Energy Savings Calculator
0203 193 8888
0203 193 8888
Estimated reading time: 9 minutes
If you installed solar panels in recent years without a battery, you’re far from alone, and you’ve probably noticed electricity prices climbing while Smart Export Guarantee rates for the electricity you export have been falling. It’s a natural moment to wonder whether you can add a battery now rather than starting from scratch. The good news is that retrofitting is common and, in the vast majority of cases, straightforward. This article covers everything you need to know: whether your existing system is compatible, the difference between AC and DC coupling, realistic 2026 costs, how to choose the right battery size, and what to check before booking an installer.
Almost any existing solar PV system can have a battery added, regardless of how old the panels themselves are. Panels generate electricity in largely the same way whether they were fitted last year or a decade ago, so panel age on its own is rarely a barrier to retrofitting.
Two factors matter far more than panel age: the type of inverter you already have, and whether there’s enough space for the a suitable battery location that meets applicable installation and fire-safety requirements.. These two things, more than anything else about your existing system, determine how straightforward and how expensive your retrofit will be.
It’s worth understanding the difference between hybrid inverters, which are already battery-ready, and standard string inverters, which need extra equipment to work with a battery. If you’re not sure which you have, the quickest way to check is to locate your inverter’s model number on its spec sheet or casing, or simply ask the installer who originally fitted your system.
DC coupling connects the battery on the DC side of the system, usually through a compatible hybrid inverter. An existing standard solar inverter may need replacing. It’s a more involved piece of work, since the inverter you already have comes out and a new one goes in.
AC coupling takes a different approach. Rather than replacing anything, it adds a separate “storage inverter” alongside your existing inverter, with no swap required. Your original equipment keeps doing exactly what it’s always done, while the new unit manages the battery independently.
AC coupling is usually cheaper and less disruptive for retrofits, since there’s less equipment to replace and less rewiring involved, while DC coupling is marginally more efficient overall, because electricity doesn’t need to be converted back and forth between the two inverters.
As a simple rule of thumb, most homeowners with a standard string inverter should start by comparing AC-coupled options, since it’s typically the quicker and more cost-effective route for a retrofit, and it’s worth a specific conversation with an installer before ruling out DC coupling altogether.
Current cost ranges for a retrofit in 2026 run roughly as follows: £3,000 to £5,500 for a 5kWh AC-coupled battery, £5,000 to £8,500 for a 10kWh unit, and £7,500 to £10,500 for a larger 13.5kWh system such as the Tesla Powerwall 3.
Retrofitting typically costs somewhat more than adding a battery at the same time as new panels, since the electrical work and commissioning can’t be combined into a single visit the way they can with a brand new installation. That additional cost is usually modest rather than dramatic, and it’s often outweighed by the benefit of not having to wait to upgrade your system.
It’s worth noting that batteries currently qualify for 0% VAT until March 2027, which represents a genuinely time-sensitive saving. Retrofitting sooner rather than later means locking in that saving rather than risking a change to VAT rules further down the line.
Urban or London properties may see an extra £800 to £1,500 added to a quote, for wiring upgrades or difficulties with access that are more common in denser housing, so it’s worth factoring this in if you live in a flat or a terraced property with limited space around the consumer unit.
Sizing a retrofit battery should be based on your household’s daily electricity usage, not just how much your solar panels generate. A battery that’s too small will fill up and empty quickly without making much difference to your bills, while one that’s too large may rarely be used to its full capacity.
Checking your evening and overnight usage patterns matters particularly, since that’s typically when the battery discharges to cover electricity your panels aren’t currently generating. If your household uses most of its electricity after dark, a larger battery is more likely to earn its cost back through genuine savings.
Among typical UK households, 5 to 10kWh tends to be the most common range, though the right figure for your home depends entirely on your own usage. It’s worth asking an installer to review your historic generation and export data, available through a smart meter or your inverter’s app, before settling on a specific size, rather than guessing based on a generic recommendation.
Adding a battery generally means updating your property’s MCS certificate to reflect the new equipment, since the certificate needs to describe your system as it currently stands rather than how it looked when first installed. Ask the installer to confirm which certification and commissioning documents the battery installation requires, and check your export supplier’s requirements.
SEG payments are usually unaffected by adding storage, though your installer should notify your energy supplier and the DNO as part of the retrofit, so that your records stay accurate and your payments continue without interruption.
A fresh DNO notification, sometimes referred to as G98 or G99 depending on the size involved, may be required depending on your total system size once the battery is added. Your installer must establish the correct DNO application or notification route before work proceeds and obtain approval before connection where required. This is a standard part of a compliant retrofit rather than anything to be concerned about.
Using an MCS-certified installer is the simplest way to keep all of this paperwork compliant, and to avoid any risk to your SEG eligibility through an incomplete or incorrect notification.
A professionally installed retrofit should not affect your existing solar panel warranties. The panels themselves are unaffected by work carried out on the inverter or the addition of a battery alongside them.
It’s worth checking your inverter warranty terms carefully, though, particularly if you’re switching to a hybrid inverter as part of a DC-coupled retrofit, since replacing the inverter is a more significant change than simply adding a battery alongside an existing one.
It’s sensible to get written confirmation from your installer that your existing warranties remain valid after the retrofit, rather than assuming this is automatically the case. Keep this documentation alongside the paperwork from your original installation and the new paperwork for the battery, so everything is in one place if you ever need to refer back to it.
Most retrofit projects follow a fairly predictable timeline, typically taking one to two days on site for the installation itself.
Before any work begins, a site survey or consultation confirms your inverter’s compatibility and the best location for the a suitable battery location that meets applicable installation and fire-safety requirements.. This step is where an installer will also flag anything unusual about your property that could affect the quote.
On the day, electrical safety checks are carried out alongside the installation, and any consumer unit or wiring upgrades identified during the survey are completed at this stage. The job finishes with commissioning the system, setting up the monitoring app, and a handover where the installer talks you through how everything works.
Choosing an MCS-certified installer matters for any retrofit work, both for the quality of the installation and to keep your paperwork, warranties and SEG eligibility in order.
It’s worth getting multiple quotes and comparing how different installers approach AC versus DC coupling for your specific setup, since recommendations can vary and it’s useful to understand the reasoning behind each one rather than accepting the first suggestion you receive.
Ask specifically about warranty coverage on both the new battery and any replacement inverter, since these are two separate things that can have different terms and different providers behind them.
Finally, check installer reviews with an eye specifically for retrofit experience, not just new installs. Retrofitting carries its own particular challenges around compatibility and existing wiring, and an installer who’s done many new installs isn’t automatically as experienced with the specific demands of adding a battery to a system that’s already in place.
Yes, in the vast majority of cases. Panel age itself is rarely a barrier to adding a battery, since the two factors that really matter are your existing inverter type and whether there’s space for the a suitable battery location that meets applicable installation and fire-safety requirements.. Older panels generally continue to generate electricity perfectly well alongside a newly added battery, so it’s worth getting a compatibility check rather than assuming your system is too old to retrofit.
For most retrofits, AC coupling tends to be the simpler and more cost-effective option, since it adds a separate storage inverter alongside your existing one rather than requiring a full replacement. DC coupling is marginally more efficient and may suit certain setups better, particularly where a full inverter upgrade makes sense anyway. The right answer depends on your specific inverter and circumstances, so it’s worth discussing both options directly with an installer before deciding.
Most retrofit installations take one to two days on site, following an initial survey or consultation to confirm compatibility and plan the work. The exact timeline depends on the complexity of your property and whether any additional wiring or consumer unit upgrades are needed. Straightforward AC-coupled retrofits with good access tend to be at the quicker end of that range.
Yes, in almost all cases. You can generally retain SEG eligibility, but your export income may fall because you export less electricity. Check your supplier’s battery-storage and grid-charging terms before installation. Using an MCS-certified installer is the most reliable way to make sure all of this is handled correctly and your SEG eligibility isn’t put at risk.
Most domestic battery retrofits fall under permitted development and don’t require planning permission, though this can vary depending on your property type and location, particularly for listed buildings or properties in conservation areas. Your installer should be able to confirm whether your specific property needs anything beyond standard building regulations and electrical compliance checks.
Retrofitting a battery is a practical, increasingly popular upgrade for homeowners who installed solar-only systems in recent years. With the right guidance on coupling, sizing, and installer choice, it can significantly boost how much of your own solar generation you actually use, rather than exporting it cheaply and buying it back later at a higher price.
If you’re wondering whether your system is compatible, book a free, no-obligation retrofit assessment with The Solar Co to find out and get a tailored quote.
Back to Blog