Methodology
How the calculator works
This page explains the assumptions and formulas used by the UK Home Battery Savings Calculator.
What the calculator estimates
The calculator estimates whether charging a home battery using cheaper off-peak electricity and using that stored energy during more expensive peak-rate periods could reduce electricity costs.
It is designed as a first-pass estimate, not a guaranteed financial forecast.
Main calculation
The calculator first works out how much battery energy is useful during peak-rate periods.
This matters because a battery only saves money when it replaces electricity that would otherwise be bought at the higher peak rate.
Off-peak charging cost
Batteries are not perfectly efficient. If the battery has 90% round-trip efficiency, more energy must be bought from the grid than the battery later delivers.
Peak-rate cost avoided
The calculator estimates the cost of the peak-rate electricity that the battery could replace.
Saving per cycle
The saving per cycle is the difference between the peak-rate electricity avoided and the cost of charging the battery off-peak.
Annual saving
Annual saving is estimated by multiplying the saving per cycle by the number of battery cycles per year.
Payback period
Payback period is the estimated number of years it would take for the annual saving to recover the installed battery cost.
If the annual saving is zero or negative, the calculator shows the payback as not profitable.
Break-even battery cost
The break-even battery cost estimates the maximum installed cost that would break even over the warranty period, using the current annual saving estimate.
What is included
- Battery usable capacity
- Useful peak-period usage
- Peak electricity rate
- Off-peak electricity rate
- Battery round-trip efficiency
- Battery cycles per year
- Installed battery cost
- Warranty period
What is not included
- Solar generation modelling
- Export payments
- Standing charges
- Battery degradation over time
- Finance costs
- Tariff exit fees
- VAT differences
- Backup power value
- Future electricity price changes
Important limitation
The calculator is deliberately simple. It is designed to help you compare rough assumptions before speaking to installers, not to replace a detailed home energy assessment or professional advice.
Best next step
Use the home battery savings calculator with your own quote, tariff rates and usage assumptions.
If you already have installer quotes, use the home battery quote comparison worksheet to compare installed cost, usable capacity, warranty and estimated payback.