Energy data

What a Green Tariff Actually Buys

Contents

The residual mix, beside the average

There is one electricity grid, and everybody is connected to it. The power arriving at your home came from whatever happened to be generating at the time — so no supplier can send you renewable electricity specifically, and none of them claim to when pressed.

What a green tariff buys is a claim on the national total. Every unit of renewable generation earns a certificate, suppliers buy and retire those certificates against their customers’ usage, and once a unit is claimed nobody else may count it. The electricity left over — unclaimed by anyone — is the residual mix, and it is what your supply is officially counted as if your tariff makes no green claim.

The government publishes both figures once a year. They are strikingly different.

Disclosure period 2025/26 · 1 Apr 2025 to 31 Mar 2026

What the electricity nobody claimed is actually made of

Across the United Kingdom, electricity supplied over this period was counted as 48.0% renewable. Once every renewable certificate had been allocated to a supplier selling a green tariff, what was left for everybody else in Great Britain was 10.8% renewable. That leftover is the residual mix, and it is what your electricity is counted as if your supplier makes no green claim.

The residual fuel mix for Great Britain beside the United Kingdom average fuel mix, by fuel, for disclosure period 2025/26.
Fuel	Residual mixGB · no green claim	EveryoneUK average	Difference
Natural gas	65.3%	31.4%	+33.9 pts
Coal	11.4%	4.7%	+6.7 pts
Renewables	10.8%	48.0%	−37.2 pts
Nuclear	7.1%	13.7%	−6.6 pts
Other fuels	5.4%	2.2%	+3.2 pts

The residual mix covers GB because Fuel Mix Disclosure is a Great Britain supply licence condition; the average covers the UK. Part of the difference between the columns is that change of boundary, not only the certificate accounting.

Carbon intensity by fuel

Averaged over everything the United Kingdom generated, 147 grams of CO₂ per kilowatt hour in 2025. These are generation emissions only — not the fuel supply chain, and not imports.

Other fuels969 g/kWh
Coal2024 figure — none published for this year921 g/kWh
Natural gas365 g/kWh
Nuclear0 g/kWh
Renewables0 g/kWh

Where a fuel shows an earlier year, the publisher gave no figure for this one — for coal, because no coal-fired electricity was generated in the United Kingdom at all — and directs that the earlier value be used. It is shown with the year it belongs to rather than restated as this year’s.

Transmission and distribution losses 2025/26: ×1.1098; Every kilowatt hour that reaches a meter needs about 11% more generated, because that much is lost in the wires on the way.
High-level radioactive waste: 0.006 g/kWh; Published alongside the carbon figures, across all United Kingdom generation rather than nuclear alone.

This is not the generation mix published in DUKES or Energy Trends. It is compiled on an attribute basis, over electricity supplied rather than generated, with imports allocated using the European Attribute Mix — so it is a different measurement of the same year and the two will not agree.

DESNZ replaces this table each August rather than adding to it, so only the current disclosure period is published. This page will build a history from 2025/26 onwards as each new period lands.

Contains public sector information licensed under the Open Government Licence v3.0 Fuel mix disclosure data table · Open Government Licence v3.0

How a certificate makes electricity green

When a wind farm generates a megawatt hour, Ofgem issues it a Renewable Energy Guarantee of Origin — a REGO. That certificate can be sold with the electricity or separately from it. A supplier that retires enough REGOs to cover what its customers used may describe that supply as 100% renewable, regardless of where the electricity itself was bought.

This is why a supplier with no wind farms and no renewable contracts can sell a fully renewable tariff. Certificates have generally traded for a small fraction of the cost of the electricity they describe, so the green label can be a modest line on a wholesale invoice rather than a change to what gets built.

The consequence shows up in the numbers. As more supply has been claimed, the renewable share left in the residual mix has at times fallen almost to nothing: DESNZ put it at 0.2% for the 2023/24 disclosure period and 0.3% for 2024/25, while the country’s own renewable generation kept climbing through both. The two figures moved in opposite directions because they are measuring different things — one what was generated, the other what was left unclaimed.

What this does and does not tell you

It does not tell you your electricity is dirty. It is the same electricity as everybody else’s, and Britain’s actual generation mix is cleaner every year — coal-fired generation reached zero in 2025.

It does tell you what the label is worth. A tariff backed by bought-in certificates and a tariff backed by the supplier’s own wind farms carry the same “100% renewable” wording and are not the same product. If that distinction matters to you, the questions worth asking a supplier are whether it owns generation, whether it holds long-term contracts with named sites, and whether its certificates came bundled with the electricity or were bought separately.

And if the aim is genuinely lower-carbon electricity rather than a lower-carbon label, when you use power does more than which tariff you are on. That is on when to use electricity in your region.

Why this disagrees with the generation mix

The percentages above will not match the generation figures published elsewhere on this site, and that is not an error in either. This table counts electricity supplied, on an attribute basis, with imports allocated using the European Attribute Mix — a Europe-wide average that carried far more coal in 2025 than Britain did. The generation mix counts what British power stations actually produced.

That import allocation is why coal appears in the residual mix at all in a year when no British coal plant ran. The publisher notes the divergence itself, which is why both numbers appear on this site with the difference stated rather than one of them quietly dropped.

For what was physically on the grid, see Britain’s electricity right now.

Frequently asked questions

What is the residual fuel mix?

It is the fuel mix assigned to every unit of electricity that no supplier has claimed as renewable. Suppliers selling green tariffs retire renewable certificates covering their customers’ usage, and those units are removed from the national pool. The residual mix is what is left in the pool afterwards, and it is the mix your supply is counted as if your tariff makes no green claim.

Does a green tariff mean renewable electricity reaches my home?

No, and no supplier can make that true. There is one grid and the electrons in your wires come from whatever was generating nearby at that moment. What a green tariff buys is a certificate — a Renewable Energy Guarantee of Origin — for each unit you use, retired on your behalf so nobody else can count it. That is an accounting claim about the national total, not a physical claim about your supply.

Is buying a green tariff pointless, then?

Not pointless, but weaker than the marketing suggests. Certificates can be bought separately from the electricity itself and cost a small fraction of a bill, so a supplier can source entirely from gas and still sell a "100% renewable" tariff. Tariffs backed by the supplier’s own generation, or by long-term contracts with specific wind and solar farms, do more than ones backed by bought-in certificates — and the difference is not visible in the label.

Why is there more coal in the residual mix than Britain burns?

Because of imports. Electricity arriving through the interconnectors is counted using the European Attribute Mix, which was 32% coal in 2025 — far more coal-heavy than Britain’s own generation, which included no coal at all. That import share lands in the residual mix and lifts its coal figure well above anything actually burned here.

Where do these numbers come from?

The Department for Energy Security and Net Zero publishes them once a year as the Fuel Mix Disclosure data table, under the Electricity (Fuel Mix Disclosure) Regulations 2005. It is the same table suppliers are required to use when they tell you what their own fuel mix was. It is published under the Open Government Licence.

Why do these percentages not match your generation mix pages?

Because they measure different things. This table is compiled on an attribute basis over electricity supplied, with imports allocated by the European Attribute Mix. The generation mix in DUKES and Energy Trends measures what British power stations actually produced. Both are correct and they will not agree — the publisher’s own note says so.