Retrofitting storage to an existing commercial solar array is often one of the better-returning battery projects, because the generation asset is already in place and under-used. Many solar-only commercial sites export a large share of their midday output at a low rate and then re-import in the evening at the full retail price. A battery sized to that surplus captures the difference.
The design 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. The actual export profile should be measured before choosing, because sizing to the headline PV kW rather than the real surplus is the classic retrofit mistake: it either strands capacity or under-delivers.
How Solar-Plus-Storage Retrofit is delivered
Delivery normally 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 the agreed export capacity); commission and prove the control strategy against the measured surplus.
What this installation includes
- Adds storage to an existing commercial rooftop or ground-mount PV array so more of the generation is used on site; the gain depends on the measured export surplus
- 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
- Published cost range
- About £20,000–£40,000 at 100 kWh; £80,000–£250,000+ at 200–500 kWh
Budget and capital allowances
A retrofit battery is priced by capacity: published UK ranges put a 100 kWh system at about £20,000–£40,000 installed and a 200–500 kWh system at about £80,000–£250,000 or more. Whether the existing solar inverter, metering and export limit can be reused is what most changes a retrofit quote. On tax, the Annual Investment Allowance covers up to £1m of qualifying spend a year in full; above that, take advice on whether the battery is main-pool plant or special-rate expenditure. These are published budget ranges, not quotes: the cost guide gives the sources. See also capital allowances and grants and funding.
Siting, fire and compliance
The added battery needs the same fire discipline as any installation: the IET Code of Practice, BS EN/IEC 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; check whether MCS certification applies at your system size. BS EN/IEC 62619/62933; fire assessment for the added battery.
For the documents behind these requirements, see commercial battery fire safety: the UK standards.
Getting it connected and signed off
Delivering a solar-plus-storage retrofit means someone 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. Check that the installer's quote says who that is. A proper installation ends with documented battery storage commissioning and an O&M handover pack. Read an independent view on whether it is worth it.
Solar-Plus-Storage Retrofit: at a glance
| Attribute | Typical for this installation |
|---|---|
| Power / capacity | 50 kW / 100 kWh-500 kW / 1 MWh |
| Siting | n/a |
| Published cost range | About £20,000–£40,000 at 100 kWh; £80,000–£250,000+ at 200–500 kWh (sources) |
| Connection | G99 (G100 limitation where constrained) |
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Common questions
How much does a commercial battery installation cost in the UK?
Published UK ranges for 2026 put a 100 kWh commercial battery at about £20,000–£40,000 installed and a 200–500 kWh system at about £80,000–£250,000 or more (roughly £300–£500 per kWh). Systems up to 50 kWh cost more per kWh (about £400–£800), and containerised systems of 500 kWh and above fall to around £140–£240 per kWh, so a 1 MW / 2 MWh container works out at roughly £280,000–£480,000 before major grid works. These are budget ranges, not quotes. Cost depends on the power-to-energy ratio, switchgear and protection works, siting and civils, fire safety and any grid-connection contribution. On tax, the Annual Investment Allowance covers up to £1m of qualifying spend a year; above that the treatment depends on how the battery is classified, so confirm it with your adviser.
How is a commercial battery sized before installation?
By survey, not by rule of thumb. Power (kW) is sized to the peak to shave or the load to support; energy (kWh) to how long that peak lasts. Many behind-the-meter systems are specified at one to two-and-a-half hours (for example 250 kW / 500 kWh). A competent installer analyses at least 12 months of half-hourly meter data, confirms the incoming supply capacity and available fault level, and settles 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. At the end you should hold the test results, the settings schedule, the electrical and fire certification and a planned O&M regime, which are the records your insurer and auditor will expect.
Can a battery be retrofitted to existing commercial solar?
Yes. The battery should be sized to the measured daytime export surplus, then AC- or DC-coupled to the existing inverter and metering. A G99 variation (or new application) is usually needed for the added storage, with G100 limitation to stay within the existing agreed export capacity. How much more of the solar output is used on site depends on the surplus and the load profile, so ask for the estimate to be worked from your own half-hourly data.
Do we need planning permission to install a commercial battery?
It depends on the size and siting of the enclosure and on where the site is: England, Wales and Scotland have different planning rules, and listed-building or conservation-area constraints can apply. Larger containerised and standalone systems increasingly need full planning permission and fire-and-rescue consultation under NFCC guidance, with attention to separation, access and noise. Confirm the route with the local planning authority before any plant is ordered.