Instantaneous water heaters: how much hot water each size gives
By Dale @ ukaircontracker · facts checked
Quick answer
An instantaneous water heater warms water only as it flows, so there is no tank to keep hot. Its power sets the flow: raising water by 30°C, a 3 kW unit gives about 1.4 litres a minute, a 9.5 kW unit about 4.5 and a 10.8 kW unit about 5.2. Anything over 3 kW needs its own electrical circuit.
| Power | Flow at 30°C rise | Flow at 45°C rise | Per hour at full power | Supply |
|---|---|---|---|---|
| 3 kW | 1.4 L/min | 1.0 L/min | 79p | 13 A plug possible |
| 7.5 kW | 3.6 L/min | 2.4 L/min | £1.97 | Dedicated circuit (about 33 A) |
| 9.5 kW | 4.5 L/min | 3.0 L/min | £2.50 | Dedicated circuit (about 41 A) |
| 10.8 kW | 5.2 L/min | 3.4 L/min | £2.84 | Dedicated circuit (about 47 A) |
Flow = kW ÷ (0.0697 × temperature rise). A 30°C rise takes 10°C mains water to 40°C. Costs at 26.32p/kWh. Winter mains water is colder, so flow at the same temperature is lower in winter.
Why power limits the flow
Heating one litre of water by one degree takes a fixed amount of energy. A heater can only add energy as fast as its rating allows, so the more water passes through each minute, the less each litre is warmed. That is why an electric shower runs cooler and weaker in winter, when the incoming mains water is colder.
Running cost
Cost per use is small because the heater only runs while the tap is open. A 9.5 kW shower running for 8 minutes uses about 1.3 kWh, roughly 33p at the current price cap.
Installation
Units above 3 kW draw more current than a 13 A plug allows and need a dedicated circuit with the right cable size and protection. That is work for a qualified electrician, and in England and Wales a new circuit is notifiable under Part P of the Building Regulations.
Common questions
- What size instantaneous water heater do I need for a shower?
- Most electric showers are 8.5–10.8 kW. At a 30°C rise a 9.5 kW unit gives about 4.5 litres a minute, and a 10.8 kW unit about 5.2.
- Are instantaneous water heaters cheaper to run?
- They waste less, because no stored water cools down between uses. The energy to heat each litre is the same as for any electric heater.
- Can I plug an instantaneous water heater into a socket?
- Only 3 kW models or smaller. A 3 kW unit gives a low flow, suitable for hand washing at one sink.