Telling the network operator: G98 and G99

The wires in the street belong to a distribution network operator, and it has an interest in what gets connected to them. A solar array pushes current back into a network designed to deliver it in one direction. A heat pump adds a substantial load to a supply sized decades ago. The installer has to tell the operator about both.

The installer does this. It is a requirement of both Microgeneration Certification Scheme (MCS) installation standards, and it is not optional. But it is worth understanding, because it can affect what you are allowed to install.

Connect and notify, or apply and wait

There are two engineering recommendations, both published by the Energy Networks Association (ENA), the body the network operators share. Which one applies depends on size.

G98 covers small installations, up to 16 A per phase, which on a single phase supply works out at 3.68 kW. The figure that counts is what the inverters can export, added together, not the size of the panels: a 4.4 kWp array on a 3.68 kW inverter is a 3.68 kW installation, which is why most arrays of around 4 kWp fall inside it. Under G98 the installer connects the system first and notifies the operator within 28 days.

G99 covers anything larger. Under G99 the installer must get the operator's approval before connection. It may approve, approve with conditions, or require reinforcement work at the customer's expense. Most domestic installations qualify for one of its fast track routes, on which the operator answers within 10 working days; larger ones go through the standard application, which takes longer.

The distinction is a real one for a householder. A 4 kWp array is generally a formality. A 6 kWp array on a 5 kW inverter is an application, and its outcome is not certain in advance. The connection route calculator works out which applies from the inverters' ratings.

Export limitation

Where a network cannot take the full output, the usual solution is an export limitation device. It monitors the connection and tells the inverter to reduce output if export would exceed an agreed figure. A 6 kWp array might be permitted on the basis that it never exports more than 3.68 kW.

An export limiter does not turn a G99 installation into a G98 one. The array is still an application; the limiter is what qualifies it for the fast track, and the limiter itself has to be a type-tested one that meets a third ENA document, G100.

That sounds worse than it usually is. Export is capped only at the moments when generation is highest and household demand lowest, which is a small part of the year. The annual loss is often a couple of per cent. But the installer should tell you about it: a quotation for a system that will be export limited ought to say so and state what the limit costs in output.

Batteries and heat pumps

A battery is a generator as far as the network is concerned, because it can export, so it is notifiable in the same way. The assessment adds together the capability of every inverter in the house. Adding a battery to an existing solar installation can therefore move the whole thing over the G98 threshold.

A heat pump is the reverse: a load, not a generator. MIS 3005-D requires the contractor to notify the network operator and to satisfy themselves that the electricity supply is adequate. This matters most in older properties with a 60 A or 80 A main fuse. There, a heat pump, an electric shower and an electric vehicle charger together can draw more than the supply was built for. The remedy is usually a fuse upgrade or a load management device, and both are cheaper to discover before the design is fixed.

In practice

For most domestic solar installations the notification is invisible: the installer submits it, the operator acknowledges it, and a copy arrives with the handover documents.

Most installers now do all of this through the ENA's Connect Direct service, which asks for the details before anything is installed, even for a system G98 would allow to be reported afterwards, and approves many of them on the spot. Each operator sets its own limits for what it approves automatically, and they vary from place to place.

Where it becomes visible is when the answer is no, or not at that size. That is more common in areas with a lot of existing generation, where the local network is already carrying as much export as it comfortably can. It is worth asking early whether a G99 application will be needed and what the operator's current position is in your area. Finding out after you have paid a deposit leaves you in an awkward position.