Smaller solar projects in England and Wales no longer need a transmission impact assessment in most places, as long as they are under 5MW. For commercial rooftops and small ground-mount schemes, that can mean a quicker, simpler route to a grid connection.

What is a transmission impact assessment?

A transmission impact assessment (TIA) checks whether a project connecting to the local distribution network would need work on the national transmission network too. For smaller projects, waiting for that check could add months to a connection.

What changed

  • The threshold for needing a TIA in England and Wales rose from 1MW to 5MW.
  • Ofgem approved the change (known as CMP446) on 12 May 2025.
  • At 39 of 225 grid supply points, the threshold stays at 1MW because of fault level issues.
  • NESO said 337 projects (682MW) already in the queue could go ahead without a TIA as a result.
  • Scotland has its own, lower threshold and was not part of this change.

What it means in the South West

  • Projects under 5MW, such as larger commercial rooftops, farm-scale ground-mount and private wire schemes, are often outside the TIA process.
  • Check which grid supply point your site sits under. If it is one of the 1MW exceptions, the old rules still apply.
  • The local network still has to have room. Headroom in parts of the South West is tight, so an early grid check is still worth doing.

What we do

We check the local grid and the TIA position as part of every site screen, so you know early whether a project is likely to need one.

Sources

Correct at the time of writing (1 October 2026). Rules change, so check the latest guidance or talk to us about your site.