Home & Construction⏱ 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.
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