per litre stored.
Mixing ratio: (stored temp - cold water temp) / (mixed temp - cold water temp)
Cold water temp (UK average): 10°C
Hot cylinder: 65°C, mixed to 40°C
Mixing factor = (65 - 10) / (40 - 10) = 55/30 = 1.83
For 341 litres at 40°C:
Cylinder size needed = 341 / 1.83 = 186 litres
Standard UK cylinder sizes: 90L, 120L, 150L, 180L, 210L, 250L
Recommendation: 210 litre cylinder for this household
Rule of thumb shortcut:
45-60 litres per person for standard use (showers)
60-80 litres per person for bath users
4-person household, mix: 4 x 55 = 220 litres (matches calculation)
Immersion Heater Recovery Time
Immersion heater: 3kW standard, some 1kW or 6kW
Energy to heat a cylinder:
Q = mass x specific heat x temperature rise
Q = 180 kg x 4,186 J/kg°C x (65-10)°C = 41,427,900 J = 41.4 MJ
Time to heat = Energy (kWh) / Power (kW)
Converting J to kWh: 41,427,900 / 3,600,000 = 11.5 kWh
At 3kW: Time = 11.5 kWh / 3 kW = 3.83 hours = ~3h 50 min
At 6kW immersion: 1h 55 min to heat from cold
Off-peak heating (overnight tariff at 7.5p/kWh):
11.5 kWh x £0.075 = £0.86 per full cylinder heat
vs daytime rate (28p/kWh): 11.5 x £0.28 = £3.22
Off-peak saving: £2.36 per day = £861/year if heated daily at off-peak
Heat Pump Hot Water Cylinders
Heat pump water heater (air source): COP 2.5-3.5
Extracts heat from room air to heat water — more efficient than direct electric.
At COP 3.0:
Energy to heat: 11.5 kWh thermal
Electricity needed: 11.5 / 3.0 = 3.83 kWh electrical
At 28p/kWh: 3.83 x £0.28 = £1.07 (vs £3.22 for standard immersion)
Annual saving vs standard immersion (daily heating): £780/year
Capital cost: £800-£1,500 more than standard cylinder
Payback vs standard immersion (off-peak): 3-5 years
Payback vs standard immersion (daytime rate): 1-2 years