Can you add a battery to existing solar panels?

Can I add a battery to my existing solar system?

Usually, yes: MCS says a certified installer can connect a battery to an existing system, and that you should check the battery is compatible with it [1]. Six things about your system decide how it is done. The installer checks them at the survey; you can find most of them on the inverter label, your FIT statement and in the meter cupboard.

AC or DC coupling: which is better for existing solar?

Neither is better in general; for existing solar the choice usually follows the inverter you already have. MCS's battery standard covers both: a battery with its own inverter, and one on the DC side of an inverter that also serves the panels [7].

The network limit is the total AC output of the generating equipment, so a 3 kW battery with its own inverter beside a 3 kW solar system takes a single-phase home past 16 amps per phase, and G99 applies [7].

On the Feed-in Tariff, do not agree to a battery that sits before the generation meter and can charge from the grid unless the installer shows, on the diagram, how the generation meter will still count solar only. With ordinary meters, Ofgem says neither generation nor metered export payments could be made in that set-up, and deemed export, which is worked out from the generation meter reading, goes with them [2]. Most smart meters are not the bi-directional meters that can fix this [2].

Will a battery affect my feed-in tariff?

1. Tell your FIT supplier about the battery and ask what it needs; our advice is to do this before the work [3][20].
2. Get the wiring diagram showing where the battery sits against the generation meter [2].
3. Establish whether the battery will charge from the grid, and what that means for your payments in your set-up [2].
4. Confirm which payments the arrangement keeps: generation payments, and deemed or metered export [2][4].

Why those four matter is below.

Not necessarily. Ofgem says storage can be added to a FIT installation without affecting its accreditation, as long as the scheme's requirements, including metering, are still met [2][3]. That is not the same as saying payments are unaffected: two things decide it, where the battery sits relative to the generation meter and whether it can charge from the grid.

• Generation payments. They are paid only if your FIT supplier can be satisfied the generation meter measures solar only [2]. That is clear when the battery can charge only from the panels or sits after the generation meter; where it cannot be shown, generation payments stop [2]. Ofgem adds that battery losses can reduce the generation recorded in some compliant set-ups, so payments are likely to fall [2].
• Deemed export. Where the generation meter measures solar only, deemed export payments continue with a battery [2]. Once an export meter is commissioned, export normally can no longer be deemed; but for an installation of 30 kW or less with a battery and a smart meter, where the solar share of export cannot be isolated, it may continue on deemed export [4].
• Export tariffs are a separate choice. To take a Smart Export Guarantee tariff you first opt out of FIT export payments, deemed or metered; generation payments carry on [3]. You can switch FIT export in or out no more than once every 12 months, and claiming both for the same capacity knowingly could be fraud [3]. If the export meter cannot separate solar export from grid electricity sent back through the battery, an SEG supplier does not have to make SEG payments at all; ask yours before the battery goes in [5][2]. Some battery tariffs end FIT export payments: Octopus's Flux terms say FiT payments for your exported energy "will be stopped" [21].

Telling your FIT supplier. Report the battery as a change to the installation, with an updated single line diagram showing where it sits and the metering, details of the meter readings used for FIT, the battery's details with its installation and commissioning date and supporting evidence, and, for an MCS-certified FIT installation, the declaration forms in Ofgem's guidance; a bi-directional meter needs its own details [3][2]. Octopus, for example, asks for the schematic line drawing and its battery storage declaration [20]. There is no way to get Ofgem's assurance in advance; changes are assessed once made and notified [2]. FIT and the Smart Export Guarantee are Great Britain schemes [3][5].

How much does it cost to add a battery to existing solar panels?

Using government median prices, a 5 kWh battery works out at about £6,500 installed and a 10 kWh battery about £8,900, by our calculation from the 2025-26 median price per kWh of nominal capacity for each size band [16].

Illustrative calculation by the Worthwatt team: size x the DESNZ 2025-26 median for its band, MCS-certified batteries in existing UK homes, installation and VAT where it applied, solar panels excluded [16]. Not quoted prices for those exact sizes.

DESNZ does not separate batteries added to existing panels from those fitted with new ones [16], and the figures may not include a replacement inverter, if yours cannot take a battery, or backup equipment. MCS says fitting a battery with new panels is often the most cost-effective option [1]; we found no official figure for how much more a retrofit costs, or for the price gap between AC and DC coupling. A battery added to existing panels has been zero-rated for VAT since 1 February 2024, until 31 March 2027, with 5 per cent due from 1 April 2027; bought without installation it is standard-rated [15]. Prices by size are on solar battery costs.

How is a solar battery installed?

Installing a solar battery runs in six steps with an MCS-certified installer, from survey to telling your FIT supplier. Until 31 March 2027, installers on the current MCS scheme follow the 2021 battery standard and those on the redeveloped scheme the 2025 one [10][6][7].

1. Survey. A competent person checks the building suits a battery and that the work keeps it within the building regulations [7], and the installer looks at where the battery can go.
2. Quote and estimate. Under the current MCS scheme, the details before the contract include whether the battery is AC or DC coupled, whether it uses your existing inverter, and its usable capacity [6]. MCS's savings estimate method, used on the redeveloped scheme, works for a battery added to existing panels; it adds your panels' contribution back to the electricity you buy before working out the estimate [8].
3. Network notification. The installer handles G98 or G99, full or fast-track [7]. For some batteries, depending on the total connected generation, the fast track aims to confirm within 10 days or less, against 45 otherwise, but not for a battery set up to run the house in a power cut [12].
4. Fitting and commissioning. The system is commissioned to a documented procedure [7]. Backup in a power cut needs an isolator that disconnects the backed-up circuits from the grid while the battery runs them [7].
5. Handover. You get the login details for any internet-connected parts [7], a handover pack, and an MCS certificate within 10 working days of completion [1] (30 calendar days under the redeveloped scheme [9]).
6. Tell your FIT supplier. If you get Feed-in Tariff payments, report the battery with an updated single line diagram, your meter readings, the battery's details and installation date with evidence, and, for an MCS-certified FIT installation, the declaration forms [3].

Is it worth adding a battery to my solar panels?

On the savings from storing solar alone, not in our worked examples: the savings did not repay the estimated battery cost within the 10 to 12 years the Energy Saving Trust gives as a typical battery life [17]. Those examples assume each stored kWh loses its export payment; on FIT deemed export it does not, so savings are higher, but by our calculation the same households still take about 17 and 23 years to repay. That also assumes the battery does not reduce your generation payments, which losses before the generation meter can do in some set-ups, and so lower deemed export, which is worked out from it [2]. A lower quote, high evening use or cheap overnight charging can change that; on the Feed-in Tariff, check the meter rules before charging from the grid. The sums, and the price at which a battery would pay back, are on is a solar battery worth it; what size battery do I need covers sizing.