G98 vs G99: Complete UK Solar Grid Connection Guide 2026

G98 and G99 are the two Energy Networks Association Engineering Recommendations that govern how solar PV systems connect to the UK low-voltage distribution network. Getting the wrong one — or missing a notification — can result in the DNO requiring disconnection or imposing conditions retroactively.

This guide explains exactly when each standard applies, what the process involves, and what the technical requirements are for each.

The Core Distinction: 16 Amps Per Phase

Everything in G98 vs G99 comes down to a single threshold: 16 amps per phase of export current.

The 16A threshold is derived from the maximum current a standard UK domestic supply fuse will carry. Below this level, the generating plant is considered small enough to connect without prior DNO review.

Calculating Export Current

The calculation is straightforward:

Export current (A) = Inverter AC output power (W) ÷ Phase voltage (V)

For single-phase at 230V:

For three-phase at 230V per phase:

The Combined Capacity Rule

The 16A threshold applies to the total export current at the connection point — not just the new installation. This is the combined capacity rule.

If a property already has a 3 kW solar system (13A single-phase) and the owner wants to add a 2 kW system (8.7A), the combined total would be 21.7A — above the 16A threshold. The new 2 kW system therefore requires G99 pre-approval, even though it would qualify for G98 in isolation.

The combined capacity rule applies to:

• Multiple solar PV arrays on the same connection
• Solar PV plus wind generation on the same connection
• Battery inverters configured to export to the grid
• Any other generating plant at the same metering point

When calculating combined capacity, use the maximum simultaneous export from all generators, not the sum of their rated capacities (if they physically cannot all export at maximum simultaneously due to interlocking controls, this can be demonstrated to the DNO).

G98: Fit-and-Inform Process

G98 uses a fit-and-inform approach. The installer notifies the DNO that the system has been or is about to be installed — the DNO does not need to approve the design in advance.

Pre-Commissioning Notification

The installer submits a pre-commissioning notification to the DNO. Timing varies by DNO: some require notification before commissioning, others accept it on the day. The notification includes:

• Installation address
• Installer name and MCS certificate number (if applicable)
• Inverter make, model, and rated AC output
• Total system capacity
• Confirmation that the inverter holds G98 type-tested certification
• Expected commissioning date

The 28-Day Rule

Once the pre-commissioning notification is received, the DNO has 28 days to raise a formal objection. An objection can only be raised on technical grounds — if the connection would cause a power quality or safety issue that cannot be resolved through standard mitigation. In practice, objections to standard residential G98 systems are rare.

If no objection is received within 28 days, the installation is deemed accepted and the system can operate on an ongoing basis.

Post-Commissioning Notification

After commissioning, the installer must submit a post-commissioning notification confirming:

• The system has been installed as notified
• The inverter protection relay settings match the G98 type-test certificate
• An Electrical Installation Certificate has been issued
• The system commissioning date

Some DNOs require the G98 commissioning test record to be submitted with the post-commissioning notification. This includes the results of the protection relay verification — confirming the inverter disconnects at the correct voltage and frequency trip points.

G99: Pre-Approval Process

G99 requires formal DNO approval before installation begins. The process is structured in two stages.

Stage 1: Design Submission

The installer submits a complete design application to the DNO. The DNO then has 45 working days from receipt of a complete application to issue a connection offer, request more information, or reject the application.

Stage 1 documents typically required:

The single-line diagram must show: the PV array, DC isolator(s), inverter(s), AC isolator, protection relay, metering point, and connection to the DNO network. It must use standard IEC or BS EN electrical symbols.

Stage 2: Commissioning

After receiving DNO acceptance of the Stage 1 application, installation can proceed. On commissioning:

1. The inverter is programmed with the protection settings agreed in the Stage 1 approval
2. A protection relay test is carried out (see the G99 Commissioning Test guide)
3. The commissioning test results are documented
4. A post-commissioning notification is submitted to the DNO, including the test results and Electrical Installation Certificate

The DNO may specify a witness test for larger systems. For most residential and small commercial G99 systems, a witnessed test is not required — the installer certifies the results.

Protection Settings

All inverters used in G98 and G99 installations must comply with the protection relay requirements of the relevant standard.

G98 Protection Settings

G98 inverters are type-tested against fixed protection settings:

G98 inverters carry a type-test certificate confirming they meet these settings. The installer does not program relay settings — they are fixed at the factory.

G99 Protection Settings

G99 gives the DNO more control over protection settings. The Protection Settings Schedule submitted with the G99 application documents the settings that will be programmed into the inverter. Standard G99 default settings include:

RoCoF (Rate of Change of Frequency) protection at 1 Hz/s is the key G99 requirement that does not apply to G98 systems. RoCoF detects abnormal changes in grid frequency that indicate islanding — a situation where the inverter continues to energise a section of the network after the grid has disconnected. The 1 Hz/s setting is the standard default, but the DNO can specify a different threshold based on local network conditions.

The DNO may also specify modified voltage trip settings for a particular connection point — for example, a higher over-voltage trip threshold on a network with known high voltage conditions, or a tighter RoCoF setting on a network with limited generation headroom.

G100: DC-Coupled Battery Storage

G100 is a separate ENA Engineering Recommendation that applies to DC-coupled battery storage systems — where a battery is connected on the DC side of a solar inverter. G100 governs how such systems export to the grid and sets requirements for export control to prevent the battery from pushing more current onto the network than the inverter’s rated export capacity.

If your project includes DC-coupled batteries, you need to:

1. Confirm whether G100 applies alongside G98 or G99 (depends on total export capacity)
2. Declare the battery in the notification or application
3. Confirm the export control method that prevents combined PV + battery export from exceeding the inverter’s rated capacity

AC-coupled batteries (with their own separate inverter) are assessed independently — the battery inverter is treated as a separate generating unit for the purposes of G98/G99/G100 compliance. See the Battery Storage Compliance guide for full details.

Per-DNO Differences

All six UK DNOs use the same G98 and G99 standards, but their submission processes differ.

UKPN and NGED have the most developed online portals and typically process standard G98 notifications and G99 applications fastest. Northern Powergrid still accepts — and sometimes requires — email submissions for certain application types.

Each DNO guide covers the specific portal URL, required form fields, and current processing timelines:

• UK Power Networks
• NGED
• Northern Powergrid
• Electricity North West
• SP Energy Networks
• SSE Networks

How Solar Design Software Helps with G98 and G99

Solar software built for UK compliance can automate most of the documentation burden for G98 and G99.

A compliant solar design software workflow produces:

• The export current calculation (confirming G98 or G99 applies)
• A single-line diagram in the format DNOs require for G99 Stage 1
• The Protection Settings Schedule with all relay parameters populated
• The system summary for the G98 or G99 notification form
• Inverter data sheet packaged with the correct ENA certification reference

For G99 applications in particular, having these documents in a consistent format reduces the risk of the DNO requesting additional information and restarting the 45-day clock.

See the Solar Design Software UK comparison for a detailed review of available tools.

For the step-by-step G99 application process, see the G99 Application Guide.

Frequently Asked Questions

What is the G98 vs G99 threshold?

The threshold is 16 amps of export current per phase. Below 16A per phase, G98 (fit-and-inform) applies. Above 16A per phase, G99 (pre-approval) applies. For single-phase installations, this is approximately 3.68 kW of inverter AC output. For three-phase, it is approximately 11 kW.

How does the combined capacity rule work?

The 16A threshold applies to the total export current at the connection point from all generators combined — not just the new system. If existing generation already uses some of the 16A allowance, the remaining headroom is reduced accordingly. Adding a small new system that pushes the combined total above 16A per phase triggers G99.

How long does a G99 application take?

The DNO has 45 working days to respond to a complete G99 application. In practice, most straightforward applications are decided in 4–8 weeks. The clock restarts if the DNO requests additional information. Submitting a complete, well-formatted application on first submission is the best way to avoid delays.

What protection settings does G99 require?

G99 requires Rate of Change of Frequency (RoCoF) protection at 1 Hz/s, Loss of Mains (LoM) protection, and voltage and frequency trip settings as specified in the Engineering Recommendation. The exact settings are documented in a Protection Settings Schedule submitted with the application. The DNO may specify modified settings for particular connection points.

Does G100 apply to my battery storage project?

G100 applies to DC-coupled battery storage systems — batteries connected on the DC side of the solar inverter. If your project includes DC-coupled batteries, confirm G100 compliance alongside G98 or G99. AC-coupled batteries with a separate inverter are assessed independently under the standard G98/G99 framework.