A Distribution Network Operator (DNO) heat map is a red/amber/green snapshot showing where the local electricity network has spare capacity for new connections. Each of Great Britain's six DNO groups publishes its own map covering primary and bulk supply substations, with red flagging constrained areas and green flagging headroom. The maps are indicative, not binding: they are the right starting point for site screening, but a formal application is still required to confirm what a connection will cost and how long it will take.
Introduction
Heat maps are the most useful free tool a renewable developer has for screening a site for grid viability. They sit on every DNO's website, refresh monthly or six-monthly, and reveal in seconds whether a substation is already constrained. Yet they remain opaque to anyone outside the connections industry. This article explains what is on a heat map, how to read it, and where its limits lie.
What is a DNO heat map?
A DNO heat map is an interactive web map (sometimes paired with a downloadable spreadsheet) showing the capacity available for new connections at each substation in a network operator's licence area. A Distribution Network Operator (DNO) is the regional company that delivers electricity from the high-voltage transmission network to homes and businesses, and owns the assets shown on the map.
There are six DNO groups in Great Britain, covering fourteen licence areas:
- National Grid Electricity Distribution (NGED): the Midlands, South West, South Wales
- UK Power Networks: London, the South East, the East of England
- SP Energy Networks: Central and Southern Scotland (SPD) and Merseyside, Cheshire, North Wales and North Shropshire (SPM)
- Scottish and Southern Electricity Networks (SSEN): the North of Scotland and Southern England
- Northern Powergrid: the North East and Yorkshire
- Electricity North West (ENWL): the North West of England
Each DNO publishes its own heat map. The underlying data follows similar conventions, but the interfaces, update frequencies, and naming differ.
How to read the colour coding
Every DNO heat map uses a red/amber/green (RAG) traffic-light system to indicate spare capacity at a substation. The colours guide users to areas where a connection is more likely to be achieved without significant network reinforcement (NGED, 2025).
The general convention across DNOs is:
- Green: substantial headroom. A connection of typical size is likely feasible without reinforcement.
- Amber: some headroom, but approaching capacity. A connection may trigger partial reinforcement, additional studies, or a flexible offer.
- Red: at or beyond firm capacity. New connections will likely require reinforcement, a non-firm offer, or no offer in the short term.
Exact thresholds vary. SP Energy Networks defines demand headroom as firm capacity plus capacity added by planned interventions or flexibility services, minus forecast demand, with negative values flagging a forecast constraint (SP Energy Networks, 2025).
Key terms behind the colours
Heat maps assume the reader understands a handful of grid terms. The essentials are:
Firm capacity
The maximum load a substation can support while operating safely with one circuit or transformer out of service. For primary substations, firm capacity is generally the rating under single-circuit-outage conditions (SP Energy Networks, 2025). It is the number against which headroom is measured.
Headroom
Firm capacity minus existing connected load (or generation) and minus any contracted but not-yet-connected capacity in the queue. Northern Powergrid publishes thermal headroom for both demand and generation, refreshed monthly (Northern Powergrid, 2025).
Demand vs generation headroom
Heat maps almost always show two separate views. Demand headroom is the capacity available for new loads such as EV chargers, heat pumps, factories, and data centres. Generation headroom is the capacity available for new generators including solar farms, wind, and batteries exporting to the grid. A substation can have ample demand headroom and zero generation headroom, because the limiting constraint differs in each direction.
Bulk Supply Point (BSP) and Primary substation
A Bulk Supply Point transforms transmission voltage (typically 132kV) down to 33kV. A Primary substation transforms 33kV down to 11kV, the voltage most distributed generation sites connect at. The smaller the project, the more relevant primary-level data is.
Active Network Management (ANM)
In constrained areas, DNOs sometimes operate an Active Network Management scheme, which monitors network limits in real time and curtails flexible generators on a last-in, first-out basis when constraints occur. NGED uses ANM in areas with "multiple complex constraints" to release otherwise unavailable capacity (NGED, 2025). ANM zones are usually flagged as a separate layer on the map.
What the data looks like across the six DNOs
| DNO | Map name | Update frequency | Granularity |
|---|---|---|---|
| NGED | Network Opportunity & Development Map | Six-monthly (LTDS) plus quotation statistics | BSP, Primary, Distribution |
| UK Power Networks | Network Infrastructure and Usage Map (NIUM) | Continuous via Open Data Portal | BSP, Primary |
| SP Energy Networks | SPD and SPM Heat Maps | Six-monthly [VERIFY] | 11kV, 33kV, 132kV |
| SSEN | Generation Availability and Network Capacity Map | Quarterly [VERIFY] | GSP, BSP, Primary |
| Northern Powergrid | Generation and Demand Availability Maps | Monthly | EHV and HV |
| Electricity North West | Heatmap Tool (Excel + web) | Monthly [VERIFY] | BSP, Primary |
The richest underlying datasets sit on the DNOs' open data portals. UK Power Networks, SSEN, and Northern Powergrid all publish raw substation-level data as CSV, GeoJSON, or via API.
UK policy context: why heat maps matter more in 2026
Heat maps have moved from a niche planning tool to a near-essential one, driven by two policy shifts.
Ofgem's TMO4+ connections reform, approved in April 2025, replaced the first-come-first-served queue with a two-gate system. Gate 1 offers are non-binding and indicative; Gate 2 offers are firm, with confirmed location, cost and connection date, but only for projects that meet readiness and strategic-need criteria (Ofgem, 2025). The reform raises the value of upfront site screening: picking the wrong site now costs delay, not just money.
The Clean Power 2030 Action Plan, published in December 2024 and updated in April 2025, targets clean electricity meeting 95% of GB demand by 2030 (DESNZ, 2025). Ofgem has also committed to mandating networks to publish more real-time data on capacity, constraints and queue positions, meaning heat maps will become more accurate and more central, not less (Ofgem, 2025).
The limits of heat maps
Heat maps are indicative. Electricity North West states this directly: the tool "cannot replicate the level of accuracy provided by the detailed assessment carried out as part of the formal application process" (ENWL, 2025). Three specific limits matter:
- They lag the queue. Even monthly updates run weeks behind the live position; contracted capacity at a substation can shift materially between refreshes.
- They mask sub-substation constraints. A primary may show green headroom, while the 11kV feeder serving your specific site is constrained. Heat maps rarely show feeder-level data.
- They do not price reinforcement. A red zone signals reinforcement is likely, but not whether it is a £40,000 transformer tap change or a £4 million circuit rebuild. That figure emerges only from a G99 application or pre-application study.
Use a heat map as a first-pass screen, not a final answer.
Why Feasibly makes heat-map analysis faster
Reading one heat map for one site is straightforward. Screening fifty sites across multiple DNO areas, with different colour schemes, cadences, and terminology, is not. It is typically several days of GIS consultant time. Feasibly pulls live DNO capacity data into a single natural-language interface, so a developer or community energy group can ask "which of these sites have generation headroom above 5MW at the nearest primary substation?" and receive an answer in minutes. The data is the same; what changes is the time and expertise required to read it.
Conclusion
A DNO heat map is the cheapest and fastest tool for filtering sites by grid viability. The colour codes are consistent in principle (green for headroom, amber for caution, red for constraint), but data, voltage levels, and definitions vary across the six UK DNO groups. As connections reform tightens the rules on which projects progress to firm offers, reading these maps correctly is becoming the difference between a project that connects in three years and one that does not connect at all. Heat maps are not the answer, but they are the right first question.