Why Your DNO Refused Your Solar Application (And What to Do)

Last updated: 19 May 2026 — Charles Fletcher, MCS-trained PV engineer, Spectrum Energy Systems

Why Your DNO Refused Your Solar Application (And What to Do)

A DNO refusal sounds like a hard stop. In practice it’s usually a negotiation. The DNO is saying ‘we can’t accept this exact specification on this exact site’ — not ‘you can’t have solar’. The job is to find a modification that satisfies both the customer’s generation goal and the DNO’s network constraints.

The three common reasons for refusal

1. Insufficient local network capacity. Your local cables and transformer can’t accept the proposed export volume on top of existing connections. Most common reason. Particularly common in rural areas with older infrastructure or urban areas with lots of recent solar uptake.

2. Engineering non-compliance. The proposed inverter isn’t on the certified G99 list, the single-line diagram has errors, protection settings are wrong, or earthing arrangement isn’t compliant. Usually a paperwork fix; the install itself can still proceed.

3. Phase imbalance / network stability concerns. Single-phase install proposed in an area where local network already has heavy single-phase load. Resolution typically involves three-phase upgrade or reduced single-phase export.

Fix 1: Reduce export capacity

The simplest fix is to cap inverter export. If the DNO will accept 5kW export but not 11kW, fit an inverter that’s limited to 5kW — even if the panel array generates more, the inverter doesn’t allow more than 5kW to flow to the grid. The surplus is absorbed by the battery (if fitted) or curtailed (if not).

This is genuinely lossless on most installs because your battery is absorbing midday surplus anyway. The grid only sees the export-limited figure. Common solution on rural sites with weak local cables.

Fix 2: Fit smart export limiting

A step beyond fixed export caps is dynamic export limiting that monitors local voltage and frequency and tapers export only when the network is approaching its limit. The DNO sometimes accepts higher headline capacity if smart limiting is fitted, because the system actively respects local network state. Spectrum fits Solis hybrids with dynamic export limiting where it’s the right answer.

Fix 3: Pay for network reinforcement

For larger commercial installs where reduction isn’t acceptable, the DNO will cost network reinforcement — upgrade of cables, transformer change, sometimes a new connection. The customer pays. Costs range from £5,000 for minor upgrades to £100,000+ for major reinforcement.

Worth the cost when:

• The system is large enough that reinforcement cost amortises across the install’s revenue
• The customer has a long-term operational interest in the site (industrial, agricultural)
• The reinforcement also benefits other electrical needs (EV fleet charging, heat-pump heating)

Not worth the cost when:

• The reinforcement exceeds 10–15% of the install’s capex
• The customer’s payback maths can’t absorb the addition
• A smaller export-limited install does most of what was wanted at a fraction of the cost

Fix 4: Upgrade to three-phase

If you’re currently on single-phase and the DNO can’t accept the export volume on that phase, upgrading to three-phase typically unlocks 3× the capacity on the same supply. Three-phase upgrade costs £2,000–£5,000 for typical domestic; commercial sites are often already three-phase.

Worth doing if you also want EV charging, heat pumps, or higher kW solar in the future.

Fix 5: Resubmit with engineering corrections

If the refusal is engineering-paperwork rather than capacity, just fix the documentation. Common engineering corrections:

• Swap to a certified G99 inverter
• Correct single-line diagram — show isolators and meter correctly
• Update protection settings to match current engineering recommendations
• Confirm earthing arrangement

Resubmission timeline is fast — DNOs typically re-review within 5–10 working days of resubmission.

How to avoid refusals in the first place

• Use an MCS-certified installer with G99 experience. Engineering errors that trigger refusal are usually avoidable. Spectrum’s applications are pre-checked against the DNO’s acceptance criteria.
• Run a pre-application enquiry for large installs. The DNO can do a non-binding capacity check before the formal G99 application. Useful on commercial sites where you want to know likely capacity before designing.
• Specify export-limited from the start. Lower headline export = lower refusal risk. Designs that already match the network typically sail through.
• Use a Solis hybrid that’s on the certified G99 list. Most Spectrum specifications use Solis precisely because the certified-list compliance is sorted.

Commercial prospects: the £500 refundable DNO check

For commercial customers, the unknown grid capacity is often the biggest single uncertainty. Spectrum’s £500 refundable DNO application service runs a G99 application before you commit to the full install, so you know exactly what export capacity the DNO will approve before deciding on the build size.

The £500 is credited against the install if you proceed with Spectrum. Without this service, most commercial customers face the choice of (a) designing on assumption then potentially redesigning if DNO refuses, or (b) running the DNO application themselves, which is an engineering documentation exercise most non-installers find impractical.

Local network capacity in the East Midlands

National Grid Electricity Distribution publishes generation capacity heatmaps for its network. Rural Lincolnshire, parts of north Nottinghamshire and the Peak District fringes have constrained capacity in places. Urban Nottingham, Derby and Leicester have variable capacity by local substation. The map isn’t a guarantee — specific feeders and transformers vary — but it’s a useful pre-application sanity check.

FAQs

Related reading

• G99 application solar: how DNO approval works
• Solar grid connection: G98 vs G99 explained
• Solar panel installation cost UK 2026
• Solar PV capital allowances

For the full Spectrum service overview see contact our team.