G98 and G99 are the two engineering recommendations from the Energy Networks Association that govern how generation equipment connects to the UK distribution network. Every grid-connected solar install falls under one or the other, and picking the right route is one of the most important decisions you'll make on the job.
The rules aren't complicated, but the edge cases are where installers come unstuck. This guide walks you through the thresholds, the documents, the timelines, and the situations that don't fit neatly into either box.
The Core Difference in One Sentence
G98 is a notification after the install. G99 is an application before the install.
That single distinction has enormous implications for your project timeline. A G98 system can be commissioned on the day of install and the paperwork follows afterwards. A G99 system cannot legally be commissioned until the DNO has approved the connection in writing, which can take anywhere from a few weeks to several months.
The Threshold: Per Phase, Not Total
The defining number is 3.68 kW per phase. This is where the routing splits:
- Up to 3.68 kW per phase → G98 (a notification)
- Above 3.68 kW per phase → G99 (a full application)
The word "per phase" matters more than installers sometimes realise. On a single-phase supply, you have one phase, so 3.68 kW total is the ceiling. On a three-phase supply, you have three phases, so the ceiling becomes 11.04 kW total (3.68 × 3), provided generation is balanced across all three phases.
Practical example: a 10 kW solar array on a three-phase supply is a G98 notification, not a G99 application, as long as the inverters distribute output evenly across all three phases. The same 10 kW system on a single-phase supply is firmly in G99 territory.
Quick Reference: When Each Form Applies
| System scenario | Route |
|---|---|
| 3.68 kW single-phase, no battery | G98 |
| 4 kW single-phase, no battery | G99 |
| 10 kW three-phase, balanced | G98 |
| 12 kW three-phase, balanced | G99 |
| 3.6 kW single-phase + 5 kW battery | G99 (see batteries below) |
| Any system over 50 kW | G99 (full network study territory) |
How G98 Works
What you submit
- Customer name and address
- MPAN
- Inverter make, model, and rated output
- Total installed generation capacity
- Commissioning date
When you submit
After commissioning, typically within 28 days. There's no pre-approval requirement, which is the entire point of G98. You can install on day one and notify after the fact.
How long it takes
DNOs usually acknowledge G98 notifications within a few working days. There's no design engineer review and no approval letter, just a confirmation that the notification has been received and recorded on the network.
How G99 Works
What you submit
- Everything in a G98, plus:
- Single Line Diagram (SLD) showing the full electrical arrangement
- Letter of Authority signed by the registered electricity account holder
- Equipment specifications for every inverter, battery, EV charger and other connected device
- Proposed export capacity
- Supply details: phase type, cutout rating, existing connected load
When you submit
Before the install. You must have written approval from the DNO before any generation equipment is commissioned. Installing without approval is a breach of your connection agreement and can lead to enforced disconnection.
How long it takes
- Under 50 kW: typically 25 to 45 working days, though this varies significantly between the six DNOs
- 50 kW to 1 MW: 45 to 65 working days
- Above 1 MW: 3 to 6 months once network studies are involved
The Edge Cases That Trip Installers Up
Batteries push systems into G99
A battery is a generation source as far as the DNO is concerned. Even on a sub-3.68 kW solar install, adding a 5 kWh battery with a 3 kW inverter often takes the combined rated output above the G98 threshold. The application has to be routed as a G99.
If the customer is "planning to add a battery later", declare it now. Resubmitting as a G99 retrospectively because a battery has been added is a much bigger job than getting it right first time.
Three-phase systems with unbalanced output
The 11.04 kW three-phase G98 ceiling assumes balanced output across all three phases. If your inverter setup loads one phase heavier than the others (more than 3.68 kW on any single phase), you're back in G99 territory regardless of total system size.
Multiple generation sources on one site
A solar array plus a wind turbine, a CHP unit, or a backup generator all need declaring together. The combined output across all sources is what determines the route, not solar alone.
EV chargers
EV chargers above 32A often need declaring to the DNO under a separate ENA process, regardless of whether the solar is G98 or G99. If you're installing solar and an EV charger on the same visit, both need to be reflected on the application paperwork.
Export-limited systems
Larger inverter capacity that's electronically export-limited to under 3.68 kW per phase may qualify for G98, but only with G100-compliant limiting equipment and proper documentation on the SLD. Get this wrong and the DNO will reject the route, push it back to G99, and you're starting over. We design G100 arrangements regularly and know which DNOs accept which limiting configurations.
What Happens If You Pick the Wrong One
Submitting a G98 when you needed a G99 has two big consequences:
- The DNO rejects the notification once they realise the system size, and the customer's connection is technically non-compliant until a G99 is approved retrospectively
- The install date you've already given the customer slips, often by weeks, because you now have to wait for full G99 approval before commissioning
Submitting a G99 when you only needed a G98 is less damaging but still costly. You'll wait weeks for approval on a job you could have commissioned on day one, and you may incur application fees for a route that wasn't required.
How to Get the Routing Right Every Time
Three checks, done at survey, will catch nearly every routing decision:
- Confirm the supply type. Single-phase or three-phase, taken from the customer's meter or cutout, not assumed from the postcode.
- Calculate per-phase output. Total inverter rated AC output divided by the number of phases the system loads. If any phase exceeds 3.68 kW, it's a G99.
- Add up every generation and storage source. Solar, battery, generator, EV charger, anything that can push power onto the network. Combined rated output is what the DNO assesses.
On every application we process, this routing is the first thing our engineers verify, before a single document is drawn. It takes minutes upfront and saves weeks at the back end.
The Bottom Line
G98 and G99 aren't interchangeable. One is a courtesy notification, the other is a full pre-approval process with real engineering scrutiny attached. Routing the job correctly is the most important decision on the application, and it's the one most likely to trip installers up on systems that sit close to the threshold.
If you'd rather not work through phase calculations, battery declarations, and per-DNO routing quirks on every job, that's exactly the kind of work we take off your plate, on a fixed-price basis, with full visibility from submission to approval.