Can I add a battery to my existing solar panels?
Short answer
Usually yes. Newer string inverters are often kept with an AC-coupled battery; older ones may be swapped for a hybrid inverter. Microinverter or optimiser systems normally need AC or same-brand batteries. The extra inverter often pushes total capacity past 3.68 kW single-phase, so a G99 application is common.
Last checked 24 September 2026 · 3 sources
What should I know before deciding?
MIS 3012 note: 3 kW PV + 3 kW battery on one phase exceeds 16 A so G99 applies. FiT systems: report storage to your FiT licensee.
What is the difference between AC-coupled and DC-coupled batteries?
A DC-coupled battery shares a hybrid inverter with the solar panels, which is efficient when fitting solar and battery together. An AC-coupled battery has its own inverter and connects to the house wiring, so existing solar stays untouched. Both are in scope of the MCS battery standard MIS 3012.
MIS 3012:2025 scope lists both use cases. MIS 3012 classes 1-4 set who is responsible for compatibility (Class 3/4: installer confirms PCE/BMS compatibility). AC coupling adds conversion losses; DC coupling can affect FiT metering if replacing a FiT inverter.
Will adding a battery affect my Feed-in Tariff payments?
Not if it is wired correctly. Ofgem says generation payments continue if the generation meter measures only your solar, so a battery charged from the grid must not sit before that meter. Deemed export can continue on the same basis. You should tell your FiT licensee when you add storage.
DC-coupled retrofits need a single meter able to net; multiple meters to calculate net output not allowed. Deemed export for FiT ≤30 kW = 50% of generation for PV.
Do I need permission from the network operator for a battery?
Up to 16 A per phase in total (about 3.68 kW single-phase, including existing solar) the installer just notifies the DNO within 28 days (G98). Above that, G99 applies: some set-ups need only notification if software-limited, but most need approval before installation, sometimes with an export limit.
G99 SGI-1: units up to 32 A design capacity limited to 16 A aggregate - notify (Form A3-3). SGI-2: aggregate <32 A/phase with G100 export ≤16 A - DNO answers within 10 working days. SGI-3: aggregate <60 A/phase with export ≤32 A - initial assessment within 10 working days.
What to do next
Ask the installer to show the proposed location, DNO route, backup arrangement and commissioning documents before work starts.
Sources
- MCS — mcscertified.com/wp-content/uploads/2025/02/MIS-3012-2025-V1.0.pdf
- Ofgem — www.ofgem.gov.uk/sites/default/files/docs/2018/06/final_storage_guidance_0.pdf
- dcode.org.uk — dcode.org.uk/assets/250307ena-erec-g99-issue-2-(2025
Last checked: 24 September 2026