Power Cut Load Blocks Explained: What Your Block Letter Means

Somewhere in the paperwork held by your electricity network operator, your home has a letter next to it. It runs from A to U, and almost nobody knows theirs. That letter only matters in one very specific situation: if Great Britain ever ran short of electricity so badly that the government had to start switching off supply on purpose, in an organised rota, to keep the grid standing up.

That situation is not happening. It has not happened in this country since the 1970s. But the plan for it is real, it is public, and it is surprisingly detailed — right down to which three-hour window your street would lose power in, on which day, and how bad things would have to get first.

This guide explains what that plan says, where your block letter comes from, and — just as importantly — what the checker above genuinely can and cannot tell you. If you'd rather look at what the grid is doing right now, our UK Grid Live tool shows Britain's live generation mix instead.

What the Electricity Supply Emergency Code is

The Electricity Supply Emergency Code, usually shortened to ESEC, is the UK government's plan for sharing out electricity when there is not enough to go round. The current version was revised in April 2026 and published by the Department for Energy Security and Net Zero (DESNZ), replacing the previous 2019 edition.

It is worth being precise about when it applies, because this is where most confusion starts. ESEC is only used in a declared national electricity supply emergency — a genuine shortfall between the electricity the country can generate and the electricity it is using. It is not the plan for storm damage. It is not the plan for a substation fault. It is not the plan for planned maintenance on your street. Those are all handled by your local network operator, entirely separately, and far more often.

ESEC exists because the alternative to organised disconnection is disorganised disconnection. If demand outstrips supply badly enough, the grid frequency falls and protection systems start tripping plant offline automatically — which can cascade into a much larger, much less controlled blackout. Rota disconnection is the deliberate, planned version: painful, but survivable, and shared out.

The code's own stated aim is that available supplies are "shared as equitably as possible amongst all non-protected sites". Everything else in it follows from that principle.

What a load block is

Every electricity distribution network in Great Britain is divided into 18 groups of roughly equal demand. Each group is about 5% of the network's load. These are the load blocks, and every connected customer is assigned one based on where their supply physically connects to the network.

The blocks are lettered, but not with a straightforward run of the alphabet. The 18 letters actually used are:

A, B, C, D, E, G, H, J, K, L, M, N, P, Q, R, S, T, U

F, I and O are skipped. The code does not explain why, but it follows a long-standing convention in engineering and logistics of avoiding letters that are easily misread: I and O look like 1 and 0, and F can be mistaken for E on a poor-quality printout or a hand-written switching schedule. When someone is reading instructions in a control room during a national emergency, that sort of thing matters.

Three points worth understanding about your letter:

• It follows the network, not the map. Your block is determined by your point of connection, not by your postcode, your council area or your street. Two neighbouring houses can in principle sit in different blocks if they are fed from different points.
• It is specific to your network operator. Great Britain has 14 electricity distribution licence areas, run by six operator groups. A "Block P" in Yorkshire and a "Block P" in Cornwall are not the same set of customers, and are not connected to each other. Each operator runs its own rota procedures built on the same 18-block structure.
• It can change. Networks get reconfigured, new estates get connected, substations get rebuilt. Block assignments are not carved in stone.

Two different rota plans — and which one you would actually get

This is the part that has changed most recently, and it is the single most important caveat on this page.

The April 2026 code sets out two different rota plans, not one:

• The Low-Level Rota Disconnection Plan (LLDP) is for short disconnections lasting hours rather than days — typically targeting the morning and evening demand peaks, when a relatively small percentage reduction is needed.
• The Variable Rota Disconnection Plan (VRDP) is for sustained disconnection running continuously across a full 24-hour cycle, over a longer period, when a much larger percentage reduction is needed. This is the plan published as Annex 1, and it is the one the checker above is built on.

And here is the crucial sentence, from the code itself: "In most scenarios, it is expected that the LLDP will be used."

The LLDP works completely differently from the published table. Rather than following a fixed rota, it simply moves sequentially through the blocks starting at A, and it carries over between events. If block K was the last one disconnected during one emergency, the next incident starts at block L. That design spreads the burden fairly over time, but it means your position in the queue depends on something nobody can know in advance: where the previous event happened to stop.

So the honest position is this. The grid published in this tool shows the VRDP — the plan for a serious, sustained, higher-percentage emergency. That is the scenario the government has published a detailed public table for, and it is genuinely informative. But for a shorter, milder event, which is the more likely of the two, your block's position would be set by the LLDP's rolling sequence instead, and no table can tell you that ahead of time.

Both plans use the same 18 load blocks and the same eight three-hour periods. Your block letter is useful either way. It is the timing that differs.

How the Variable Rota Disconnection Plan decides who goes first

The day is divided into eight periods of three hours each. They do not start at midnight. The rota day begins at 00:30, so the periods run 00:30–03:30, 03:30–06:30, and so on through to 21:30–00:30. That last window is the reason the checker above marks some cells "+1" — it starts one evening and finishes after midnight the next day.

For every one of those eight periods, on every one of the seven days of the week, Annex 1 publishes an ordered list of all 18 blocks. That is 56 separate orderings. The government sets a severity level from 1 to 18 depending on how big the shortfall is, and the rule is simple:

At severity level n, the first n blocks on that period's list are disconnected.

So at level 1, one block in 18 is off during each period — roughly 5% of demand. At level 6, six blocks are off, about a third. At level 18, every block is off: a total shutdown of non-protected supply.

Because every one of those 56 lists contains all 18 letters, every block appears in every period. There is no block that is permanently safe at a given time of day. What differs is how far down the list your block sits — in other words, how bad the shortage has to get before it reaches you. That is exactly what the numbers in the checker's grid are showing you.

The decision-making runs on a daily cycle. The National Energy System Operator (NESO) advises government by 14:00 each day on the rota level it thinks is needed for the following 24 hours; DESNZ confirms the plan by 15:00; and detailed activation schedules go out to the network operators by 17:30 the evening before.

What each severity level means

The levels are not a vague scale of alarm. They are a direct measure of how much demand has to be removed, divided into 18 roughly equal slices.

Level 9 is the worst case for the network operators themselves, because nine blocks have to be switched off while nine different blocks are switched back on simultaneously. The code notes that this changeover can take up to an hour, which is one reason the published timings are described as nominal.

The levels above that describe scenarios that would be a national crisis by any measure. They exist in the plan for completeness, not because anyone expects to use them.

Why your block letter tells you which half of the week

Here is the genuinely useful pattern in the published table, and it is not something the code spells out in plain language — it emerges from the tables themselves.

The code states that the VRDP is "designed to concentrate disconnection periods on the three days (i.e., Monday, Tuesday and Wednesday or Thursday, Friday and Saturday), for any one Load Block". The idea is fairness over time: if you are going to lose power repeatedly, it is better to lose it on a predictable few days than scattered unpredictably across every day of the week.

When you work through the published tables, that split turns out to be extremely clean, and it falls almost exactly down the middle of the alphabet:

• Blocks A to K take their low-severity disconnections on Monday, Tuesday and Wednesday.
• Blocks L to U take theirs on Thursday, Friday and Saturday.

At the lowest levels the separation is absolute — there is no crossover at all. Sunday sits outside the split entirely, which is consistent with the code naming only those two three-day groups.

So if your letter is in the first half of the alphabet, you are in the early-week group. If it is in the second half, you are in the late-week group. It is the single most memorable thing your block letter tells you.

Who is protected, and who is not

Not everybody is on the rota. ESEC sets out a category of protected sites, which are kept on supply for as long as physically possible. A site qualifies if it meets one of three tests: a national or regional critical need, a public health and safety issue, or the potential for catastrophic damage to high-value plant.

In practice that covers hospitals, water and sewerage treatment, airports and ports, gas terminals, telecommunications infrastructure, some food manufacturing, and industrial processes that would be destroyed rather than merely paused by a sudden loss of power — think glass furnaces or aluminium smelters.

Protected sites are recorded on a Protected Sites List (PSL) held by each network operator. Under the 2026 code they are simply excluded from the load blocks altogether. Older records — including the dataset behind this checker — used the block letter V to mark them, which is why a V result here means "protected", not "block V".

Two important caveats, both of which the checker will tell you if your postcode comes back as protected:

• Protection attaches to a site, not a postcode. If a hospital shares your postcode, the hospital is protected. Your flat across the road may not be.
• Protected does not mean unaffected. The code expects protected sites to keep supply but still to reduce their electricity usage during disconnection periods.

The Protected Sites List is a matter for large infrastructure, and it is not something a household can join. The household equivalent is the Priority Services Register, which is a different thing entirely and genuinely worth signing up for — see the preparation section below.

How this differs from an ordinary power cut

If your power is off right now, this page is almost certainly not the reason. Ordinary power cuts and ESEC rota disconnections have almost nothing in common beyond the symptom.

If you have lost power unexpectedly, call 105. It is free, it works from any phone in Great Britain, and it connects you to whichever network operator covers your area. You can also report and track outages through Power Cut 105 or your operator's live outage map — those are the genuine live tools, and this page is not a substitute for them.

Has this ever happened in Britain?

Rota disconnections on this pattern were last used in Britain in the 1970s, during the industrial disputes that led to the Three-Day Week in early 1974 and the power cuts of the preceding years. Anyone old enough to remember candles on the kitchen table and a television that shut down early is remembering a version of this system in action.

Since then, the plan has been maintained, revised and rehearsed, but not triggered. It came closest to public attention in late 2022, when the electricity system operator published a winter outlook that included — as an unlikely scenario rather than a forecast — the possibility of rota disconnections if gas supplies to Europe were severely constrained. That scenario did not come to pass. The searches that spiked at the time are a large part of why people still ask what block they are in.

It is worth holding both facts at once: the plan is real and detailed, and it has sat unused for half a century. If you want a sense of how comfortable the margin usually is, our live grid tool shows generation against demand in close to real time, and our energy news covers the policy side.

What this checker can and cannot tell you

We would rather be straightforward about the limits than let the tool imply more precision than it has.

What it can tell you: which load block a postcode was recorded as being in, and — from the government's published Annex 1 — the severity level at which that block would first be disconnected in each three-hour window under the Variable Rota Disconnection Plan.

What it cannot tell you:

• Which plan would actually be used. As set out above, the code expects the Low-Level plan to be used in most scenarios, and that one follows a rolling sequence rather than a published table.
• Exactly when your power would go off. The three-hour windows are nominal. Network operators stagger the actual switching to avoid slamming huge blocks of demand on and off at once, and real timings can move by up to an hour.
• What your own network operator would do. All 14 licence areas run their own rota procedures. We apply the single national default sequence to every postcode because that is the best available public information, but your operator's plan is the authoritative one.
• Whether you would definitely lose supply. The code explicitly allows operators to keep areas containing significant embedded generation connected, even where the published plan says otherwise — and notes that this protection is not guaranteed either.
• Anything about Northern Ireland. ESEC is a Great Britain instrument. Northern Ireland's network is run by NIE Networks under separate arrangements, with its own system operator and its own emergency planning. There is no GB load block for a Northern Irish postcode.
• Anything current. There is no live feed of ESEC activation, because ESEC is not activated.

About one in twelve mainland GB postcodes in our data has no usable block letter at all. Where that is the case we say so rather than guessing, because a wrong letter would hand you the wrong days entirely — which is worse than no answer.

How to prepare without panicking

The honest advice here is that this is not a scenario worth losing sleep over. It is, however, a reasonable prompt to do the sort of basic preparation that is equally useful for a storm — which is a far more likely reason to lose power.

• Join the Priority Services Register. If someone in your home relies on powered medical equipment, is of pensionable age, or has a disability or chronic illness, this is by far the most useful thing on this list. It is free, and both your supplier and your network operator keep one. Ofgem explains how to join your supplier's Priority Services Register.
• Know your operator and save 105. It works for every power cut, whatever the cause.
• Keep a torch that is not your phone. Head torches are better than hand torches if you are trying to do anything.
• Have a battery pack for your phone, charged. Mobile masts have their own backup, but it is finite.
• Know how your heating behaves. Most gas boilers need mains electricity for the pump and controls, so a power cut usually means no heating either — worth understanding whatever your gas tariff looks like.
• Think about what is in the freezer. A full freezer left closed holds temperature for around 24 hours.

None of that is specific to rota disconnection. That is rather the point: the preparation that helps in an organised national emergency is the same preparation that helps on an ordinary windy Tuesday.

If this has prompted you to look at your energy set-up more broadly, our bill checker will tell you whether you are overpaying, and you can compare electricity deals, compare gas or look at dual fuel tariffs from there. Households generating their own power are less exposed to all of this, which is part of the appeal of green energy tariffs and home generation — our home energy guide covers the options. Small businesses have their own continuity considerations, covered in our business energy section, and there are more explainers in our gas and electricity guides.

Where our data comes from

Two separate sources sit behind this tool, and they have different standing.

The rota schedule — the table of which block goes when, at which severity level — comes directly from Annex 1 of the Electricity Supply Emergency Code, revised April 2026. That document is Crown copyright and is reusable under the Open Government Licence v3.0. We generate our copy of the table directly from the published document rather than transcribing it by hand, and we check it automatically against the structural rules the document itself guarantees, so a copying error cannot quietly creep in. We also compared the April 2026 table against the 2019 edition it replaced: the 56 rows are identical, so the sequencing itself has not changed.

The postcode-to-block mapping is a separate dataset, collected in October 2022, and it is not covered by that licence. It is now several years old. Postcodes created since then are missing, and network changes since then are not reflected. Treat the block letter as indicative rather than authoritative.

If you need a definitive answer about your own supply — particularly if you are making decisions about backup power, medical equipment or a business continuity plan — your electricity distribution network operator is the organisation to ask. They hold the real list. You can find out who yours is by calling 105, or from the gas and electricity hub if you want to start with your tariff instead.