How voltage reduction can ensure system stability | National Energy System Operator
Summary
NESO explains its voltage reduction capability, where DNOs cut distribution voltage by 3-6% to reduce demand during emergencies. Testing with DNO groups shows a 3% voltage reduction yields approximately 1.2% demand reduction. Full DNO-wide testing is planned to refine the process.
Why it matters
This is an existing emergency tool, not a new mechanism. The 1.2% demand reduction per 3% voltage cut is modest but real, and quantifying it through structured testing strengthens the control room's toolkit for tight margins without resorting to disconnection.
Key facts
- •Stage 1: 3% voltage reduction; Stage 2: further 3% to maximum 6% total
- •3% voltage reduction yields approximately 1.2% demand reduction on average
- •Two test phases completed (Northern and Southern DNO blocks)
- •Full simultaneous all-DNO test planned
- •Constant resistance devices (kettles, heaters) see largest demand reduction; constant power devices (servers) see none
Areas affected
Memo
What this is about
NESO has published an explainer on voltage reduction, the emergency tool that allows Distribution Network Operators to cut voltage on their networks by 3-6% to reduce electricity demand during supply shortfalls. This is not a new mechanism. It sits in the control room toolkit between commercial actions (calling on reserve plant, interconnector flows) and disconnection. The publication matters because it discloses testing results: structured trials with DNO groups have quantified the demand response, giving the control room harder numbers to work with when margins tighten.
The context is winter adequacy. GB's de-rated capacity margins have been narrowing as thermal plant closes faster than firm replacement connects. Voltage reduction is one of the last non-disconnection levers available. Knowing precisely how much demand it sheds, rather than relying on engineering rules of thumb, makes the difference between a controlled response and a gamble.
Key points
The mechanism. DNOs reduce distribution voltage in two stages: Stage 1 cuts 3%, Stage 2 cuts a further 3%, to a maximum 6% total reduction. Lower voltage means lower power draw from resistive loads (kettles, older heaters), moderate reduction from constant-current devices (LED lights, chargers), and negligible reduction from constant-power devices (servers, industrial equipment). The mix of load types on any given network determines the actual demand reduction achieved.
The test results. NESO has completed two phases of testing, covering Northern and Southern DNO blocks. A 3% voltage reduction produced approximately 1.2% demand reduction on average. That ratio (0.4% demand reduction per 1% voltage reduction) is lower than the theoretical maximum for a purely resistive load, which reflects the growing share of constant-power electronics in GB's demand mix. The number is modest but real: on a 35 GW winter evening peak, 1.2% is roughly 420 MW, equivalent to a mid-size gas plant.
What the ratio tells you about demand composition. The 0.4 coefficient is itself a data point. A decade ago, before widespread LED lighting, EV chargers, and heat pump inverters, the ratio would have been higher. As the load mix shifts further toward power electronics, voltage reduction becomes progressively less effective. NESO does not say this explicitly, but the implication is clear: this tool has a shelf life. Its value as an emergency lever will erode as electrification deepens.
Communication and automation. The tests validated not just the demand response but the communication chain between NESO's control room and DNO automated voltage control systems. In an emergency, speed matters. The ability to instruct all DNOs simultaneously and have voltage reduction implemented through automated systems, rather than manual switching, is operationally significant.
What this is not. This is not demand-side response. It is not voluntary. Consumers do not participate or choose. Voltage reduction happens to them, invisibly. The kettle takes slightly longer to boil. The old bar heater runs slightly cooler. Most consumers would not notice a 3% cut. It is a blunt instrument, but it works without any consumer action, market signal, or contractual arrangement, which is precisely its value in an emergency.
What happens next
Full DNO-wide test. NESO plans to run a simultaneous test across all DNO licence areas. Previous tests covered Northern and Southern blocks separately. A system-wide test will reveal whether the 1.2% average holds when all regions reduce simultaneously, or whether regional variation in load composition produces a different aggregate result. No date has been published for this test.
Detailed results publication. NESO states that more detailed results and analysis from the completed tests are "due to be shared soon." This should include regional breakdowns, time-of-day variation, and potentially the demand reduction coefficient for Stage 2 (6% voltage cut), which may not scale linearly.
Winter 2026/27 readiness. The practical question is whether voltage reduction is formally included in NESO's Electricity Margin Notice procedures for next winter with updated, test-validated parameters rather than legacy assumptions. If the full DNO test completes before autumn, the control room will have a better-calibrated tool for tight margin events.
Longer-term erosion. NESO should be tracking the voltage reduction coefficient over time. If the 0.4 ratio is falling as the load mix shifts toward constant-power devices, the control room needs to know. A tool that delivers 420 MW today may deliver 300 MW in five years. Planning assumptions should reflect the trajectory, not just the current snapshot.
Source text
How voltage reduction can ensure system stability | National Energy System Operator Show/Hide Menu Toggle Add to favourites Close tooltip Sign in to add this page to your favourites Sign in or register Show favourites Close Close tooltip Sign in or register to manage your favourites Sign in or register Help You are now signed in Visit My NESO account to view and manage your dataset subscriptions. Maybe later Go to your account How voltage reduction can ensure system stability Security of Supply 8 May 2026 - 4 minute read Ensuring a stable and secure electricity supply across Great Britain is one of our key roles as the organisation that operates GB’s electricity system. Our control room experts balance supply and demand every second of every day of every year, making sure you have the energy you need, when you need it. Great Britain has one of the world’s most secure electricity systems and we work constantly in the background to make sure demand doesn’t outstrip generation. We’re developing a way to manage this through voltage reduction, a vital tool for balancing the grid during emergencies and ensuring we maintain our world leading security of supply standards. What is voltage? Think of electricity like water running through your garden hose. Voltage is the push that keeps electricity moving through cables, just like water needs pressure to get from your tap to the end of your hose. Opening your tap to full allows more mass flow (more area for the same pressure to push water through), which creates high pressure and the water flows strongly to give you a powerful jet of water. Like this, high voltage enables electricity to flow strongly through cables, but it can still move without the tap being fully open i.e when voltage is reduced. So, what is voltage control? Voltage control allows Distribution Network Operators (DNOs), the companies that transport electricity from the high-voltage national transmission system to homes and business, to reduce the voltage running through cables, which lowers the demand of the network when there isn’t enough electricity generation on the system or in the event of anomalies like low frequency or thermal overloads. In this instance, voltage control, is a specific action DNOs can perform in an emergency. Voltage control is also something we do constantly at NESO to manage voltage on Great Britain’s electricity system to keep it between statutory limits. Voltage must be carefully controlled because too high or too low levels can damage equipment or cause power cuts. We manage this in real time by coordinating power stations, renewable generators, and specialised equipment on the grid. This includes instructing generators to adjust their output, switching reactive power devices like capacitors and reactors on or off, and using advanced monitoring systems to track conditions across the network. How it works Voltage reduction is implemented in stages, with stage one involving a 3% reduction and stage two a further 3% to a maximum total of 6%. The principle is simple: reducing voltage decreases the power consumed by devices, thereby reducing overall electricity demand. The devices we have at home and in businesses respond differently to voltage reduction: • Constant Resistance Devices (e.g., kettles, old heaters): Experience a significant reduction in demand. • Constant Current Devices (e.g., LED lights, chargers): Show a moderate reduction. • Constant Power Devices (e.g., servers, precision industrial equipment): Typically, do not reduce demand. Why don’t we run the electricity system at lower voltage all the time? Running the network at low voltage all the time would waste a large amount of energy lost by heat and make the system inefficient. On the transmission network, electricity can travel in two ways; high voltage with low current or low voltage with high current. To constantly run the network at low voltage, we’d need more current to be generated and carried though the cables. The energy lost when it’s not being pushed through the cables at high current turns into heat in the wires, which is inefficient can put the network at risk. We have one of the world's most secure electricity networks in Great Britain, but operating at low voltage all the time would mean redesigning pylons, substations, cables and wires, which would increase costs. Operating the system at high voltage is always going to be the most efficient way to run the transmission network, getting our electricity moving from where it’s generated to homes and businesses fast, and reducing waste. Does it work? To ensure we know voltage reduction can be an effective way of managing demand we conduct regular tests with DNOs. During these tests, specific DNO groups are instructed to reduce voltage by 3-6% through automated systems. This not only tests communication between us but also validates the expected demand reduction. So far, we’ve conducted successful tests in two phases focusing on Northern and Southern blocks. The tests have consistently shown that voltage reduction leads to a measurable demand reduction, although the extent can vary. On average, a 3% voltage reduction has resulted in approximately a 1.2% demand reduction. More detailed results and analysis from these tests are due to be shared soon and we plan to run a test involving all DNOs at the same time to refine emergency processes as well as improve the effectiveness. Voltage reduction is more than just a technical exercise; it's a testament to the collaborative efforts of energy stakeholders to maintain a resilient and operable energy system. As technology and demand patterns evolve, continuous testing and adaptation are essential to meet future energy challenges. Similar reading Security of Supply Strengthening energy security through international… 30 Apr 2026 - 3 minute read On 20 and 21 April, NESO hosted its first International Security Roundtable in London. We brought… Security of Supply Great Britain’s electricity system is prepared for summer 14 Apr 2026 - 3 minute read In this year’s Summer Outlook, you can find out how we expect secure and reliable electricity… Security of Supply Delivering reliability for GB consumers: the Capacity… 20 Mar 2026 - 3 minute read Britain’s electricity system depends on a diverse mix of resources to keep the lights on during… logo--facebook