DNO G98 and G99 Applications: Under 3.68kW Notification vs Over 3.68kW Full Application Process

Summary

The G98 and G99 engineering recommendations govern the connection of small-scale generation (including solar PV) to the UK low-voltage distribution network. They are published by the Energy Networks Association (ENA) and adopted by all UK DNOs.

G98 was introduced in 2019 to simplify small-scale connections. For most domestic solar PV systems (4kWp or less), G98 notification is all that is required — a straightforward online form submitted after installation. However, many installers incorrectly believe any system under 3.68kW export capacity is automatically G98-compliant without any notification. The notification is required — it is not optional. Missing it is a grid breach.

Key Facts

• G98 — ENA Engineering Recommendation G98; covers single-phase generation up to 3.68kW (16A) export capacity; notification required within 28 days of connection
• G99 — ENA Engineering Recommendation G99; covers systems above G98 thresholds; full application required; DNO approval needed before connection
• 3.68kW threshold — the export capacity limit for G98: 16A × 230V = 3,680W; this is the inverter's maximum export rating, not the panel array size
• 11kW three-phase threshold — for three-phase connections, the G98 limit is 11kW (16A per phase × 400V × √3 ≈ 11kW)
• Notification vs application — G98 is notification (tell the DNO after connecting); G99 is application (get approval before connecting)
• 28-day rule (G98) — the G98 notification must be submitted to the DNO within 28 days of connection; failure is a breach of the grid connection agreement
• DNO approval period (G99) — DNOs have up to 45 working days (9 calendar weeks) to approve or reject a G99 application; in practice, most domestic G99 applications are processed in 2–4 weeks
• Export limitation — a 4kWp solar array with an inverter configured to export-limit at 3.68kW qualifies for G98, even though the panel capacity exceeds 3.68kWp
• Three-phase G99 — a single-phase system with multiple inverters in parallel that together exceed 3.68kW requires G99
• Aggregated capacity — if a second solar PV system is added to a property that already has solar PV, the total combined export capacity determines G98 or G99 eligibility
• Automatic disconnection — G98/G99-compliant inverters must disconnect automatically if grid voltage or frequency goes outside specified limits (under/over voltage and under/over frequency protection)
• Anti-islanding — a key safety requirement: the inverter must detect loss of grid supply and disconnect immediately; this prevents the solar system energising a dead section of grid

Quick Reference Table: G98 vs G99 Key Differences

Detailed Guidance

Understanding the Export Capacity vs Array Size Distinction

The 3.68kW G98 threshold applies to the inverter's export capacity — not the solar panel array size (kWp).

Example 1: G98-eligible

• 4.2kWp solar panel array (say, 14 × 300Wp panels)
• Inverter: 3.68kW (3,680W) single-phase export capacity, inverter export limited to 3.68kW
• Total export capacity = 3.68kW → G98 notification required

Example 2: G99 required

• 4.2kWp solar panel array
• Inverter: 5kW single-phase, no export limit set
• Total export capacity = 5kW → G99 application required before connection

Export limiting: Setting the inverter to export-limit at 3.68kW means the inverter will not export more than 3.68kW to the grid regardless of solar generation. Excess generation above household consumption + 3.68kW export will be clipped (wasted). For most households, this rarely occurs in practice, but it is a function of the inverter setup. Most modern inverters support export limiting via the commissioning software.

Note: export limiting the inverter to 3.68kW means self-consumption is not limited — the array can generate and be consumed by the household at whatever rate the panels produce. Only the export to grid is capped.

G98 Notification Process

For qualifying systems (≤3.68kW single-phase export):

Step 1: Complete the installation Install the solar PV system fully (panels, inverter, DC isolation, AC connection, earthing, protective devices). Commission and test the system. The inverter must be G98/G99-compliant — check the ENA's G98 Type Test register to confirm the inverter model is on the approved list.

Step 2: Submit the G98 notification Most DNOs accept G98 notifications through their online portals. The notification form requires:

• Installation address
• Inverter make, model, and serial number
• Total generation capacity (kWp) and export capacity (kW)
• Date of connection
• Installer name and contact details (MCS certificate number recommended)

Submit within 28 days of connection.

Step 3: DNO acknowledgement The DNO will acknowledge the notification. Under G98, the DNO can object within a specified period (typically 28 days) if the connection causes network issues (overloaded local network, voltage rise problems). In practice, objections are rare for standard domestic installs.

Step 4: Customer record-keeping Keep a copy of the notification and the DNO's acknowledgement. Include these in the customer's handover documentation.

G99 Application Process

For systems requiring G99 approval:

Step 1: Contact the DNO Before starting the design, contact the local DNO's connections team. For most domestic G99 applications, you can complete the online pre-application enquiry to check whether there are any known network constraints at the address.

Step 2: Technical application Submit the G99 application with:

• System design: array kWp, inverter make and model, export capacity, mounting details
• Single-line diagram of the proposed connection
• Protection settings for the inverter (G98/G99 protection parameters)
• Site address and metering details

Most DNOs have an online G99 application portal.

Step 3: DNO technical review The DNO reviews the application against their local network capacity. They check:

• Whether the proposed export capacity will cause voltage rise on the local LV network above acceptable limits
• Whether the network has capacity for the new connection
• Protection co-ordination

Step 4: Approval or rejection The DNO may:

• Approve unconditionally — most common for modest G99 domestic applications
• Approve with conditions — e.g., require export limiting, or specify protection settings
• Reject — rare for small domestic systems; requires upgrade of the local network before connection is possible

Step 5: Connection and commissioning After approval, install and commission the system. Submit a commissioning notification to the DNO confirming the connection has been made as approved.

Inverter G98/G99 Compliance

Type Testing: G98 and G99 require inverters to be type-tested to confirm they include the specified protection functions (under/over voltage, under/over frequency, anti-islanding, ROCOF protection). The ENA maintains a register of type-tested inverters.

Common protection parameters (G98 defaults):

• Under voltage: disconnect if V < 184V for >2.5s
• Over voltage: disconnect if V > 264V for >1s; V > 273V for >0.5s
• Under frequency: disconnect if f < 47Hz
• Over frequency: disconnect if f > 52Hz
• ROCOF (Rate of Change of Frequency): disconnect if df/dt > 1Hz/s

For G98, the inverter's default type-tested settings are normally acceptable. G99 applications may specify modified protection settings to suit the local network.

Common Mistakes and Compliance Issues

Installing without any notification: Some installers fail to submit even the G98 notification. This is a breach of the connection conditions. While enforcement is rare, it can cause issues when the customer sells the property (solicitor's searches may reveal the connection breach).

Incorrectly claiming G98 eligibility: An installer with a 5kW inverter who believes the 4.2kWp array qualifies for G98 because the panels are "only 4.2kWp" is wrong. The inverter export capacity determines G98/G99 eligibility, not the panel array kWp. If the 5kW inverter is not export-limited to 3.68kW, G99 is required.

Missing the 28-day G98 deadline: The G98 notification must be submitted within 28 days. Many installers forget or deprioritise the paperwork. Build the notification submission into the commissioning checklist and complete it on the day of commissioning.

Not checking the G98 type test register: If the inverter is not on the G98/G99 type test register, it is not compliant — even if it has all the correct protection settings. Check the ENA register before installing.

Frequently Asked Questions

My customer wants a 6kWp system. Does that require G99?

It depends on the inverter configuration, not the panel capacity. If the inverter is set to export-limit at 3.68kW, the system qualifies for G98. If the inverter exports at its full rated capacity (e.g., a 5kW or 6kW inverter not export-limited), G99 is required. Many installers of larger domestic systems use export limiting to maintain G98 eligibility and simplify the process.

How long does a G99 application take?

DNOs have 45 working days (approximately 9 calendar weeks) to process a G99 application. In practice, most straightforward domestic G99 applications (say, 10kWp) are processed in 2–4 weeks. Commercial applications, especially for systems in congested suburban networks, can take the full 45 days. Always allow for this in project programmes.

Does the G98 notification cost anything?

No. G98 notification is free. G99 applications may involve a DNO application fee, typically £100–£300 for residential-scale systems; larger commercial applications may be more.

Do I need to notify the DNO if I add a battery to an existing G98-notified solar system?

It depends on whether the battery adds export capacity. A battery that is only charged from the solar array and household supply (AC-coupled, no grid export from battery) may not require a new notification. A battery that can export to the grid (V2G, or excess battery discharge) changes the export capacity calculation and may trigger a new notification or G99 application. Check with the DNO and your MCS certification body.

Regulations & Standards

• ENA Engineering Recommendation G98 — notification of small embedded generators; up to 3.68kW single-phase

• ENA Engineering Recommendation G99 — application for embedded generators above G98 thresholds

• BSI PD CLC/TR 50549-1 and -2 — requirements for generating plants to be connected to LV networks

• MCS 012 — inverter type testing requirements for MCS-certified solar installations

• ENA Engineering Recommendation G98 — energynetworks.org — full G98 text and type test register

• ENA Engineering Recommendation G99 — energynetworks.org — G99 application guidance

• UK Power Networks G98/G99 applications — example DNO portal

• mcs certification solar — MCS certification requirements for solar installers

• solar pv commissioning handover — commissioning documentation including DNO notification

• solar battery storage installation — battery storage and DNO notification implications

• ev charger brands compared — G99 interaction with large EV charging installations