renewables
Home battery storage
A home battery costs £3,500 to £8,000 installed, and the size that suits most households is between 5 and 10 kilowatt hours. What makes the numbers work is usually not solar at all. It is a cheap overnight tariff, charging the battery at a few pence and running the house on it through the expensive part of the day.
What this covers
What it costs. A 5 kilowatt hour battery installed is £3,500 to £5,500 and a 10 kilowatt hour system £5,500 to £8,000. Full cost breakdown.
What the job covers
- Adding storage to an existing solar array
- AC coupled retrofit where the inverter stays
- Sizing storage against the household's evening load
- Charging from a cheap overnight tariff
- Backup circuits that stay live in a power cut
What is involved
There are two ways to fit one. DC coupling puts the battery on the same inverter as a solar array and is the efficient route when both go in together. AC coupling gives the battery its own inverter and leaves an existing solar system untouched, which is what almost every retrofit ends up being, and it is also what makes a battery possible in a house with no panels at all. The battery, its inverter and its isolation are wired back to their own way in the consumer unit, and a current transformer clamp on the incoming tails tells the system what the house is doing.
Sizing is a question about the evening, not about the roof. Look at how many units the house uses between the end of a cheap overnight window and the start of the next one, because that is what the battery has to carry. For most households that is 6 to 12 kilowatt hours, and buying beyond it puts capacity in the garage that never discharges. A heat pump or an electric car changes the answer completely, and in opposite directions: the heat pump wants more storage, the car is already a battery with its own tariff.
Backup during a power cut is a separate feature and is not included by default. A standard installation shuts down when the grid goes, because feeding a dead network is dangerous to whoever is repairing it. Backup needs an automatic transfer arrangement and a separate board for the circuits that stay live, usually lighting, the fridge and a few sockets rather than the whole house. It is worth asking for at the design stage and expensive to retrofit.
Choosing someone for the job
Ask whether the quote is AC or DC coupled and what that means for an existing array. Ask for the usable capacity rather than the nameplate figure, because they differ. Ask what the warranty guarantees in cycles or throughput rather than only in years. Ask whether power cut backup is included, and if it is, exactly which circuits stay live.
Common questions
Do I need solar panels to have a battery?
No. A battery charged from a cheap overnight tariff saves money in a house with no panels at all, and for many households that is the larger part of the saving even when panels are present.
What size battery should I get?
Work from the evening consumption between cheap tariff windows rather than from the roof. Most households land between 5 and 10 kilowatt hours. Capacity that never discharges earns nothing.