Installing solar PV on Tier III and Tier IV data centres without breaking your uptime guarantee
The only reason most data centre operators haven't added rooftop solar is installer competence, not solar economics. Here's exactly how we do it without touching your SLA.
Why most solar installers cannot work on Tier III/IV facilities
The Uptime Institute's Tier III and Tier IV classifications define fault-tolerant, concurrently maintainable infrastructure. Tier III requires N+1 redundancy on all electrical paths — every critical system has at least one backup. Tier IV requires 2N redundancy with full fault tolerance — any single failure anywhere in the electrical infrastructure must not interrupt operations.
Installing solar PV on a Tier III or Tier IV facility requires modifying the AC electrical infrastructure — specifically, connecting the inverter output to the facility's switchgear. If this work is performed incorrectly, or if commissioning is managed without understanding the downstream electrical topology, it can create a momentary single-point-of-failure that the Tier classification was designed to prevent. Standard commercial solar installers — who work on warehouses, factories, and offices — do not encounter this constraint in their normal work. Their commissioning methodology does not account for it.
The consequence is a real market failure: excellent solar economics on data centres (flat 24/7 load, high self-consumption, 5–7 year payback) are not being captured because most installers cannot deliver the work safely. We built our practice specifically to solve this.
Our six-stage commissioning methodology for live data centre environments
Stage 1: Electrical topology survey and protection relay coordination
Before any installation work begins, our engineers produce a full single-line diagram of the facility's existing electrical infrastructure — from the utility intake (11 kV or 33 kV depending on facility scale), through main HV switchgear, transformers, LV distribution boards, UPS systems, and PDUs to the critical load. This topology is essential for determining where the solar inverter connects to the AC system without creating a single-point-of-failure, and for specifying the G99 Protection Relay settings required by the DNO at the point of connection.
We submit the G99 application to the DNO at this stage — not after installation, which is the mistake that causes project delays of 8–16 weeks. G99 pre-notification takes 28 days minimum; we start the clock during survey, so it clears before we arrive on site to commission.
Stage 2: Roof structure assessment under live operations
Roof access on a live data centre is governed by change management procedures, permit-to-work systems, and in many cases security clearance requirements. We work within your existing change management framework — we do not treat the roof as freely accessible construction space. Our structural assessment is performed by a certified structural engineer (not the installation team) and includes a load distribution model that accounts for existing plant (CRAC units, chillers, cooling towers, cable routes) before any additional load from the PV array is placed.
For facilities with weight-restricted roof structures — common on older colocation conversions and industrial buildings repurposed as data centres — we specify lightweight frameless racking systems achieving 8–12 kg/m² system weight, versus the 15–20 kg/m² of standard racking. This extends the addressable roof area on constrained structures.
Stage 3: DC cabling and array installation (zero AC involvement)
The array installation phase — mounting structures, panels, DC string wiring — involves no connection to the facility's AC electrical system. There is zero risk to operational systems during this phase. We perform all DC work during standard working hours and within normal change management windows. DC circuits are tested for continuity and insulation resistance (IR testing) and open-circuit voltage verified before any inverter connection is attempted.
DC cable routes through the building are agreed in advance with your facilities and security teams. All penetrations through fire-rated compartments are sealed with approved intumescent materials and documented in the as-built record. Cable routes avoid critical areas — UPS battery rooms, fuel storage, generator plant — and are segregated from mains AC supply cables per IEE Wiring Regulations (BS 7671) Section 712.
Stage 4: Inverter installation and pre-commissioning
Inverters are installed, configured, and pre-commissioned on the DC side before any AC connection. Internal protection settings — anti-islanding, under/over voltage, under/over frequency — are configured to DNO requirements from the G99 approval document. This is verified by our protection engineer before the AC connection change management window opens.
For Tier IV facilities with 2N UPS infrastructure, we specify string inverters (not central inverters) to maintain string-level fault tolerance. A single inverter failure on a string inverter system reduces generation by the affected string's capacity — typically 5–10% of total — without affecting the critical load. A central inverter failure on the same system takes down 100% of solar generation simultaneously.
Stage 5: Phased AC connection during approved maintenance windows
The AC connection is the only stage that touches live electrical infrastructure. We schedule this within your existing planned maintenance windows — typically 2–4 hours at night or over a weekend. The connection methodology varies by facility type:
- Tier III (N+1): We connect to the non-critical supply path during a planned maintenance window when the critical load is confirmed as running on the redundant path. The connection is made, protection relay tested, and the inverter brought to output. The critical load is then transferred back across both paths.
- Tier IV (2N): Connection is made to an independently fused distribution board on one power path, with the critical load maintained on the second power path throughout. No transfer of critical load is required. The inverter is commissioned and verified while the critical load is unaffected.
We do not connect solar PV directly to UPS input. The inverter connects to the pre-UPS distribution — typically the LV busbar — on a dedicated MCB rated for the inverter output current, protected by an RCBO where required by the system design. UPS input remains on mains grid; solar reduces the grid draw upstream of the UPS.
Stage 6: DNO commissioning, G99 testing, and handover
DNO commissioning (G99 protection test) is arranged with the relevant Distribution Network Operator and performed by a qualified protection engineer. The test confirms anti-islanding protection and voltage/frequency protection settings within the DNO's approved tolerance bands. We provide the G99 commissioning test report, DNO acceptance letter, and MCS commissioning certificate as part of the handover pack.
Handover documentation includes: as-built single-line diagram, panel layout drawing, string configuration record, inverter configuration settings backup, Protection Relay settings record, structural engineer sign-off, fire compartment penetration schedule, O&M manual, warranty documentation, and monitoring platform credentials.
Fire strategy: BS 8519 and data centre-specific requirements
Data centres present specific fire risk considerations for solar PV installations. BS 8519 (Selection and installation of fire detection and fire alarm systems for buildings) applies to data centres and requires that solar PV system design does not compromise the existing fire detection, suppression, and evacuation infrastructure. We produce a fire strategy note as part of the design package, reviewed by a competent fire engineer, covering:
- DC cable isolation methodology (rapid shutdown requirement) — ensuring that in a fire event, DC voltage on roof cables can be reduced to a safe level within 30 seconds to protect fire crews. We specify rapid shutdown-compliant inverters or module-level electronics where the roof access plan requires it.
- Panel arrangement and maintenance access paths — fire brigade access routes across the roof must be maintained at minimum 1.0m width, with emergency equipment access routes agreed with the facility's fire safety team.
- Suppression system interference — roof-mounted panels must not obstruct the operation of roof-mounted suppression heads (gaseous, sprinkler, or foam) or interfere with the thermal detection triggers of any roof-level fire detection system.
- Electrical isolation points — clearly labelled DC isolation switches at accessible locations for fire brigade use, in addition to the DNO grid isolation.
BPSS clearance and security protocol
All personnel accessing your data centre site hold current Baseline Personnel Security Standard (BPSS) clearance. We do not apply for clearance as a reactive response to your security team's request — we hold it as a company standard, and every engineer and project manager on our team is pre-cleared before any project mobilisation. Clearance certificates and DBS check references are provided to your security team at the pre-mobilisation stage, not the morning of first site access.
We follow your site security procedures precisely — tailgating policy, visitor logging, laptop and device policy, photography restrictions, and NDA requirements. Our project managers are experienced in the access control systems used by major colocation operators (including Equinix, VIRTUS, CyrusOne, Ark Continuity, and NTT) and by carrier-neutral exchange operators. We brief all installation personnel on site-specific security protocols before first access, and we operate a zero-tolerance policy for protocol violations.
Our Tier III/IV installation track record
Across 350+ commercial solar installations, we have delivered 24+ MW of commissioned capacity. Our data centre portfolio spans hyperscale campus systems above 1 MW, medium colocation facilities in the 200–750 kW range, and enterprise data suites from 50 kW upward. In every case, the installation was completed without a single unplanned outage to the critical load.
We carry professional indemnity insurance of £5 million, public liability of £10 million, and employer's liability of £10 million. Our installations are backed by a 10-year Insurance-Backed Warranty on workmanship. We hold MCS Commercial Certification, NICEIC Approved Contractor status, and ISO 9001/14001/45001 certification — all independently verifiable against the issuing bodies.
If you would like to speak directly with a technical director from a comparable data centre installation we have delivered, we can arrange a reference call under NDA. We believe the most reliable evidence of our capability is the operational experience of operators who have been through the commissioning process with us.