Adding a battery to existing solar panels

The harder question isn't whether it's possible. It's which of the two wiring routes suits your system, what a retrofit does to Feed-in Tariff payments if you're still on FIT, and whether your existing inverter is actually worth keeping. Here is how we'd answer each, and what we check before quoting.

Can any solar panel system take a battery later?

In practice, yes, provided the roof structure and the panels themselves are sound. The battery doesn't attach to the panels at all. It sits downstream, usually in a utility room, garage or loft space with the inverter, and is wired into your consumer unit and meter setup. What limits a retrofit is the condition and type of your existing inverter and consumer unit, not the panels.

The one case where it gets more involved is a very old string inverter that was never designed to talk to a battery and has no manufacturer "hybrid" equivalent. Then the practical answer is to replace the inverter as part of the job rather than bolt a battery onto something that was never meant to host one.

AC-coupled or a hybrid inverter swap: which retrofit route?

Most retrofits onto a system that's still performing well go the AC-coupled route, because it leaves a working inverter alone. We'd only recommend a hybrid inverter swap where the existing inverter is near end of warranty, already giving fault codes, or isn't one we can get parts or firmware support for. That's a judgement call a survey makes on your actual inverter, not a rule of thumb.

What happens to your Feed-in Tariff payments?

Nothing changes to the generation payment itself. Your FIT generation tariff is paid on what your generation meter records, and that reading doesn't move just because a battery now sits between your panels and your consumption.

What does need to happen is telling your FIT licensee you've added storage. Ofgem's guidance for FIT generators says battery storage can be added without affecting your accreditation, provided the installation keeps meeting the scheme's metering rules, and that you report the change to your FIT licensee, with an updated single-line or schematic diagram showing how the storage is wired in and the meter readings you're using to claim payments. Where the battery can also be charged from the grid rather than only from your panels, the fix is usually a bi-directional meter that nets the two off, so your generation reading stays accurate. We'll talk you through exactly where the battery sits relative to your meter during the survey, and flag anything that needs a call to your licensee.

Do you need to notify the grid (G98 or G99)?

Usually yes, because a battery brings its own inverter, and any inverter connecting to the network needs a notification regardless of whether it's generating or storing. Which form applies depends on the combined capacity of your solar inverter and the new battery inverter together, not just the battery on its own.

Under 3.68kW (16A) per phase combined, it's a G98: a "connect and notify" process where the installer tells the district network operator (DNO) within 28 days of commissioning. Above that threshold, it's a G99, which needs the DNO's approval before installation goes ahead, not after. We handle the paperwork either way as part of the job; you don't submit anything yourself.

Will an older inverter cause problems?

Sometimes, and it's worth being blunt about it rather than assuming every system is retrofit-ready. Inverters wear out well before panels do, and an inverter nearing the end of its working life, one that's already had a repair, or an early model with no hybrid or battery-ready firmware path, is the main reason a straightforward AC-coupled job turns into "replace the inverter first." We'd rather tell you that at survey stage than fit a battery against an inverter that's going to need replacing within a year regardless.

What does the survey check?

• Your existing inverter: make, age, condition, and whether it has a manufacturer battery-ready path or needs replacing.
• Your consumer unit and meter setup, including where the generation meter sits and whether the battery can be wired to charge downstream of it.
• Space and ventilation for the battery and any new inverter, indoors or in a suitable outbuilding.
• Roof and panel condition, only to confirm nothing about the existing array needs attention before new equipment goes in.
• Combined inverter capacity, to work out whether the job needs a G98 or a G99 notification.

What it costs

Pro-Fit doesn't publish a fixed price for battery retrofits on our guide price table, because the right answer depends on your existing inverter, your consumer unit, and which battery suits the job. It's priced in writing after a free survey, the same process we use for a new solar and battery install from first call to handover: survey, a written quote with the price fixed, 14 days to change your mind, then installation and handover with the monitoring app set up.

Batteries we fit come from Fox ESS, Sigenergy, Sunsynk and Powervault, named on the written quotation so you know exactly what's going in. Workmanship carries a 2 year guarantee from Pro-Fit, the battery up to 12 years from the manufacturer (confirmed in your handover pack), and the inverter per the terms on your quote. 0% VAT on qualifying installations until 31 March 2027, then 5%.

Pro-Fit is Flexi-Orb certified and an HIES member for solar and battery work, so the retrofit is covered by the same standards body as a full new install, even though the panels themselves went up with someone else.