An indoor cabinet BESS is the right install where capacity is modest and there is no external hardstanding — offices, retail units, small manufacturers and multi-tenant buildings. The cabinets sit in an existing plant room or switchroom and integrate tightly with the local distribution boards and metering, which keeps cable runs short and the electrical design clean.
The trade-off is that the battery is now inside an occupied building, so the fire strategy is the governing constraint. The room has to be compartmented to the building fire strategy, ventilated for both cooling and any off-gassing, and covered by detection tied into the building alarm. We design that room, not just drop cabinets into it, and we get landlord, tenant and insurer consent before we start in a multi-let building.
How we install indoor cabinet bess
Delivery runs: confirm the switchroom or battery-room compartmentation, ventilation and detection; upgrade fire separation where needed; install and anchor the cabinets; terminate into the local distribution and metering; commission protection and any islanding transfer; then witnessed testing. Access and manual-handling routes are checked at survey because indoor plant rooms are often tight.
What this install includes
- Wall-mounted or floor-standing cabinets installed in a plant room, switchroom, or dedicated battery room for smaller commercial sites
- Suits offices, retail units, small manufacturers, and multi-tenant buildings where an external compound is not practical
- Fire detection, thermal monitoring, and room separation designed for an occupied building
- Tighter integration with existing distribution boards and metering
Typical indoor cabinet bess installation
- Power / capacity
- 30 kW / 60 kWh-250 kW / 500 kWh
- Siting
- plant room / switchroom
- Project value
- £45,000-£300,000
- Payback
- 7.5 years
What a indoor cabinet bess installation costs
Expect £45,000-£300,000 for a indoor cabinet bess installation, at a 7.5-year payback once demand-charge and self-consumption value is modelled from your data. Qualifying plant gets 100% Annual Investment Allowance to £1m and a 50% first-year allowance thereafter; storage is special-rate, not eligible for full expensing, and general commercial premises do not get 0% VAT. See the cost guide, capital allowances and grants and funding.
Fire strategy and standards
Indoor installs demand the most careful fire design: compartmentation to the building fire strategy, adequate ventilation, dedicated detection linked to the building alarm, and separation from occupied and escape areas, all to PAS 63100:2024 principles and BS EN 62619 / BS EN 62933. We produce the fire risk assessment and coordinate it with the building’s existing fire strategy.
G99 for systems above 16 A per phase (~3.68 kW single-phase); most commercial cabinets. Battery-room fire compartmentation, detection, and ventilation to PAS 63100 principles and the building fire strategy. BS 7671 electrical installation; BS EN 62619/62933 cell and system safety. Landlord/tenant and insurer consent in multi-tenant buildings.
DNO connection and witnessed commissioning
The connection sets the programme on any indoor cabinet bess installation. We lodge the G99 application early, add a G100 export/import limitation where capacity is tight, and prove the metering and CT design so peak-shaving works from day one. Every install ends with a documented, witnessed commissioning and an O&M handover pack. Read our honest view on whether it is worth it.
Indoor Cabinet BESS Installation: at a glance
| Attribute | Typical for this install |
|---|---|
| Power / capacity | 30 kW / 60 kWh-250 kW / 500 kWh |
| Siting | plant room / switchroom |
| Project value | £45,000-£300,000 |
| Simple payback | 7.5 years |
| Connection | G99 (G100 limitation where constrained) |
Get a free indoor cabinet bess installation 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
What fire safety standards apply to a commercial battery installation?
We design to PAS 63100:2024 principles for installation and fire protection, BS EN 62619 for cell safety, and BS EN/IEC 62933 for system safety, with NFCC guidance for larger and grid-scale sites. We specify lithium-iron-phosphate (LFP) cells for their thermal stability, and engineer separation distances, detection, thermal monitoring, and appropriate compartmentation or bunding. A fire risk assessment is produced and the insurer, and where necessary the fire authority, engaged before installation.
Do you handle the DNO application and grid connection, or do we?
We handle it as part of our scope: the G99 application, single-line diagrams, protection settings, half-hourly metering arrangement, DNO liaison, and witness-test coordination, plus the G100 limitation design where the network is constrained. You get one accountable contractor across survey, design, connection, install, and commissioning rather than having to coordinate a battery supplier, an electrician, and the DNO yourself.
Can you install a battery without shutting down our operations?
In most cases yes. The tie-in to your existing switchgear is planned around your operations using temporary supplies, out-of-hours working, or a short pre-agreed outage only where unavoidable. We establish your supply arrangement, available fault level, and switchroom access at survey so the connection method is designed, method-statemented, and risk-assessed before we mobilise. Minimising disruption is a design-stage decision, not an on-the-day improvisation.
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.