What is an emission factor API?

A web service your software calls with an activity, a unit and a year, such as 1,000 kWh of UK electricity in 2026, which returns the emission factor and usually the kg CO2e. It saves you loading and updating the government spreadsheets yourself.

Where do official UK emission factors come from?

The Department for Energy Security and Net Zero (DESNZ) publishes the UK government greenhouse gas conversion factors for company reporting each June, under the Open Government Licence v3.0. The 2026 set was published on 11 June 2026, with a full spreadsheet and a flat file for automatic processing.

Can I put the DESNZ factors in a commercial product?

Yes. The Open Government Licence lets you copy, adapt and exploit the information commercially, including in your own product or application, as long as you include the attribution statement, link to the licence where you can, and don't suggest the government endorses you.

Which year's factors should my software use?

The set for the reporting year the activity falls in, not simply the latest. Store the factor year and factor ID with every result, so a past report can be reproduced and doesn't change when a new set is published.

Can every carbon dataset be resold through an API?

No. Licences differ. The DESNZ factors (Open Government Licence) and Ember's electricity data (CC BY 4.0) can be used commercially with attribution, but NESO's live Carbon Intensity API terms forbid selling, leasing or sublicensing the API or access to it.

Emission factor APIs: putting official UK factors into your own software

An emission factor API returns the official factor, and usually the kg CO2e, for an activity, unit and year, so your own software doesn't have to load and update the government spreadsheets. For UK activities the source is the DESNZ conversion factors, published each June under the Open Government Licence v3.0; the 2026 set changed UK grid electricity by 26%, which is why every result must be stored with the factor year it used.

Updated 19 October 2026 by the Carbon Recycling team. Figures use the official UK government conversion factors and are checked against the current gov.uk guidance.

What an emission factor API does

An emission factor turns an activity into emissions: kWh of electricity, litres of diesel, miles driven or nights in a hotel, multiplied by a factor in kg CO2e per unit. An API (application programming interface) is a way for one piece of software to ask another for that factor over the web. Your software sends something like "electricity, UK, kWh, 2026" and gets back the factor, its unit, the year of the set it came from and, if you send a quantity, the kg CO2e.

Who needs one: software companies adding carbon figures to an expense, fleet, energy, procurement or accounting product; in-house teams building carbon into their own systems; and consultants and accountants who want the same figures in their own spreadsheets and tools every time. If you just want your own footprint, you don't need an API; see our guide to calculating your business carbon footprint.

The official datasets

UK: the DESNZ conversion factors

The UK government's greenhouse gas conversion factors for company reporting are published by the Department for Energy Security and Net Zero each June. The 2026 set came out on 11 June 2026, as a full spreadsheet for people and a flat file "for automatic processing only", alongside a methodology report and a major changes report. They cover Scope 1, 2 and 3 activities, and GOV.UK lists them as relevant to the government's reporting guidelines, including SECR. GOV.UK says they may be used for other purposes "but users do so at their own risk". Carbon Recycling holds every set from 2002 to 2026; see UK emission factors back to 2002 and what changed in the 2026 factors.

They come under the Open Government Licence v3.0. Our post on using the UK conversion factors in your software goes through what the licence allows clause by clause.

Electricity outside the UK

DESNZ's factors are for UK activity. For sites in other countries, Carbon Recycling uses each country's own government figure where one is published under a licence we have checked, currently Australia, Canada, China, France, Germany, New Zealand and Singapore, and otherwise Ember's yearly electricity data, an independent dataset released under CC BY 4.0. US sites use the US EPA's eGRID figures by power region, which as a US government work are in the public domain. The electricity factors by country page lists them, and our post on official overseas electricity factors explains the switch to government figures.

Data that can't be resold

Not every carbon dataset can go into a product you charge for. NESO's Carbon Intensity API, which gives live and forecast carbon intensity for the GB grid, is free to use, but its terms say you cannot "sell, lease, or sublicense the Carbon Intensity API or access to it". We show it free in the app and never sell it. Other datasets that came up in our research were ruled out because of their licence or cost. Our data sources page lists every source we use, what its licence allows, and the ones we don't use.

The rule of thumb: before you put any dataset behind an API, read its licence for three things: commercial use, redistribution of the figures themselves, and the attribution it requires.

Build your own or use an API?

The DESNZ spreadsheet is free, so building your own lookup is a real option. It comes down to how many factors you need, how many years, and who keeps it up to date.

	Build from the spreadsheet	Use an API
Best when	You need a handful of factors (say electricity, gas and car miles) for one or two years	You need many activities, past years, other countries, or the same figures in several products
Each June	Download the new set, map any changed rows and IDs, test, release	The provider loads the new set; you choose when to use it
Corrections	You have to notice them. In July 2026 DESNZ republished the 2026 flat file because some blank values had been shown as 0	The provider should apply them and say what changed
Other countries	Find, licence-check and load each source yourself	Depends on the provider: check each figure's source and licence is named
Cost	Free data, plus your developers' time every year	A subscription

If you build your own, use the flat file for loading and the full set for reading the notes: the guidance on each worksheet explains when each factor applies, and that is where most mistakes come from.

Version by factor year

This is the part most home-built tools get wrong. Factors change every year, and some change a lot. In 2026 DESNZ also changed how it works out the electricity factor, cutting the lag behind the real grid from two years to one. 100,000 kWh of UK grid electricity is 17,700 kg CO2e with the 2025 factor (0.177 kg per kWh) and 13,096 kg with the 2026 factor (0.13096 kg per kWh), 26% less for the same use. Our post on why footprints dropped in 2026 covers what that means for reports.

Ask for a year, every time. Pick the factor set for the reporting year the activity falls in, not "latest". Our conversion factors guide explains which set to use for a year that spans two calendar years.
Store what you used. Save the factor value, unit, factor ID, set year and source with every calculated line. A report from last year must reproduce exactly, and an auditor or customer may ask.
Never overwrite silently. When a new set or a correction arrives, recalculate past years only on purpose, for example to restate a baseline, and record that you did.

Attribution

Every open licence here asks you to credit the source, in your product or application. The wording we use, from our data licence register:

DESNZ conversion factors: "Contains public sector information licensed under the Open Government Licence v3.0. Source: DESNZ, Greenhouse gas reporting: conversion factors."
Ember: "Contains data from Ember, licensed under CC BY 4.0 (ember-energy.org)."
US EPA: "Source: US EPA, eGRID and GHG Emission Factors Hub. Not required by law, but we credit it."
NESO Carbon Intensity API (shown in our app, not resold): "Carbon intensity data from the National Energy System Operator's Carbon Intensity API (carbonintensity.org.uk), CC BY 4.0."

Where listing every credit isn't practical, the Open Government Licence allows a link to a page that holds them all, which is what our data sources page is for. Whichever route you take, don't use government logos or suggest the government or any data publisher endorses your product.

How Carbon Recycling helps

Inside Carbon Recycling, every calculation already picks the factor set for its reporting year and stores the factor it used, from 2002 to 2026, with each source credited on the data sources page. The Factor API add-on, coming soon, will put the official UK conversion factors and electricity factors for other countries into your own software through a simple API. Like every add-on, it sits on top of a paid plan.

If you're an accountant or adviser putting factors into client work, the Accountant portal, also coming soon, lets you run carbon reporting for your clients from one login. Our guide to carbon reporting for accountants covers the service itself.

Where this fits

Start free: kWh to CO2e calculator
The plan: Starter, £59 + VAT a month. An add-on needs a paid plan underneath. Starter gives you the app with the same DESNZ factors, so you can check API results against a full calculation.
The add-on: Factor API, £49 + VAT a month for 10,000 calls, or £199 for 100,000, coming soon. The official UK conversion factors and electricity factors for other countries, in your own software through a simple API.
Also: Accountant portal. For firms advising clients: run carbon reporting for each client from one login, with a dashboard of where each one is up to.

Including VAT at 20%: Starter £70.80 a month; Factor API £58.80 a month for 10,000 calls, or £238.80 for 100,000. Each add-on is its own subscription on top of a paid plan: a 12-month minimum, paid monthly, or yearly upfront for 10 months' price. The 14-day free trial lets you add data and see your reports, but printing, saving and downloading need a paid plan.