Retrofit Battery Storage to Existing Solar

Almost every battery you can add to an existing solar array falls into one of two families, and choosing correctly is the biggest single decision in the whole job. It decides whether your current inverter stays or goes, how much you pay, and how much of every stored unit you get back.

AC-coupled retrofit

An AC-coupled battery brings its own inverter/charger and sits on the household AC side of your existing system. Your current solar inverter is left exactly as it is, still turning panel DC into AC for the house; the battery simply watches the mains and stores surplus. Because it works alongside any solar inverter, an AC retrofit is the simplest, least disruptive and usually cheapest route — the array is never touched. The trade-off is a second conversion step (DC to AC to DC), so real-world round-trip efficiency lands around 85–90%. For most Wiltshire homes with a healthy 3–6kW string inverter still inside its warranty, this is the route we recommend.

DC-coupled (hybrid) retrofit

A DC-coupled battery connects to the panel-side DC bus of a hybrid inverter, which means your existing solar inverter is removed and replaced with a single unit managing panels, battery, house load and export together. Solar charges the battery DC-to-DC with no intermediate AC step, lifting round-trip efficiency to roughly 92–97% and giving you one app for everything. It costs more because you are buying an inverter as well as a battery, and the array has to be re-commissioned to the new unit — but if your existing inverter is over 8–10 years old or already faulty, replacing it as part of the upgrade often makes better sense than paying for a battery inverter on top.

The honest rule of thumb: AC-couple a system that already works well; DC-couple (hybrid) a system whose inverter is near the end of its life or that you want fully unified. We assess both on every survey rather than defaulting to whichever battery we happen to stock. If you are still weighing up whether storage stacks up at all, our is battery storage worth it breakdown runs the numbers before you commit, and our dedicated battery storage page covers the products in full. This retrofit route is a subset of our wider solar and battery service.

Before we quote a single kWh, we survey the system you already have. Four things decide what can be fitted and at what cost.

1. Your inverter type and size

String inverters (SMA, Solis, Fronius, Growatt and similar) AC-couple easily and can also be swapped for a hybrid if you prefer DC coupling. SolarEdge systems can take a matched DC battery or AC-couple. Enphase micro-inverter arrays can only ever be AC-coupled — there is no shared DC bus to tap, so a hybrid swap is off the table. We also check the inverter's kW rating and age: a 4kW unit installed in 2015 nearing its warranty end is a strong candidate for a hybrid replacement, whereas a healthy 5kW unit from 2022 is better left in place with an AC battery.

2. DNO notification (G98/G99)

Adding storage changes what your property can push onto the grid, so it must be notified to your Distribution Network Operator. Small AC retrofits usually qualify for the simpler G98 notification; a hybrid inverter swap or a larger system needs a G99 application, which we handle for you. Skipping this is not optional and can invalidate your Smart Export Guarantee payments.

3. The CT clamp — the brain of a retrofit

Every AC-coupled retrofit needs a current transformer (CT) clamp fitted around your incoming meter tail. This measures import and export in real time so the battery only ever soaks up genuine solar surplus and discharges to cover the house without spilling stored energy back to the grid for a few pence. A mis-sited or missing CT clamp is the most common reason a retrofit "doesn't seem to save anything" — placement matters, and it is a job for a qualified installer, not a plug-in accessory.

4. Backup: gateway or no gateway

A standard retrofit keeps your lights on only while the grid is up. If you want the battery to carry the house through a power cut, you need a backup gateway (such as Tesla's Backup Gateway) and a consumer-unit changeover, which adds cost and space. Many homeowners skip it and simply bank the daily import savings. We map your consumer unit and meter position on survey so there are no surprises. Homes we survey across Melksham, Chippenham and Devizes vary hugely in board layout, and that dictates the cleanest install path.

Once we know whether you are AC- or DC-coupling, the battery itself is chosen around your electricity habits, roof output and available wall or floor space — not brand loyalty.

The batteries we fit in 2026

Sigenergy SigenStor is our workhorse for modular retrofits. It stacks from around 5kWh upward in the same footprint, so a household can start modestly and add modules later as an EV or heat pump arrives. It carries a 10-year warranty on LFP (lithium iron phosphate) chemistry and can be configured as an all-in-one hybrid for DC coupling or paired as AC storage. Tesla Powerwall 3 is the choice where you want maximum single-unit power: 13.5kWh, up to 11.5kW continuous output (enough to run almost the whole house at once), 97.5% inverter efficiency and a 10-year, unlimited-cycle warranty guaranteeing 70% capacity retention. Multiple Powerwalls stack if you need more.

A note on GivEnergy

We are frequently asked about GivEnergy. For transparency: GivEnergy Ltd entered administration in April 2026 and has ceased trading, meaning its manufacturer warranties and cloud support are no longer being honoured directly. Existing GivEnergy hardware keeps charging and powering homes — the battery does not stop working — but we are not fitting new GivEnergy systems, and if you own one we can advise on monitoring and any future replacement. We would rather tell you this plainly than sell a product whose backing has gone.

Sizing: bigger is not automatically better

The right capacity is the one your solar can actually refill and your evenings can actually drain. A 4kWp array generating roughly 3,900kWh a year produces meaningful summer surplus but little in December, so oversizing to 20kWh+ on a modest array leaves you paying for capacity that sits half-empty for months. As a guide, a typical three-to-four-bedroom home with a 4–5kWp array is well matched by 9–13kWh of storage; add capacity only if you run a home EV charger, work from home, or plan to charge cheaply overnight on a time-of-use tariff. Our engineer-led survey models your last 12 months of consumption against your roof before recommending a size. For a wider brand comparison, our guide to the best home batteries in the UK for 2026 weighs Sigenergy, Tesla and the alternatives side by side.

A battery-only retrofit is a smaller, cleaner job than a full solar install, and 2026 tax rules work in your favour.

What it costs

As a 2026 guide, an AC-coupled retrofit typically runs £2,995–£7,000 installed for 5–10kWh, covering the battery, inverter/charger, CT clamp, circuits and any DNO variation. A DC-coupled / hybrid retrofit sits higher at roughly £7,000–£12,000 because it includes replacing your solar inverter. A single Tesla Powerwall 3 lands around £9,500–£10,500 fully installed. Crucially, 0% VAT applies to domestic solar and battery storage — including standalone battery retrofits — until 31 March 2027, so adding storage now avoids a 20% charge that returns afterwards. The exact price table below shows how cost scales with capacity.

How the savings actually work

A retrofit battery earns its keep through arbitrage: it stores solar you would otherwise export for as little as 4–16p/kWh under the Smart Export Guarantee (a good rate today is around 10.8p) and uses it in the evening to displace grid import at roughly 27p/kWh. On a time-of-use tariff it can also charge from cheap overnight rates and discharge across the expensive peak, widening the gap further. That spread is why storage payback has shortened even as SEG rates vary — you are capturing the difference between ~27p import and ~11p export on every stored unit. Spreading the cost is straightforward too: our solar finance options let you fund a retrofit against the monthly bill savings rather than upfront.

Pair it with an EV charger

If you drive electric, a retrofit is the natural moment to add a smart charger. A Zappi can divert genuine solar surplus straight into the car (its eco+ mode charges only from excess generation), while an Ohme leans on smart tariffs to pull the cheapest overnight electricity — either can work in concert with your new battery so solar, storage and car are prioritised sensibly. See how we integrate the two on our solar EV charging page. As MCS-certified, NICEIC-approved installers led by engineer Matt Butler, we fit retrofits and chargers across a 30-mile radius of Melksham — including Portishead, Thornbury and Calne. For local pricing context, see solar panel costs in Wiltshire.