Best battery for existing solar panels: AC-coupled retrofit picks

Quick answer: For most existing solar arrays the Tesla Powerwall 3 is the strongest retrofit buy: 13.5kWh, AC or DC coupling, a 10-year warranty and a unit price of around £7,395 plus roughly £1,000 to £2,500 installation (Heatable, checked July 2026). If you want the longest warranty, Enphase’s IQ Battery 5P runs 15 years. Modest arrays should start smaller than 13.5kWh.

Why AC-coupled is the retrofit route

Your existing panels feed a string inverter that turns DC into AC. An AC-coupled battery has its own inverter and connects on the AC side, so nothing about your original system changes. No rewiring of strings, no replacing a working inverter, and if you’re on the old Feed-in Tariff your generation meter reads exactly as before. That last point matters: FIT payments are based on what you generate, and an AC-coupled retrofit leaves that measurement alone. We’ve covered the wiring choices in our retrofit guide, and the deeper technical trade-offs sit in our full battery comparison.

We haven’t tested these units — this is based on published specifications and warranty terms, checked July 2026. Batteries installed by a contractor also carry 0% VAT until 31 March 2027.

The picks

Best all-round: Tesla Powerwall 3

13.5kWh of storage, 11.5kW of continuous power, and it accepts both AC and DC coupling, so it retrofits cleanly today and could absorb a future panel upgrade directly. The warranty is 10 years with a guarantee of at least 80% capacity retention. Heatable listed the unit at £7,395 with installation typically adding £1,000 to £2,500 (checked July 2026), and it’s expandable to 54kWh with additional units. The catch is power you may not use: 11.5kW output is more than most homes draw, and a single Powerwall is overkill for a small 2.5kWp array.

Best warranty: Enphase IQ Battery 5P

The IQ 5P is AC-coupled by design, with 5kWh usable per module, 3.2kW continuous output and a 15-year or 6,000-cycle warranty (enphase.com, checked July 2026). That’s the longest mainstream battery warranty in the UK, five years past Tesla and most of the field. It’s modular, so you can start at 5kWh for a small array and add capacity later; Upvolt Energy puts a typical single-module retrofit around £5,000 installed (checked July 2026). The weak spot is output: 3.2kW per module won’t run a kettle and an oven together off-grid, so heavy-load homes need two modules or a different pick.

The cautionary tale: GivEnergy All in One

The All in One packs 13.5kWh with up to 7.2kW peak output (3.6kW continuous), and until this year it was the obvious British pick. Then GivEnergy Ltd entered administration on 9 April 2026, ceased trading, and stopped honouring its 12-year hardware warranty. Installed units keep working and the app runs on under a separate software company, but a new purchase from remaining stock comes with no manufacturer warranty at all — whatever the discount, price that risk in. Our GivEnergy review and manufacturer failure guide cover what owners should do now.

Size to your surplus, not the brochure

Here’s where we differ from most battery marketing. If your 4kWp array exports maybe 5 to 8kWh on a good summer day, a 13.5kWh battery will rarely fill from solar alone. It only earns its price if you also charge it cheaply overnight on a smart tariff and discharge through the day, which is a tariff-arbitrage decision as much as a solar one. Our smart tariff comparison for solar and battery homes runs those numbers. If you just want to stop exporting summer surplus for pennies, one Enphase 5P module is plenty, and at the current 26.32p/kWh price cap every stored kilowatt-hour you use instead of importing is worth about 26p minus whatever export payment you gave up.

A few practicalities before you sign anything. Your installer must notify your distribution network operator (a G98 or G99 application depending on export capacity), which is routine but can add weeks on constrained networks, so ask where your application stands. Think about location too: lithium batteries prefer garages, utility rooms and sheltered outdoor walls over hot lofts, and heavier units like the Powerwall need a solid mounting surface. And you’ll want a smart meter reporting half-hourly, because most of the payback comes from time-of-use tariffs that require one.

Estimate your own payback with our calculator, or see the battery storage product guide for the basics.

FAQs

Will a battery affect my Feed-in Tariff payments?

An AC-coupled battery fitted after your generation meter doesn’t change what the meter records, so FIT generation payments continue as before. Tell your FIT licensee about the change anyway, and avoid DC-coupled retrofits that rewire the generation side without specialist advice.

Do I need to replace my inverter?

No. That’s the point of AC coupling: the battery brings its own inverter and your existing one keeps doing its job. If your string inverter is near the end of its life anyway, a hybrid replacement plus DC battery is worth pricing as an alternative.

How long until a retrofit battery pays for itself?

It depends on your tariff, export rate and usage pattern, and typical estimates run to a decade or more on self-consumption alone. Aggressive time-of-use charging shortens that considerably. Treat vendor payback claims as best-case scenarios.