Telling the network operator: G98, G99 and why it matters

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, and the operator has to be told about both.

The installer does this. It is a requirement of both 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, and which applies depends on size.

G98 covers small installations, up to 16 A per phase, which on a single phase supply works out at about 3.68 kW. Most domestic solar arrays of around 4 kWp fall inside it. Under G98 the installation is connected first and the operator is notified within 28 days.

G99 covers anything larger. Under G99 the operator's approval must be obtained before connection, and the operator has a defined period to respond. It may approve, approve with conditions, or require reinforcement work at the customer's expense.

The distinction is a real one for a householder. A 4 kWp array is generally a formality. A 6 kWp array is an application, and its outcome is not certain in advance.

Export limitation

Where a network cannot take the full output, the usual solution is an export limitation device, which monitors the connection and instructs 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.

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 it should be disclosed, and a quotation for a system that will be export limited ought to say so and say what it costs in output.

Batteries and heat pumps

A battery is a generator as far as the network is concerned, because it can export. It is notifiable in the same way, and the assessment considers the combined capability of all the inverters together, not each in isolation, so adding a battery to an existing solar installation can move the whole thing over the G98 threshold.

A heat pump is the reverse: a load, not a generator. MIS 3005-D requires it to be notified to the network operator, and requires the contractor to satisfy themselves that the electricity supply is adequate for it. This matters most in older properties with a 60 A or 80 A main fuse, where a heat pump, an electric shower and an EV charger together can exceed what 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.

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, because finding out after a deposit has been paid is an uncomfortable place to be.