Hot Water Heat Pump Calculator

A dedicated domestic hot water (DHW) heat pump is a self-contained unit that heats water using an integrated heat pump compressor. It extracts heat from surrounding air (typically a garage or utility room) and delivers it to a built-in or connected cylinder. Units are typically 1.5-3 kW and can be retrofit without major plumbing changes.

DHW heat pumps typically declare a COP of about 2.0-3.4 when heating water to 55C, measured to BS EN 16147 (the rating standard for domestic hot water heat pumps, tested at a 10C cold-water inlet). COP is lower than for space-heating heat pumps because of the higher target temperature. It is better in warmer locations (plant or utility rooms) and when heating to lower temperatures (45-50C with thermostatic mixing valves). The COP this tool shows is an indicative estimate; use the unit’s EN 16147 declared COP for the real figure.

A typical 2 kW DHW heat pump can reheat a 200-litre cylinder from cold in approximately 4-6 hours. For daily use (partial reheat), recovery time is typically 2-3 hours. This is adequate for most households but may require scheduling for large families.

An unheated space like a garage is ideal — the heat pump extracts heat from the air, cooling and dehumidifying the space. A utility room or plant room also works. The space should be at least 20 m³ and well-ventilated. The unit produces cool, dry exhaust air which can be beneficial in summer.

Yes. An immersion heater has a COP of 1.0 (100% efficient). A DHW heat pump achieves a COP of roughly 2.0-3.0, meaning it uses around 50-66% less electricity for the same amount of hot water. At the Ofgem Q3 2026 electricity cap (26.11p/kWh) this calculator estimates an annual saving of roughly £400-540 for a family of 3-4 replacing immersion-only heating (the exact figure depends on demand, COP and your unit rate — use the result above). It also provides dehumidification in the installed space.