This calculator isolates the tariff gap: the largest amount available from buying the same quantity of energy at a cheaper time. It is a screening tool, not a savings promise.
How to use the result
Take the annual gap and subtract the effect of storage losses, conversion efficiency, peak-rate top-ups and any export income forgone. Then compare the remainder with purchase, installation, finance and maintenance costs.
Where to find the inputs
Use unit rates from your latest tariff document or bill. Do not use the direct-debit amount: it includes a mixture of consumption, standing charge and account balance.
What is deliberately excluded
The tool does not assume a product, household profile or national average. It also does not compare gas and electricity on input price alone; boilers and heat pumps deliver different amounts of useful heat from each input kWh.
Run scenarios, not one forecast
Try a narrow and wide tariff gap. Then vary daily heat demand. If the investment only works under the most optimistic combination, the decision is exposed to tariff changes.
Your next step
Ask the installer for a predicted annual input in kWh and the proportion expected to charge off-peak. Put those two assumptions beside the quote.
Sources
Primary and independent references checked by our editorial team.
- Energy price cap unit rates and standing charges — Ofgem Checked 10 Nov 2026
- Economy 7 tariff guidance — Ofgem Checked 10 Nov 2026