Heat battery running costs: use your own tariff (UK)

A heat battery's running cost starts with a simple multiplication: energy charged into the store, in kWh, times the unit rate at the time it charges. The hard part is estimating the energy needed and comparing the same amount of useful heat.

The short answer

Use rates from your bill, not a national headline. Then separate the maximum value of moving energy into a cheap period from the real saving after storage losses, controls and the performance of the system being replaced.

Our calculator deliberately does not assume a product efficiency. Add a maker's published figure only when the test conditions match the proposed use.

A worked method, without invented prices

1. Find annual or daily hot-water and heating demand in kWh.
2. Record the off-peak rate, peak rate and exact cheap window.
3. Multiply the energy expected to charge off-peak by each rate.
4. Adjust for published system losses and any energy charged outside the window.
5. Compare with the useful heat cost of the existing system.

What changes the result

The tariff gap

A wide gap between peak and off-peak prices creates more room for storage to help. A low off-peak rate is not enough on its own; check whether the day rate or standing charge has increased and whether the charge window is long enough.

The amount shifted

Savings only apply to energy actually moved from a dearer period. Solar energy that would otherwise have been exported needs a different comparison from grid electricity.

Losses and controls

All hot stores lose some heat. Poor scheduling can also trigger top-up charging at the expensive rate. Ask the installer to state the control strategy in plain English.

The capital cost

An annual bill difference is not a payback calculation. Include purchase, installation, finance, maintenance and likely replacement timing. Treat very long payback periods cautiously because tariffs change.

Your next step

Download twelve months of half-hourly data if your supplier provides it. Mark the demand that the proposed system could genuinely move. That is a better foundation than multiplying one “typical day” by 365.