Home battery storage saves money by storing electricity when it’s cheap and using it when it’s expensive, so on a 7p overnight rate and a 27p peak rate, every unit it shifts is worth 20p. That only adds up if the battery is the right size, can still charge on a cold winter night, and is backed by a company that will be around to honour the warranty. This guide covers the specs that decide whether a battery pays for itself.
A battery’s job is to store cheap electricity and release it when electricity is expensive. The cheap electricity can come from your solar panels during the day or from the grid on an overnight tariff.
Take an illustrative overnight rate of 7p a unit and a peak rate of 27p. Every kWh the battery moves from the cheap window to the expensive one saves 20p. If a 13.5 kWh battery fills up and empties once a day, that’s £2.70 a day, or £985.50 a year.
Over ten years that’s £9,855 before electricity prices change. Electricity prices have risen by an average of around 6% a year over the past 25 years. At 6%, the year 1 saving of £985 grows to around £1,665 a year by year 10.
Those figures assume the battery fills and empties fully every single day, and they ignore round-trip losses. Real batteries lose some energy charging and discharging, so treat this as the shape of the maths rather than a forecast for your home.
The aim is a battery that fills up cheaply and empties itself every day. Almost every spec below comes back to that.
Capacity is how much energy a battery stores, measured in kilowatt hours (kWh). Always compare usable capacity, because the figure in a product name is often bigger than what you can use day to day.
The Sigenergy SigenStor BAT 10.0 module, for example, has a total capacity of 9.04 kWh and a usable capacity of 8.76 kWh. The Tesla Powerwall 3 is listed at 13.5 kWh, and Tesla quotes that as the usable figure on its datasheet.
If two quotes use different measures, you’re not comparing like with like. Ask each installer for the usable capacity in writing.
A battery earns its money by cycling fully, ideally once a day. So the right size is roughly how much electricity your home uses outside your cheap-rate window.
Oversizing costs you money. If your home uses 8 kWh outside the cheap window and you fit a 13.5 kWh battery, around 5.5 kWh never gets used. That’s about 40% of the battery you paid for sitting idle and dragging down the return on the part that does work.
Undersizing costs you too, just less. If your home uses 15 kWh outside the cheap window and you fit a 10 kWh battery, you’ll still buy around 5 kWh a day at the peak rate. You’re still saving, just not as much as you could.
In most cases we’d suggest sizing slightly under and reviewing a year of real data before adding more. Electricity use tends to creep up, especially once an electric car or heat pump arrives, and battery prices tend to fall. Paying for extra capacity when you need it is usually cheaper than paying for it years early.
That only works if the system can grow. The Tesla Powerwall 3, Sigenergy SigenStor and Fox ESS EVO all take expansion modules. You can model different sizes for your own home with our battery storage calculator.
Power and capacity are different things. Capacity, in kWh, is the size of the tank. Power, in kW, is how fast the tap can fill and empty it.
The inverter sets the power. It turns the DC electricity from your panels and battery into the AC electricity your home runs on, and back again when the battery charges. On many battery systems the inverter is built into the unit.
The inverter’s rating caps how much solar power the system can use at once. If you have 8 kW of panels on a 3.68 kW inverter, the panels can never deliver more than 3.68 kW. On a sunny summer day, a big share of the generation you paid for is simply lost once the battery is full.
The 3.68 kW figure is common for a reason. It’s the most you can install per phase without asking your Distribution Network Operator (DNO) for permission first. Anything bigger needs an application before installation, and DNOs can take up to 11 weeks to respond when they’re busy, though around 6 weeks is more typical.
A 3.68 kW inverter lets an installer fit straight away and get paid sooner. Some inverters, including the Powerwall 3’s, can be set to 3.68 kW in software and raised once DNO approval comes through. A fixed-rating inverter can’t, so it’s worth asking which you’re being quoted.
Not by default. A battery will only keep your home running in a power cut if the system includes a backup gateway.
Without one, the battery has to shut down with the grid, so it can’t send power down cables that someone may be working on. The gateway disconnects your home from the grid so the battery can run it on its own. This is sometimes called islanding.
Backup isn’t the same on every system. Some switch over so fast you won’t notice, and others have a short interruption. Some back up the whole house, others only selected circuits or a single socket. Some support three-phase supplies and some only single-phase.
If power cut protection matters to you, ask your installer exactly what will and won’t keep running. Our comparison of the Tesla Powerwall 3 vs Sigenergy SigenStor covers how two of the most popular systems handle it.
A battery is a box of cells and wiring. The software decides when it charges and when it empties, so it decides how much money you save.
Good battery software should know your tariff, ideally pulling prices in automatically rather than relying on cheap times typed in by hand. That matters even more on tariffs where the price changes every half hour.
It should also plan ahead. By looking at the weather forecast and your expected usage, it can work out how much to charge overnight, so it doesn’t fill up from the grid before a sunny day when solar could do the job.
The app should show clearly where your power is coming from, where it’s going and how much you’ve saved. If you can’t see what the battery is doing, you can’t tell whether it’s working properly.
If you’re planning an electric car or a heat pump in the next few years, check that it can talk to your battery. Some manufacturers keep everything in their own app with their own charger and inverter. Others connect to different brands through an open API or third-party apps.
Neither approach is wrong. What matters is that the parts of your system can coordinate, because that’s how it runs most efficiently.
Winter is when a battery should save you the most, and it’s also when cells struggle. Lithium cells become less efficient in the cold and, close to freezing, may refuse to charge to protect themselves. A battery that can’t charge overnight misses that night’s cheap rate.
Thermal management matters most if the battery is going outside. Tesla says the Powerwall 3 actively heats and cools its cells, and warms itself in advance when a cold night is forecast. Sigenergy modules have built-in heaters that switch on at around 0°C.
Many batteries have no active heating at all. If yours will sit somewhere cold, check how it handles winter before you buy. Our guide to understanding batteries in the Knowledge Bank covers the basics of how battery systems are put together.
A battery warranty usually has three numbers. These are:
Whichever limit runs out first ends the cover. One cycle is a full charge and discharge, so a warranty capped at 3,000 cycles or 10 years, on a battery that cycles once a day, actually ends after about 8.2 years.
Capacity guarantees vary too. Every battery loses some capacity with age. In the UK, Tesla’s Powerwall warranty guarantees 80% of capacity after 10 years with unlimited cycles, when the battery is used for solar self-consumption, time-based control and backup. On a 13.5 kWh Powerwall that’s at least 10.8 kWh after a decade.
A warranty is only as good as the company behind it. GivEnergy, one of the biggest battery brands in the UK, entered administration on 9 April 2026 and stopped trading. The administrator confirmed that GivEnergy Ltd will no longer honour hardware warranties.
Those batteries still work, but if one fails there’s no manufacturer to claim against. Some app features have also moved behind a paid monthly plan. We’ve had GivEnergy owners pay us to replace their system with a Tesla Powerwall so they’re not left without cover. Our post on what the GivEnergy administration means for UK homeowners covers the detail.
How quickly a manufacturer fixes faults matters as much as the paperwork. Every week you wait for a replacement is another week buying full-price electricity. Having installed over 1,000 Powerwalls, we’ve found Tesla will usually confirm a fault remotely and replace the whole unit, and we’ve consistently seen faulty Powerwall 2 units upgraded to a Powerwall 3 at no cost.
Look at how long a brand has been around, how big it is and how it handles faults. Ask your installer what warranty claims are actually like, because they’re the first people you’ll ring.
The total price on a quote doesn’t tell you much on its own, because batteries come in different sizes. Divide the total price by the usable capacity to get the price per usable kWh.
| Quote | Total price | Usable capacity | Price per usable kWh |
|---|---|---|---|
| Quote A | £5,000 | 9 kWh | £555.56 |
| Quote B | £7,000 | 13.5 kWh | £518.52 |
Quote B looks £2,000 more expensive, but it’s about £37 cheaper for every kWh of storage you get.
Make sure both quotes include the same things. If you want power cut protection, the backup gateway needs to be in both, otherwise one will look cheaper than it really is.
Sizing still matters here. A battery is only good value if you use it. If you only ever use half of it, every kWh you do use has effectively cost you twice as much.
Before you commit, check these points on every quote:
Our battery storage systems page in the Knowledge Bank lists the systems we install. If you’d like help working out which battery suits your home, Spirit Energy provides bespoke technical quotations with forecast savings for each option, so you can see what a battery is expected to return before you pay for it.