Home battery savings calculator UK

How the home battery calculator works

The calculator treats each full cycle as one discharge of the usable battery capacity. It estimates annual saving with this formula:

min(usable capacity × cycles per week, daily peak use × 7) × 52 × (peak rate − cheap rate ÷ efficiency)

Rates are in pence, so the result is divided by 100 to convert to pounds. Efficiency is entered as a percentage. Weekly discharge is capped at seven times your daily peak-rate use. The model assumes this energy replaces electricity bought at the peak rate and that the cheap-rate electricity is available long enough to charge the battery.

Example: 5 kWh battery on a cheap overnight tariff

Here is the same calculation with a simple overnight-charging example. It uses a 5 kWh usable battery, not a 5 kWh nameplate figure with unknown usable capacity.

• 5 kWhusable capacity
• 7p per kWhcheap rate
• 28p per kWhpeak rate
• 90%round-trip efficiency
• 5 full cycles per weekcycling assumption
• £1,699battery cost used

That is 1,300 kWh of discharged energy a year. After allowing for the energy lost on the way in, the estimated gross saving is £263 per year. Dividing £1,699 by that annual saving gives a simple payback of 6.5 years.

This example is deliberately transparent. It does not claim that a battery will cycle five times every week or that every stored unit will replace a peak-rate unit.

Home battery savings at different tariff spreads

The wider the gap between the rate used to charge and the rate avoided at discharge, the stronger the maths. These figures keep the other assumptions fixed at 5 kWh usable capacity, 90% efficiency, five cycles per week and £1,699 cost.

The nominal gap is not the saving per delivered kWh because charging losses are included in the calculation. All results are before installation, degradation and other excluded costs.

What affects battery payback?

Battery capacity

A larger battery can shift more electricity, but only if your home uses that energy later. An oversized battery may add cost without adding the same amount of saving. Compare usable capacity, not only the headline number.

Round-trip efficiency

Efficiency is the energy left after charging and discharging. At 90%, each 5 kWh discharge needs about 5.56 kWh from the cheap tariff. Lower efficiency means paying for more input energy to deliver the same useful output.

Cheap-rate duration

A low unit rate only helps if the cheap period lasts long enough to charge the battery. A short window, a low charge power or a tariff with conditions can reduce the cycles you can actually complete.

Your evening electricity use

The model assumes stored energy displaces expensive electricity. If your evening demand is lower than the battery output, some energy may remain unused or be exported at a different value. Your smart-meter history is a better input than a guessed cycle count.

Installation cost

Hardware price is not always the full cost. Add electrician work, a dedicated circuit, delivery, mounting or other required work to the figure you use for payback. For plug-in products, follow the maker's instructions and check whether a socket connection is actually permitted.

What this calculator does not include

This is a deliberately simple gross-saving model. It does not include standing charges, battery degradation, maintenance, finance or interest, VAT differences, costs you have not entered, export income, charge and discharge limits, missed cycles, changes to tariff rates, or the value of electricity you could have used in another way. It also does not decide whether a particular battery is safe, approved or suitable for your home.

Use the result as a ceiling for a first conversation, then check your tariff terms and actual usage. Read how home batteries work without solar and the guide to batteries and Octopus Energy.