Retrofitting storage to an existing commercial solar array is one of the highest-return installs we do, because the generation asset is already in place and under-used. Solar-only commercial sites typically self-consume just 40-60% of what they generate and spill the rest to the grid at a low export rate, then re-import in the evening at full retail. A battery sized to that surplus captures the spread.
The install decision is AC- versus DC-coupling. DC-coupling can be more efficient where the existing inverter and DC architecture suit it; AC-coupling is more flexible and less disruptive to a working array. We measure the actual export profile before choosing, because sizing to the headline PV kW rather than the real surplus is the classic retrofit mistake, and it either strands capacity or under-delivers.
How we install solar-plus-storage retrofit
Delivery runs: measure the existing generation and export profile; choose AC- or DC-coupling; integrate the battery, its inverter and metering with the existing PV system; apply for the G99 variation (and G100 export limitation to stay within agreed export capacity); commission and prove the control strategy against the measured surplus.
What this install includes
- Adds storage to an existing commercial rooftop or ground-mount PV array to lift self-consumption from 40-60% to 80%+
- AC- or DC-coupled retrofit depending on the existing inverter and metering arrangement
- Battery sized to the measured daytime export surplus, not the headline PV kW
- Often requires a G99 variation to the existing generation connection
Typical solar-plus-storage retrofit
- Power / capacity
- 50 kW / 100 kWh-500 kW / 1 MWh
- Project value
- £60,000-£650,000
- Payback
- 6.5 years
Budget and capital allowances
Budget £60,000-£650,000 for this install, with a 6.5-year simple payback. On tax, a solar-plus-storage retrofit qualifies for 100% AIA on the first £1m and a 50% first-year allowance on the balance (special-rate — not full expensing); we model capital, finance and lease routes side by side. See the cost guide, capital allowances and grants and funding.
Siting, fire and compliance
The added battery brings the same fire discipline as any install — PAS 63100:2024 principles, BS EN 62619 cells, and an updated fire risk assessment covering the new plant and its location relative to the existing PV and inverters.
G99 variation or new application for the added storage; G100 export limitation to stay within the existing agreed export capacity. Integration with the existing inverter/metering; MCS or equivalent install standard for the storage element. BS EN 62619/62933; fire assessment for the added battery.
Getting it connected and signed off
Delivering a solar-plus-storage retrofit means owning the G99 process end to end — application, protection settings, DNO liaison and G100 limitation on constrained networks — with the metering engineered for accurate control and settlement. Every install ends with a documented, witnessed commissioning and an O&M handover pack. Read our honest view on whether it is worth it.
Solar-Plus-Storage Retrofit: at a glance
| Attribute | Typical for this install |
|---|---|
| Power / capacity | 50 kW / 100 kWh-500 kW / 1 MWh |
| Siting | n/a |
| Project value | £60,000-£650,000 |
| Simple payback | 6.5 years |
| Connection | G99 (G100 limitation where constrained) |
Get a free solar-plus-storage retrofit feasibility
Responds within one working day
- 1. Free desk feasibility from your meter data and roof, no obligation.
- 2. Site survey and a fixed-price proposal, itemised in writing.
- 3. Install and aftercare by MCS-certified engineers.
- MCS Certified
- NICEIC
- RECC
- TrustMark
Common questions
How much does a commercial battery installation cost in the UK?
As a 2026 rule of thumb, fully installed behind-the-meter commercial BESS lands at roughly £400-£700 per kWh of usable capacity, falling toward £250-£400/kWh at multi-MWh scale. A 250 kW / 500 kWh system is around £150,000-£300,000; a 1 MW / 2 MWh system £600,000-£1.2m. Cost depends on power-to-energy ratio, switchgear and protection works, siting and civils, and any grid-connection contribution. Qualifying plant attracts 100% AIA on the first £1m and a 50% first-year allowance on the balance.
How is a commercial battery sized before installation?
By survey, not by rule of thumb. Power (kW) is sized to the peak you need to shave or the load you need to support; energy (kWh) to how long that peak lasts, most behind-the-meter systems land at 1.5-2.5 hours (e.g. 250 kW / 500 kWh). We pull at least 12 months of half-hourly meter data, confirm your incoming supply capacity and available fault level, and settle the connection route before specifying plant, because the DNO position can change the design.
What does the commissioning process involve?
Commissioning is a documented, witnessed handover: protection and control settings verified, G99/G100 functions demonstrated to the DNO where required, metering and monitoring proven, and cell/thermal management confirmed. You receive the test results, settings schedule, electrical and fire certification, and a planned O&M regime. The system is demonstrably compliant and you hold the records your insurer and auditor will expect.
Can you retrofit a battery to our existing commercial solar?
Yes, and it is one of the most common installs we do. We measure your existing daytime export surplus and size the battery to it, then AC- or DC-couple the storage to your existing inverter and metering. A G99 variation (or new application) is usually needed for the added storage, with G100 limitation to stay within your existing agreed export capacity. Adding storage typically lifts self-consumption from 40-60% toward 80%+.
Do we need planning permission to install a commercial battery?
Behind-the-meter enclosures on an existing commercial site are often permitted development or a minor application, subject to size, siting, and listed-building or conservation-area constraints. Containerised and larger standalone systems increasingly need full planning permission and fire-and-rescue-service consultation under NFCC guidance, with attention to separation distances, firefighting access, and noise. We confirm the planning route in the feasibility study before any plant is ordered.