If you already have solar panels, much of what they make during the day probably goes to the grid while you are out, for a small export payment. A battery keeps that energy for the evening, when you would otherwise be buying electricity at the full rate. Adding one to an existing system is common, whoever installed your panels. This guide covers what decides how it works for yours.
How a battery joins an existing system
There are two broad approaches. The battery can sit alongside your existing inverter and store the electricity your panels have already produced. Or the existing inverter can be replaced, so one unit manages the panels and the battery together. Which makes sense depends on your inverter’s type and age, and on whether you plan to add more panels later. We check before we recommend anything.
What we check first
- Your inverter. Its type and age decide how a battery connects, and whether it makes sense to replace it at the same time.
- Your export arrangement. If you are paid for exporting, or still on the Feed-in Tariff, we check how a battery fits with that before you commit.
- Your supply and space. Your consumer unit, your connection to the grid, and where the battery will sit, indoors or out.
If you are on the Feed-in Tariff
You can usually still add a battery. Your Feed-in Tariff supplier needs to be told, and the battery has to be connected so it does not affect your generation meter. We check your arrangement and explain what it means for your payments before you commit.
More than a store for solar
A battery is not limited to storing your own solar. On a time-of-use tariff it can also charge overnight at the cheap rate, which matters most in winter, when your panels make less. With a backup gateway fitted, it can also keep the lights on in a power cut. Our winter guide explains how solar and storage work together through the year.
Paperwork and VAT
Your network operator has to be told about a battery, and in many cases has to approve it before it is installed. We make the application and handle the paperwork; our G98 and G99 guide explains the difference. Batteries installed in homes in Great Britain carry 0% VAT until 31 March 2027, when the rate is due to return to 5%, including when one is added to an existing system.
Which battery?
We fit two, and both can be added alongside most existing systems. Tesla Powerwall 3 comes in whole 13.5kWh units with simple control from the Tesla app. Sigenergy SigenStor is modular, so you can start smaller and add capacity later. Our comparison guide goes into the detail.
Have your MCS certificate and a photo of your inverter to hand, then see how adding a battery works.
This guide is general information, not advice for your specific property. Every Powergy proposal is modelled on your actual roof, usage and tariff.