Efficiency is not one universal percentage. A heat battery charges, waits and releases heat, and energy can be lost or redirected at each stage. The number that matters is useful heat delivered when and where the household needs it.
The short answer
Ask for three separate figures under stated test conditions: energy taken to charge, standing heat loss over time, and useful energy delivered. Do not compare one maker's storage capacity with another maker's delivered capacity unless the definitions match.
Where energy goes
Electrical resistance charging turns electricity into heat close to the point of use. That does not mean every charged kWh reaches a tap. The casing and connected pipework release heat while the store waits; controls and pumps may use electricity; and some energy can remain below the useful delivery temperature.
Heat released inside the heated part of the home may reduce space-heating demand in winter, but it can be unwanted in summer. It is still a loss from the hot-water job and should not disappear from the calculation.
Compare the test conditions
For every published figure, look for:
- starting and finishing temperatures
- the time between charge and measurement
- ambient room temperature
- whether pipework and controls are included
- the outlet temperature and flow used
- whether capacity means stored or delivered heat
- the product configuration and software version
Standing loss becomes more important when heat sits for longer. A unit charged shortly before morning demand may perform differently in practice from one that must hold energy across a full day.
Use the number in your tariff maths
If a system needs more input energy than the useful heat it delivers, use the input figure when estimating charging cost. Then add any peak-rate top-up. The running-cost calculator deliberately shows an upper-bound tariff gap before these adjustments.
The room and pipes matter too
Product insulation is only part of the installation. Pipe runs, valves and nearby fittings can lose heat. Approved Document L addresses limiting losses from hot-water vessels and pipework; the installer should explain how exposed hot pipes will be insulated and how the installed system meets applicable requirements.
Your next step
Ask each supplier for the test report or technical sheet behind its loss and capacity claims. Put the definitions side by side before comparing percentages.
Sources
Primary and independent references checked by our editorial team.
- Approved Document L: conservation of fuel and power, Volume 1 — GOV.UK Checked 10 Nov 2026
- Insulating tanks, pipes and radiators — Energy Saving Trust Checked 10 Nov 2026