The 100 MWh threshold: how to work out your heat demand

If you manage a large commercial or public building, one number decides whether heat network zoning is likely to apply to you at all: your annual heat demand. Above the line, your building could be required to connect to a district heating network inside a zone. Below it, under current proposals, you are not covered by that requirement.

This guide explains what the 100 MWh heat demand threshold means, how to get from your bills to a reasonable estimate, and where the grey areas are.

What the threshold is

The government set out who will have to connect in its response to consultation, published 21 January 2026[source]. For existing commercial and public buildings, the relevant category is:

So two tests apply together. The building needs a wet heating system, meaning heat is moved around by hot water through radiators, underfloor pipes or coils in air handling units. And its average heat demand needs to be above 100 MWh a year[source].

Two other categories exist, for new buildings and for existing buildings with communal heating, both domestic and non-domestic[source]. Those don't depend on the 100 MWh figure. This guide is about existing non-domestic buildings.

kWh and MWh

Energy bills almost always use kilowatt-hours (kWh). The threshold is in megawatt-hours (MWh). The conversion is simple: one MWh is 1,000 kWh. So the threshold of 100 MWh a year is 100,000 kWh a year.

To put that in context, a typical home uses a small fraction of that for heating. A building that crosses 100,000 kWh of heat a year is usually a substantial one: a mid-sized office block, a hotel, a secondary school, a leisure centre, a university building or a hospital wing. But size alone doesn't decide it. A smaller building that's poorly insulated or heated for long hours can use more heat than a bigger, efficient one.

Your gas bills will show consumption in kWh even though the meter measures volume, because suppliers convert cubic metres into kWh on the bill. Use the kWh figure.

From fuel to heat demand

Here's the first subtle point. Your gas bill tells you how much fuel went into your boilers. It doesn't tell you how much heat came out. No boiler is perfectly efficient, so the heat delivered to the building is always less than the energy in the gas.

Which figure counts? The policy talks about heat demand, which most naturally means the heat the building actually uses. But the regulations will define the method, and it's possible they will work from metered fuel, delivered heat or some standard conversion. Until then, it's worth working out both.

The efficiency figure is the weak point in that calculation. The number printed on a boiler's data plate is a test figure. Real seasonal efficiency depends on the boiler's age, how it's controlled, return water temperatures and how much it cycles on and off. Your maintenance contractor or an energy consultant can give you a realistic figure for your plant. Don't just pick the number that gets you under the line.

If you already buy heat through a heat meter, for example from an existing communal or district heating system, the meter reading is already close to delivered heat, and you don't need an efficiency adjustment.

Separating heating and hot water from other uses

The threshold is about heat for the building: space heating and hot water. Many sites use gas for other things too. Common examples are commercial kitchens, laundries, swimming pool heating in leisure centres, sterilisation in healthcare settings, and process heat in labs or workshops.

Whether each of these counts will depend on the regulations. For now, try to split your consumption into three piles:

1. Space heating and domestic hot water, almost certainly in scope.
2. Uses that are clearly heat for the building but unusual, such as pool heating. Note these separately.
3. Catering and process uses, such as gas hobs and ovens, which are less likely to count.

The best way to split them is sub-metering. If you don't have sub-meters, you can estimate. Look at summer consumption, when space heating is off: what's left is roughly hot water plus catering and process use. Equipment ratings and operating hours give a rough figure for kitchens. Write down your assumptions, because someone may later ask how you got your number.

Multiple meters and multi-building sites

Many sites have more than one gas meter, and many meters serve more than one building. Before you add anything up, map it out. For each meter, note the meter number, which building or buildings it serves and what it feeds.

Then work out a figure for each building on its own and for the site as a whole. That matters because the regulations will need to say what counts as a building for this purpose. A university campus or hospital estate might have one central boiler house feeding a dozen buildings through its own pipes. On campuses, the government intends to extend the requirement to buildings containing an energy centre that supplies a campus network[source]. So if you run a campus energy centre, assume you're likely to be in scope however the individual buildings add up.

The reverse also happens. One large building with several meters, for example after an extension, may look small on each meter but be well over the threshold in total.

Why the average matters

The policy refers to average heat demand, not one year's figure. That's sensible, because heat use swings with the weather. A cold winter can push consumption up noticeably, and a mild one pulls it down.

So use several years. Three is a reasonable minimum if you have the data. Check whether any year was unusual: a period of partial closure, a major refurbishment, a change of tenant, or a boiler fault that left the building cold for weeks. Note these rather than quietly dropping the year.

If you want to go further, an energy consultant can weather-correct your figures using degree days, a standard measure of how cold a period was. That gives you a figure for a typical year, which is closer to what an average is trying to capture.

When there's no metered data

Some buildings don't have usable data: a new acquisition, a building on a shared meter, or a site where records were lost. The government has thought about this. The detail behind the policy says that where metered data is missing, demand will be estimated using energy-density benchmarks for the building's use type. In other words, a typical figure per square metre for an office, a hotel, a school and so on, multiplied by the floor area.

The benchmark figures themselves haven't been published as part of the zoning regime. What's clear is that a benchmark is an average, and your building may be better or worse than average. If you think a benchmark would overstate your demand, the remedy is to get real data. Fitting a gas sub-meter or heat meter now, and running it for a full heating season, gives you a figure you can stand behind.

Borderline cases

If your figure comes out well above or well below 100 MWh, you have your answer for planning purposes. The hard cases are buildings close to the line.

If that's you, a few things are worth doing. Get the efficiency figure checked properly rather than assumed. Sub-meter the uses that might be excluded. Gather more years of data. And think about what's changing: an energy efficiency project, a change of use or new occupancy patterns could move you one way or the other.

Remember too that the threshold is only one part of the picture. Even if your building is covered, there are grounds for exemption, such as existing low-carbon heating with an expected lifespan of more than 2 years[source]. A building heated only by direct electric panels has no wet system and wouldn't meet the first test anyway, and the proposals also list this ground for exemption: enduring technical incompatibility that cannot be fixed without significant cost, such as direct electric heating[source]. Our exemptions guide covers these.

Keep good records

Whatever your figure, write it down with the workings. Keep copies of the bills or meter data you used, the meter map, your efficiency assumption and where it came from, how you split out catering or process use, and any unusual years. Store it somewhere your successor can find it.

There's no deadline for this yet. No zones have been designated yet, and nobody can be required to connect until the regulations are in force. But if a zone is designated around your building, a well-documented heat demand figure is the single most useful thing you can bring to the table. You can test your building against all the proposed tests with our connection requirement tool, and if you'd like help, independent heat network consultants who do feasibility work can produce a defensible figure for you.