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BatteriesAdding a battery to solar you already have
A retrofit is one of the most common jobs there is, and your panels never get touched. The part that catches people out is the grid application, not the battery.
Updated 4 August 2026 · from the OVO Solar & Battery team
The short answer
You can add a battery to solar panels that are already on your roof, and it is one of the most common jobs in the trade. In most retrofits the battery arrives with its own inverter and charger built in and joins the system on the AC side, near your consumer unit. Your panels, your existing solar inverter and the wiring between them are left exactly as they are.
Your existing inverter usually stays put
An AC coupled battery works alongside whatever solar inverter you already have, whatever brand it is and however old it is, as long as it is healthy. It also does not have to live next to the existing kit. A garage, a utility room or a suitable outside wall are all normal places for it to go.
AC coupled, DC coupled, and why retrofits are usually AC
There are two ways to wire a battery into a home. AC coupled means the battery brings its own inverter and connects on the AC side, alongside the solar you already have. DC coupled means the battery and the panels share one hybrid inverter, which on a retrofit normally means taking out the inverter that is already working and replacing it.
| AC coupled | DC coupled hybrid | |
|---|---|---|
| Your existing solar inverter | Stays exactly as it is | Usually replaced with a hybrid unit |
| Where the battery can go | Anywhere sensible, it does not need to be near the panels | Tied to where the inverter sits |
| Round trip losses | Roughly 8 to 10% | Roughly 6 to 8% |
| Suits | Retrofit to working solar, microinverter systems, and battery with no solar at all | Solar and battery fitted together, or an existing inverter already due for replacement |
Round trip loss figures are typical ranges for UK home storage and vary by product and by how hard the system is worked.
The efficiency gap is real, but it is small and it is rarely a reason to remove an inverter that works. Two percentage points on a 10 kWh cycle is about 0.2 kWh a day. The case for going hybrid is a different one: if your solar inverter is already old and close to the end of its life, replacing it with a hybrid unit at the same time as the battery goes in means paying for one visit instead of two.
The grid connection is what catches people out
Your Distribution Network Operator, the company that owns the cables in your street, decides what you are allowed to connect. Under G98, a single phase connection up to 16A per phase, which is 3.68 kW, can be installed first and notified within 28 days. Anything above that needs a G99 application approved in writing before the work happens, and the operator has up to 45 working days to respond. The detail that matters on a retrofit is that the assessment is made on inverter rating, not on battery capacity, and the generation you already have counts towards it.
3.5 kW plus 3.5 kW takes you into G99
If your existing solar inverter is rated at 3.5 kW and the battery brings another 3.5 kW with it, the site is assessed at 7 kW and drops out of G98 into G99. Nothing is wrong with that. It just means an application and a decision before the install date rather than a notification afterwards, and it is the most common reason a retrofit takes longer than the homeowner expected. G98 and G99 explained walks through both routes.
Your local OVO expert checks the inverter rating and works out which application you need on the free 45 minute video design, with no home visit and a written quote on the call.
Get your free quote →What changes once the battery is in
- Your evenings stop being expensive. Solar your roof made at lunchtime covers the four to nine block instead of being exported for a few pence and bought back later at the standard rate.
- You can buy electricity when it is cheap. With a smart meter and a time of use tariff, the battery fills during the cheap overnight window and runs the house through the expensive parts of the day. That works in December as well as June, which solar on its own does not.
- Your export pattern changes. Less of what your roof makes leaves the property, so export income falls while your import bill falls further. How that trade lands depends on the export rate you are on.
The arithmetic on the overnight shift is simple enough to do on a phone. The July to September 2026 price cap puts the average electricity unit rate at 26.11p per kWh. Economy 7 night rates averaged 15.43p per kWh in April 2026, with the cheapest around 12.5p, and smart time of use tariffs price their cheap windows lower still. Buy a unit at 15.43p, lose 9% of it on the round trip, and it has cost you about 17p by the time you use it, against 26.11p to import the same unit at the standard rate. That is roughly 9p saved on every unit you shift, and a better off peak rate widens the gap.
Five things to check before you commit
What your existing inverter is rated at
It is printed on the label on the unit itself, usually in kW. That number plus the battery inverter is what decides whether you are on a G98 notification or a G99 application, so it is the first thing worth photographing.
Whether you have a smart meter
Time of use tariffs run on half hourly readings and a traditional meter cannot supply them. Without one, a battery can still store your own solar, but it cannot reach the cheap overnight rates that make up a large part of the case. Sort the meter before the battery, not after.
Which export arrangement you are on
If you are on the Feed in Tariff, the Smart Export Guarantee cannot be combined with FiT export payments, so speak to your FiT licensee before changing anything. Worth knowing too: electricity you bought from the grid and later exported from the battery is not covered by the SEG obligation, so a supplier does not have to pay for it.
What your evening and overnight usage actually looks like
Not your annual total, the part of the day your roof cannot reach. Take the annual kWh from a bill and divide by 365 for a daily baseline. Ofgem's 2026 typical values are 1,600 kWh a year for a low user, 2,500 for medium and 3,800 for high, which is 4.4, 6.8 and 10.4 kWh a day.
Who stands behind the kit
A battery warranty is a promise made by a company, and companies do not always last. The UK's best known domestic battery brand went into administration in April 2026 and stopped trading, leaving owners with hardware that still worked and no manufacturer to call. Ask who honours the warranty, and ask what covers the workmanship. Ours is the OVO guaranteed 10 year workmanship warranty.
What a retrofit battery costs
Across the UK market, installed home battery prices run at roughly £400 to £900 per kWh. That is a wide band because it covers everything from a small retrofit to a large system with backup hardware. Our own battery price is published rather than quoted case by case: £2,444 for 5.2 kWh usable, about £470 per usable kWh, fitted and commissioned. Battery storage installed at a home in Great Britain currently attracts 0% VAT, and a retrofit added to existing solar qualifies exactly as a new system does. That relief is scheduled to run until 31 March 2027, after which the rate is set to return to 5%. Full pricing sits on the costs page.
How the job runs
- A free 45 minute remote video design. No home visit, and you get a written quote on the call.
- Your local OVO expert checks the existing system, the inverter rating and the meter, then works out whether the connection is a G98 notification or a G99 application.
- Where G99 applies, the application goes in and the network operator has up to 45 working days to respond. G98 sites are notified within 28 days of the work being done.
- Install day. The battery is mounted, wired in on the AC side and commissioned. Your panels and your existing inverter are left alone.
- Certification and export. Signing up for OVO's SEG takes around four weeks and needs MCS certification or equivalent, a smart meter, and battery schematics where a battery is fitted. The Smart Export Guarantee page carries the detail.
Get your retrofit priced in writing on the call, with the grid application worked out before you commit to anything.
Get your free quote →Retrofit questions, answered
Can I add a battery to solar panels somebody else installed?
Yes. An AC coupled battery brings its own inverter, so it works alongside existing solar regardless of who fitted it or which brand it is. What matters is the rating of your current inverter, the condition of the existing installation and whether you have a smart meter. Put your postcode into the coverage map and it will tell you which local OVO expert covers your area.
Will my existing solar inverter have to be replaced?
Usually not. On an AC coupled retrofit the battery has its own inverter and charger, so the solar inverter you already have carries on doing its job. Replacement only really comes into the conversation if that inverter is old and near the end of its life, in which case swapping it for a hybrid unit while the battery is being fitted saves paying for a second visit later.
Do I need permission from the network operator?
It depends on the total inverter rating, not the size of the battery. Up to 3.68 kW per phase you are inside G98, which is fitted first and notified within 28 days. Above that you need a G99 application approved before the work goes ahead, and the operator has up to 45 working days to respond. Existing solar counts towards the total, which is why a retrofit often moves a site from G98 into G99.
Is there VAT on a retrofit battery?
Not at the moment. Battery storage installed at a home in Great Britain attracts 0% VAT, and that covers a retrofit added to existing solar as well as a standalone battery with no solar at all. The relief is scheduled to end on 31 March 2027, with the rate set to return to 5% from 1 April 2027. On a typical installation that is a difference of roughly £225 to £500.
What size battery should I add to a system I already have?
Size it against the electricity you use when your roof cannot help, which for most homes is the evening and overnight block, rather than against the size of the array. Most UK homes land somewhere between 5 and 10 kWh of usable capacity. Capacity you cannot fill and empty most days just extends your payback. We go through the method in what size battery do I need.
Will the battery keep my house running in a power cut?
Not on its own. A standard grid connected battery shuts down when the grid goes down, which is a safety rule called anti islanding rather than a fault. Keeping the lights on needs extra hardware between the meter and the consumer unit, and how much of the house it covers is a decision in itself. Our guide to gateways and whole home backup covers the options.
Does adding a battery change what I get paid for exporting?
It can. OVO publishes four SEG rates: 4p per kWh on the standard SEG, open to anyone with an eligible system; 12p on SEG Beyond Exclusive for OVO energy customers with systems under 30kW; 15p on SEG Install Exclusive where OVO installed the panels; and up to 20p where OVO installed both the panels and the battery. Rates were correct at July 2026 and are subject to change, and eligibility criteria apply to each one, so get your own position confirmed on the design call rather than assumed.
Figures and specifications in this guide are sourced below and were checked on the date shown. Rates and product specifications change; we confirm the current picture on your free design call.
Sources
- Ofgem, energy price cap 1 July to 30 September 2026, average electricity unit rate 26.11p per kWh ofgem.gov.uk
- AC coupled and DC coupled home storage compared, including round trip efficiency greenreachenergy.co.uk
- Decision rule for AC coupling on retrofits and hybrid inverters on new installations saveenergyuk.co.uk
- G98 and G99 thresholds, cumulative inverter assessment and network operator response times capture.energy
- Zero rate VAT on domestic battery storage including retrofit and standalone, to 31 March 2027 solarenergyuk.org
- Installed cost per kWh benchmarks for UK home battery storage, 2026 kilowatts.uk
- Ofgem typical domestic consumption values and the daily usage bands they imply kindenergy.co.uk
- Economy 7 average night unit rates, April 2026 energyplus.co.uk
- Smart Export Guarantee treatment of grid charged electricity exported from a battery which.co.uk
- OVO Smart Export Guarantee rates, eligibility criteria and signup requirements ovoenergy.com
- UK domestic battery manufacturer administration, April 2026, and what it meant for existing owners ess-news.com
- OVO Solar and Battery published battery pricing and specification ovosolarandbattery.co.uk
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