750 kW Solar PV on a Tier III Colocation Facility in Slough, Berkshire

Project background

The Slough–Berkshire corridor hosts the highest density of carrier-neutral colocation facilities in the UK outside of London. Four major operators maintain large campus sites within a three-mile radius of Slough town centre, served by the same SSEN (South East England Networks) primary substation. The site on this project — a single-tenant-managed, multi-tenanted colocation facility operating at Tier III reliability standards — sits within that cluster, drawing power from the same constrained grid infrastructure that makes solar feasibility in this location both commercially attractive and technically demanding.

The operator had evaluated solar PV twice before engaging us — once in 2021 and once in 2023 — and pulled back on both occasions due to concerns about grid export constraints and installation risk during live operations. Our engagement began with a feasibility study that addressed both blockers directly: a zero-export configuration to eliminate grid constraint risk, and a commissioning plan structured around the facility’s existing planned maintenance windows.

The facility

The building is a three-storey, purpose-built colocation facility of approximately 12,000 sqm gross floor area, constructed in 2009 and retrofitted to Tier III standard in 2014. The roof is a single-pitch built-up membrane roof (BUR) with a north-south orientation and a 4° pitch, accessed via internal stairwells. At the time of our survey, the roof carried mechanical plant — CRAC unit condenser coils, fire suppression pipework, and telecommunications infrastructure — across approximately 30% of its surface. Available solar PV area was 5,800 sqm after structural assessment and clear zone requirements around plant.

The facility operates at 4.2 MW average IT load on half-hourly metering. Peak demand has reached 5.8 MW in summer cooling periods. The total electricity cost in the year before installation was £4.3 million at a blended grid rate of approximately 23p/kWh.

The technical challenge: zero-export on a constrained grid

The SSEN primary substation serving this site has been operating close to its rated export capacity since 2022, driven by the substantial expansion of the local DC cluster. Under normal export-connected solar installations, surplus generation would flow back to the grid via the DNO connection. In a constrained zone, this creates connection rejection risk and can result in the DNO declining a G99 application or imposing significant export limitation relay requirements.

Our solution was a zero-export configuration using Huawei SmartLogger 3000 in combination with a set of Acrel DTSD1352 metering units at the grid import point. The system monitors net import every 100 milliseconds and uses the inverter’s reactive power curtailment and active power limitation features to ensure that generation never exceeds the real-time local load. In practice, with a 4.2 MW constant IT load, the 750 kW solar array never approaches grid export under any operational condition — the self-consumption ratio is 100% by design, regardless of instantaneous solar yield.

This approach eliminated the export constraint issue entirely. The G99 Protection Relay application to SSEN was submitted and approved in 41 working days — within the DNO’s standard 65-day target.

System design

The 750 kW array consists of 1,488 JA Solar JAM72S30-505/MR half-cut PERC modules at 505 Wp each, mounted on a ballasted Schletter FixZ low-profile aluminium racking system. The low profile (maximum 15° pitch on a 4° base) minimises wind loading on the membrane roof and avoids penetration of the roofing system at any point.

Ten Huawei SUN2000-100KTL-M2 string inverters (100 kW each) are located in weatherproof enclosures adjacent to the roof access points to minimise DC cable runs. AC output is combined in a dedicated solar generator sub-distribution board (SGSDB) and connected to the main low-voltage (LV) distribution system in the main plant room on the first floor.

The Protection Relay — a SEL-311C set to the SSEN relay settings schedule — was installed in a dedicated SSDB compartment with metering and BMS integration points.

System summary:

• Array capacity: 750 kW (1,488 × 505 Wp JA Solar PERC half-cut)
• Inverters: 10 × Huawei SUN2000-100KTL-M2
• Mounting: Schletter FixZ ballasted, 12° East-West portrait orientation
• Export control: Zero-export via Huawei SmartLogger 3000 + Acrel metering
• Grid connection: G99 Protection Relay (SSEN approved 41 working days)
• Monitoring: Huawei FusionSolar cloud platform with half-hourly resolution

Installation methodology

The data centre’s planned maintenance windows run twice per year — one in late spring, one in early autumn. Each window provides 72 hours of reduced-risk access for works near live electrical infrastructure. Our installation plan was structured to complete the entire rooftop and electrical installation within two consecutive maintenance windows six months apart.

Phase 1 (Window 1 — Spring): Structural survey sign-off, roofing contractor preparation (new sacrificial membrane where ballast blocks were to be placed), all cable tray and containment installation, DC cable pulling from strings to inverter enclosures, inverter enclosure installation, and SGSDB installation and energisation. Phase 1 left the inverters, racking, and switchgear installed but not connected to the roof array.

Phase 2 (Window 2 — Autumn): Module installation (1,488 units in 3.5 days with a crew of twelve), DC string connection, inverter AC connection at the SGSDB, G99 Protection Relay commissioning (conducted with the DNO G99 engineer in attendance), and full system energisation and performance verification.

Total on-site installation time: six days of physical work across two windows. The six-month gap between windows was deliberately used to allow the G99 Protection Relay application to complete and the relay to arrive from the manufacturer (16-week lead time at time of order).

All personnel held current BPSS clearance, CSCS Gold as a minimum, and were listed on the facility’s approved contractor register. Two members of our project management team hold SC clearance for future reference on higher-clearance projects within the same campus.

Results

The system was commissioned in the second window (Autumn 2025) and has been generating through its first winter. Based on monitored data through the first operating season:

• Annual generation (modelled P50): 675,000 kWh
• Self-consumption ratio: 100% (no export events recorded since commissioning)
• Annual electricity cost saving (year 1): £156,000 at 23p/kWh blended rate
• CO₂ avoided: 94.5 tonnes CO₂e in year 1 (using UK grid average emission factor)
• Capital cost: £918,000 (ex-VAT)
• Full Expensing tax relief (25% CT): £229,500 in year of expenditure
• Net capital cost after tax: £688,500
• Simple payback (pre-tax): 5.9 years
• Post-tax payback: 4.4 years
• Project IRR (25-year DCF): 17%

Scope 2 and sustainability outcome

The operator uses the market-based method for Scope 2 accounting under the GHG Protocol. 675,000 kWh of annual generation, backed by MCS project registration and REGO issuance through Ofgem, enables 100% of the solar fraction to be recorded as market-based Scope 2 zero. This reduces the operator’s total Scope 2 emissions by approximately 4.2% annually — meaningful progress toward a stated 2030 net zero operations target.

The facility’s largest tenants — two cloud service providers and a financial services firm — receive copies of the REGO documentation as part of their annual sustainability reporting packs, enabling them to claim a proportional Scope 2 reduction in their own Scope 3 supply chain disclosures.

Reference availability

The operator (referred to by project reference DC-SL-001 in our portfolio documentation) has agreed to take reference calls from qualified prospective customers with comparable projects — carrier-neutral colocation, similar system size, SSEN grid connection. Contact us with your project context and we will arrange a direct introduction under NDA.