Complete Guide for Commercial Solar Installations

When you plan a commercial solar PV system in the UK, one of the most important milestones is securing approval from your Distribution Network Operator (DNO) under Engineering Recommendation G99. This guide explains the essentials, typical timelines, and practical steps to keep your project on track.

What is G99?

G99 sets the rules for connecting generation (solar PV, CHP, etc.) above 16A per phase to the public electricity network. It ensures your system can operate safely and won’t destabilise the grid.

When do you need a DNO application?

Nearly all commercial-scale PV systems require DNO approval before installation/energisation. Your local DNO will be one of: UK Power Networks, National Grid Electricity Distribution (formerly WPD), SP Energy Networks, Scottish & Southern Electricity Networks (SSEN), or Northern Powergrid.

The G99 application pack typically includes

  • Single-line diagram (SLD) with inverter and protection details
  • Site plan and MPAN/supply details (LV or HV)
  • Inverter and protection type-test certificates (ENA type-tested)
  • Proposed export capacity (kW/kVA) and any export-limiting scheme
  • Protection settings (over/under voltage, over/under frequency, ROCOF/Vector Shift if required)
  • Owner’s contact details

Step-by-step process

  1. Feasibility & pre-design
    Assess available roof/land, supply characteristics, and the business case. Decide whether export limitation (to match a DNO limit) will be required.
  2. G99 application submission
    Submit the full pack to the DNO. Incomplete applications are the most common cause of delay.
  3. Connection Offer (budget/firm)
    The DNO issues an offer outlining costs, technical requirements and any reinforcement.
  4. Acceptance & detailed design
    Accept the offer and finalise design to DNO requirements. Coordinate with your Meter Operator (MOP) and any Independent Connection Provider (ICP) if applicable.
  5. Installation, settings & testing
    Install the system, configure settings to the approved values, and prepare for DNO witness testing if required.
  6. Commissioning & handover
    Complete G99 commissioning forms, as-built drawings, and O&M documentation. Keep records for compliance and insurance.

Typical timelines (indicative)

  • LV connections (simple): 6–12 weeks to Connection Offer; overall programme depends on site works.
  • Larger LV/HV or constrained networks: allow additional time for studies, approvals and reinforcement works.

Common pitfalls (and how to avoid them)

  • Incomplete data: Submit full, accurate SLDs and type-test documents.
  • Late export-limit decisions: Agree limits early and specify a G100-compliant (export-limiting) scheme where needed.
  • Scheduling clashes: Book commissioning and any DNO witness testing well in advance.
  • Metering coordination: Align with your MOP for CTs/VTs, meter changes and energisation dates.

Benefits of getting G99 right

  • Compliance & safety — protects your business and the public network.
  • Predictable costs & timelines — fewer surprises during construction.
  • Optimised export — maximise on-site generation within DNO limits.
  • Smoother commissioning — faster sign-off and earlier savings.

How we can help

We manage the entire G99 and DNO process for commercial clients—application, export-limiting design, DNO liaison, and witness testing—so your project proceeds smoothly and safely.

Thinking about a project or stuck in the process? Get in touch for a quick feasibility review.