What happens if your DNO refuses your application

What a refusal actually rests on

Two layers, and it helps to know which one you are arguing with.

The statutory layer. G99 puts it plainly: “Under Regulation 22 of the ESQCR 2002, no person may operate Power Generating Modules in parallel with a public Distribution Network without the agreement of the DNO.” That is regulation, not the operator’s preference, and it is not something an installer can waive.

The technical layer. G99 requires the DNO to “refuse to allow the connection of a Power Generating Module which does not comply with the requirements and connection process set out in this EREC G99 and which is not covered by a derogation granted by the Authority or a LON”. The Authority is Ofgem; a LON is a limited operational notification, covered below.

The technical reasons, in order of likelihood

Voltage rise. Feeding power into a network pushes local voltage up. There is only so much headroom before the supply goes outside statutory limits.

Voltage step change and flicker. G99 requires measurements against BS EN 61000-3-11, declared on the compliance form. The DNO uses those declared figures to calculate the maximum supply impedance needed to comply with EREC P28 — and G99 is explicit about where that can lead: the calculation “may show that the voltage fluctuations will be greater than those permitted and hence reinforcement of the Distribution Network may be required before the Power Generating Module can be connected.”

Harmonic distortion. Inverters inject harmonics; the network has planning limits under EREC G5.

Aggregate capacity in the area. Enough existing generation on the same feeder and the headroom is simply gone.

Notice that three of those four are properties of the network rather than of your equipment. Changing inverter brand rarely fixes them. Reducing export usually does.

The four realistic outcomes

G99’s SGI-3 procedure describes the reinforcement path directly: where further analysis is needed, “installation and commissioning must not proceed until the DNO has established if it is necessary to upgrade the network, and whether such work may be chargeable to the Generator, if the Generator wishes to go ahead.”

Two things in that sentence are worth holding onto. The DNO must establish whether it is chargeable — so you are entitled to know before committing. And “if the Generator wishes to go ahead” — the decision remains yours.

Why an export limit is usually the good outcome

This is the part that gets treated as a defeat and generally should not be.

A kilowatt hour you use is worth your import rate. A kilowatt hour you export earns your SEG rate, and in Ofgem’s latest annual report the best untied rate was 12p while many were 4.1p or less. The ratio is roughly six to one.

So a limit on the smaller of the two revenue streams costs much less than the headline capacity reduction suggests. And because Export limitation caps instantaneous export in kW rather than annual energy in kWh, a limit only binds during the brightest hours of the brightest days.

There is a second benefit. Under G99’s simplified procedures, a G100 -compliant export limitation scheme is what makes the lighter SGI-2 and SGI-3 routes available at all — so accepting a limit can be what keeps you out of a full standard application. That is covered on export limiting explained.

The three escalation routes

They are genuinely different and people conflate them constantly.

1. Redesign — the one to try first

Not an appeal at all, and usually the fastest. If the constraint is export, limit export. If the issue is total capacity, reduce the inverter’s registered capacity — G98 permits a manufacturer to restrict a micro-generator in software “provided these settings are not accessible to the Customer”, and a lower registered capacity can move an installation into a lighter procedure entirely.

Panel capacity does not have to fall with it. The limit is on what reaches the network, not on what sits on the roof.

2. Derogation — the technical route

Where the installation genuinely cannot meet a G99 requirement, a derogation from Ofgem is the formal mechanism. Note who applies: G99 says “the Generator and the DNO shall apply to the Authority for a derogation”. It is a joint application, not something you file against your operator.

While Ofgem considers it, any interim or limited operational notification “will be extended to remain in force until the Authority has notified the DNO and the Generator of its decision”.

The outcomes are set out plainly, and the third is worth reading before relying on this route:

• Ofgem grants it — the DNO issues final operational notification once other unresolved issues are cleared
• Ofgem decides one is not required — the DNO reconsiders and proceeds
• Ofgem refuses — “there will be no Operational Notification in place and the DNO will initiate a process to disconnect the Power Generating Facility”

That last outcome means a derogation application is not a free option once equipment is already running under a limited notification.

3. Complaint — the conduct route

If your objection is to how the DNO has behaved — delay, inconsistency, a charge you believe is wrong, an assessment you think is mistaken — that is a complaint, not a derogation. The sequence is the operator’s own complaints procedure first, then the Energy Ombudsman once that is exhausted or has run out of time.

A complaint will not overturn a genuine technical constraint. It is the right route for process, and the wrong one for physics.

Practical steps if you get a refusal or a condition

Get the reason in writing, specifically. “Insufficient capacity” is not a reason you can act on. Voltage rise, step change against P28, harmonic headroom or aggregate feeder capacity are, because each points at a different remedy.

Ask what export limit would be acceptable. This is the single most useful question, and it converts a refusal into a number you can evaluate.

Ask whether reinforcement is chargeable, and how much, before you commit to anything. G99 puts that determination before your decision to proceed for a reason.

Check the equipment is on the Type Test Register. A non-type-tested inverter turns a simple application into a compliance exercise, and that is fixable by changing equipment.

Do not install anyway. Beyond breaching G99, operating in parallel without agreement runs against regulation 22 of the ESQCR. It also puts your SEG arrangement and potentially your insurance in an awkward place.

Keep every document. The application, the assessment, the offer, the conditions. If this becomes a complaint later, the paper trail is the case.