TL;DR:

  • Heat pump water heaters (HPWHs) move heat from surrounding air rather than generating it, using around 70% less electricity than a standard electric tank
  • Running costs in the UK typically fall from £350–500/year (immersion heater) to £100–150/year, paying back the upside cost in 4–7 years
  • They need floor space, adequate ambient temperature (above ~5°C), and a condensate drain — worth checking before you buy

Everyone knows about heat pumps for space heating. Fewer people know that the same technology applies to water heating — and that a heat pump water heater (HPHW) is often the faster-payback, lower-disruption upgrade. You don’t need to retrofit your whole heating system. You don’t need a new cylinder. You can often replace an existing electric immersion tank in a day.

With UK electricity prices still elevated and the Boiler Upgrade Scheme focused on space heating, the domestic hot water side of energy bills is getting less attention than it deserves. That’s starting to change.

How they work

A conventional electric water heater is essentially a large kettle: resistance elements convert electricity to heat at a 1:1 ratio. You put in 1 kWh of electricity, you get 1 kWh of heat.

A heat pump water heater works differently. It contains a small refrigeration cycle — a compressor, an evaporator, and a heat exchanger wrapped around the water tank. The evaporator pulls heat from the surrounding air (even cool air contains useful heat energy). The compressor concentrates it. The heat exchanger transfers it to the water.

The result: a coefficient of performance (COP) of 2.5 to 4.0. You put in 1 kWh of electricity, you get 2.5 to 4 kWh of heat in the water. The rest comes from the air in your garage, utility room, or airing cupboard.

The running cost arithmetic

Take a household using 120 litres of hot water per day — typical for a family of three or four. An immersion heater delivering this uses roughly 1,800 kWh/year. At 24p/kWh (a reasonable current UK unit rate), that’s around £432/year.

The same hot water demand from a heat pump water heater at a COP of 3.0 needs 600 kWh/year. At the same rate, that’s £144/year. Annual saving: £288.

A quality HPHW unit costs £800–1,400 installed. Simple payback: 3–5 years. Over a 15-year lifespan, you’re looking at £3,000–4,000 in savings against the additional upfront cost. This compares favourably with most other home energy investments.

If you’re currently on a gas boiler with an indirect hot water cylinder, the maths is different — gas is cheaper per unit than electricity, so the savings are smaller. HPWHs make the most sense replacing direct electric heating.

What your home needs

Space and location. HPWHs need a footprint of roughly 0.5–0.7m² and a ceiling height of at least 2m. More importantly, they extract heat from the surrounding air — ideally from a space with a minimum ambient temperature of 5–7°C year-round. A heated utility room is ideal. An unheated garage in a cold climate is marginal. A kitchen cupboard is too small.

A condensate outlet. Like a condensing boiler or air conditioning unit, HPWHs produce condensate (water extracted from the air) that needs to drain away. You need a drain nearby, or the capacity to route a condensate pipe to one.

Electricity supply. Most HPWHs run on a standard 13A socket. The compressor draws 400–600W during operation — far less than an immersion element (2–3kW). This is generally a non-issue.

Noise. The compressor produces around 40–50dB — similar to a quiet dishwasher. Not a problem in a utility room or garage; potentially audible in a bedroom-adjacent location.

The best units available in the UK right now

Ariston Nuos Primo — widely distributed, 80–200L capacity range, reliable COP figures, good installer network. Often recommended as the entry-level safe choice.

Vaillant uniSTOR air — premium build quality, integrates well with Vaillant smart controls, higher upfront cost, strong warranty.

Stiebel Eltron WWK — German-engineered, excellent efficiency in cooler ambient conditions, longer track record in European markets.

Thermia Diplomat Optimum — strong cold-climate performance, popular in Scandinavian markets but increasingly available in the UK.

Most manufacturers offer units from 80L to 300L. A household of four typically needs 150–200L.

Smart tariff integration

One underutilised feature: most HPWHs can be scheduled or controlled via smart plugs, home energy management systems, or manufacturer apps. If you’re on an Octopus Agile or Economy 7 tariff, you can schedule heating to run during cheap overnight periods. This can cut running costs by a further 30–50%, pushing the payback period below three years in some cases.

The combination of a HPHW running on cheap overnight electricity, topped up by solar export during the day, is the most efficient domestic hot water setup currently available without specialist equipment.

Is it worth it?

For homes currently using electric immersion heating — yes, almost certainly. The payback is clear and the disruption minimal. For gas-heated homes, run the numbers for your specific tariffs. As gas prices remain volatile and electricity from renewables gets cheaper over time, the case only strengthens.

The grant picture in the UK is patchy: HPWHs don’t currently qualify for the Boiler Upgrade Scheme (which is space-heating only), but local authority schemes and energy company obligations sometimes cover part of the cost. Worth checking before you buy.

If you’re replacing an aging immersion heater in the next 12 months, this should be at the top of your shortlist.