On 12 May 2025, Ofgem approved a major regulatory change—Modification CMP446 (WACM 1)—that raises the Transmission Impact Assessment (TIA) threshold from 1 MW to 5 MW export capacity for England and Wales. This reform removes a key barrier for distributed energy projects, particularly benefiting community-scale renewables. Here’s what you need to know.
🔍 What Is a TIA—And Why Did It Need Reform?
A Transmission Impact Assessment (TIA) is a review conducted by the Electricity System Operator (ESO) to determine whether a distribution-connected generation project could significantly affect the national transmission system.
The Problem with the Old 1 MW Threshold
- Overly cautious for modern grids: The 1 MW trigger was set when distributed generation was rare. Today, with smarter grid management, smaller projects pose minimal transmission risks.
- Delays and costs: Even a 1.1 MW solar farm could face months of extra studies, coordination between DNOs and the ESO, and potential reinforcement costs—disproportionate to its actual impact.
🆕 Key Changes Under the New Rules (From 12 May 2025)
- TIA threshold raised to 5 MW (export capacity)
- Applies to new applications and queued projects not yet connected.
- Export capacity (not just generation capacity) is the deciding factor.
- A 4 MW solar farm with a 5 MW inverter? Still exempt.
- A 6 MW battery capable of exporting more than 5 MW? Subject to TIA.
- A 6 MW battery export-limited to 5 MW or less? Still exempt.
- Local network checks still apply
- Even below 5 MW, Distribution Network Operators (DNOs) can require mitigations if:
- Fault levels are exceeded.
- Voltage or thermal limits are breached.
- Example: A 3 MW wind farm in a weak grid area might need current-limiting reactors—but no TIA.
- Even below 5 MW, Distribution Network Operators (DNOs) can require mitigations if:
- Scotland keeps its 200 kW threshold (for now)
- Due to regional grid differences, Scotland’s lower threshold remains. However, discussions on alignment are ongoing.
⚡ Who Benefits Most?
The 1–5MW threshold change directly impacts projects that previously faced disproportionate hurdles:
- Community-scale solar/wind: Single 1.5–4MW projects no longer need to be split into sub-1MW phases.
- Battery storage: 4MW discharge capacity becomes more viable without TIA costs.
- Hybrid systems: Solar+storage schemes up to 5MW export gain flexibility.
Indirect benefits for smaller projects:
With fewer TIAs overwhelming the system, DNOs and the ESO can process all connections faster—including sub-1MW community installations.
🧠Bigger Picture: Aligning with Net Zero Goals
This reform supports the UK’s 2035 net-zero power grid target by:
- Accelerating connections for decentralised renewables.
- Reducing bureaucracy for projects that pose little systemic risk.
- Encouraging local energy ownership—a key pillar of the Energy Act 2023.
What’s next?
- Watch for DNOs adapting connection processes to reflect the change.
- Potential future reforms in Scotland if grid upgrades allow.
đź“ť Conclusion: A Step Forward, But Challenges Remain
Raising the TIA threshold is a win for community energy, cutting red tape for projects that never should have faced heavy scrutiny. However:
- Local grid constraints (congestion, fault levels) still need addressing.
- Queue management reforms must complement this to speed up connections further.
For now, this is a clear signal: small-scale, locally owned energy is finally being treated proportionately.
