What's the difference between green energy suppliers?

Energy companies that offer tariffs with 100% renewable electricity have different approaches to providing it. 

There are fewer firms claiming to sell 100% renewable electricity than in previous years – and when it comes to sustainability, they don't all take the same approach, either.

In August 2024, we asked energy companies in Great Britain to tell us more about the green electricity and gas they sell to homes, as well as other sustainable practices. 

As a result, we named Ecotricity,  Good Energy and Octopus Energy our Eco Providers for energy for this year.

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Can I get 100% renewable electricity to my home?

The source of the electricity that comes by wire into your home depends on where you live, but also on how much renewable generation is happening across the country at the time. On average, around 43% of our electricity nationally now comes from renewable sources.

Average UK fuel mix

1. Renewables - 43.2%
2. Natural gas - 35%
3. Nuclear - 12.7%
4. Coal - 6.3%
5. Other fuels - 2.8%

Source: Department for Energy Security and Net Zero's UK fuel mix; figures from April 2023 to March 2024.

The next released overall fuel mix is likely to look different, after the UK's last coal-fired plant closed in September 2024.

If you buy a 100% renewable tariff, it does not change the electricity that comes into your home. But it does change what part of the energy market you are paying for.

Want to start generating your own renewable electricity? Find out more about solar panels and home energy storage.

What is green gas?

Green gas is biomethane. It accounts for a very small proportion of the gas in our grid – most of it is natural gas.

To make biomethane, organic materials, such as food waste and farm waste, are broken down by bacteria in huge tanks, or digesters. The breakdown happens without oxygen, called anaerobic digestion, and the process produces biogas.

Biogas is then purified and the CO2 is removed, turning it into biomethane. Biomethane has similar characteristics to natural gas, so it can then be added into the gas grid and used in our homes. 

It can also be produced from landfill gas and the gasification of biomass. 

Biomethane is carbon neutral. It releases emissions when we burn it to heat our homes but is made from organic materials that absorbed carbon dioxide when they grew. Burning it releases the same amount of carbon dioxide, so, overall, it doesn't add more CO2 to the atmosphere.

Several energy companies sell green gas alongside renewable electricity. They include:

As with renewable electricity, green gas is not piped directly into you home if you buy a tariff that includes it. The difference with a green gas tariff is that you're paying extra towards the green gas that is added into the grid mix.

Like renewable electricity, there are certificates to show where green gas was produced. They are called renewable gas guarantees of origin (RGGOs) and biomethane certificates. 

If you’re looking to cut the carbon footprint of your heating, find out more about heat pumps.

What are the rules for renewable energy tariffs?

There is no set definition of what a renewable or green tariff is, and companies take a variety of approaches. 

To make an ‘environmental claim’ about a tariff, an energy company must:

• Show that the environmental gain is because the customer chose the tariff;
• Publish its fuel mix and information about the environmental benefit of the tariff;
• Be able to prove where its renewable energy has come from, by having enough certificates.

To sell a tariff labelled as 100% renewable electricity, an energy firm must buy enough certificates for renewable energy to match what customers on the tariff use over a year.

If a company has bought enough certificates to match all of its customers’ electricity use (on all tariffs), it will be able to say that its overall fuel mix is 100% renewable. These certificates prove that a megawatt hour of renewable electricity has been generated. They're called renewable energy guarantees of origin (REGOs).

But these certificates don’t prove that the company has generated any renewable electricity itself, nor bought renewable electricity directly from a generator. Suppliers can buy them without buying the associated renewable electricity.

Different approaches to renewable energy tariffs

There are several different approaches that energy companies take to provide '100% renewable' electricity tariffs. These are:

• Providing renewable power from generators the company operates to meet customer demand;
• Helping customers to buy stakes in renewable generation and matching their consumption to the output (for example, a customer buys a stake in a wind farm) – this is rare;
• Having a contract or trading deal to take the output of a renewable generator and incorporate it into a tariff;
• Matching customer consumption against renewable output via traded certificates (such as REGOs).

Many companies use a combination of these.

The Committee on Climate Change says that the ability to 'unbundle' REGOs from the power generated 'could mean that the supplier of the green tariff is not actually purchasing renewable electricity, but it is simply purchasing the certificate'.

Find out whether your energy company generates renewable electricity and its other sustainable practices in our 2025 energy company reviews.

Does green energy cost more?

It sometimes costs more to buy your electricity from companies that have more direct links to renewable generation. 

Many renewable generators receive government subsidies. But some energy firms buy from generators that don't have subsidies, such as smaller generators. This means that they're helping support renewable projects that might not otherwise exist. This can come at an extra cost.

Three energy companies are exempt from the price cap on default energy tariffs because they have proved to energy regulator Ofgem that they have higher costs because they support renewables, that they support renewables beyond existing subsidies, and that customers have actively chosen to buy them. 

This means that they can charge more, and their tariffs are often among the priciest available. They are:

• 100Green
• Ecotricity
• Good Energy

Green gas is more expensive than renewable electricity and there is less of it around. 

Understand the energy price cap and what it means for your bills.

How do you choose a sustainable energy company?

First, be clear about what you want or expect from the company you buy from. Then, look out for the following to help you:

• Does the company say it owns renewable generation, such as solar or wind farms?
• Can you tell if the company buys power (as well as certificates) from generators? Sometimes you’ll be able to find a section where the company asks generators to get in touch about selling power to it.
• If the company sells 100% renewable electricity, is there a clear explanation to prove this? 
• How does it help customers cut carbon and live more efficiently? Many install solar panels, batteries, heat pumps, insulation and EV chargers; others offer time-of-use tariffs to encourage you to use power at less carbon-intense times; or they incorporate other efficiency initiatives.

Don’t be swayed by environmentally friendly images, designs and phrases if they’re not backed up by factual information.

Which? Eco Providers for energy 2024

It’s often hard to tell what a company is really doing behind the scenes to support renewable energy and be more sustainable. So, in August 2024, we asked energy suppliers in Great Britain to tell us exactly how they sourced their renewable energy – and we assessed other aspects of their sustainability. 

Three companies were awarded Eco Providers for Energy: Ecotricity, Good Energy and Octopus Energy. 

All scored full marks for buying and selling renewable power and for their greenhouse gas emissions.

100Green narrowly missed out this year. It's still among the best for buying and selling renewable power, and the only one to sell 100% green gas. But customers can't get carbon-cutting tech installed by it and it doesn't yet calculate its scope 3 emissions, unlike higher-scoring firms.

The table below details the results of our findings. Swipe or scroll right to see the total score breakdown in full.

How we score energy supplier sustainability 

Generating, buying and selling renewable power is a key part of energy supplier sustainability, and worth half the points in our assessment.

But these aren't the only factors. This year, we looked at companies' greenhouse gas emissions, as well as other criteria, to get a fuller picture of suppliers' overall environmental impact.

We awarded points for:

• Buying and selling renewable power: Companies were awarded points for buying renewable power directly from generators through PPAs or similar contract arrangements, and additional points for whether it was from generators that did not receive government subsidies. Companies also gained points for supplying a greater percentage of renewable electricity than the fuel mix average, and for supplying 100% renewable electricity. 
• Generating renewable power: Points were awarded to those that generate renewable electricity. 
• Fossil fuel links: Points were deducted for generating electricity from fossil fuels or buying directly from fossil-fuel generators. We also took into account whether a supplier's parent company does this.
• Green gas: We gave a point for selling green gas, and another one if it it made up more than 5% of gas sold. 
• Time of use (TOU) products and SEG (Smart Export Guarantee) tariffs: Firms received points if they offered one or more ToU tariffs. These encourage customers to use energy at less popular times, which is when the National Grid has the most renewable energy in its mix. Suppliers that had above-average SEG rates for domestic generators, such as those with solar panels installed, also earned a point. We calculated this based on responses received.
• Low carbon installations: Suppliers earned points for providing customers with ways to reduce their home's carbon footprint by installing heat pumps, solar panels, battery storage, insulation and EV chargers. 
• Greenhouse gas emissions: We looked at companies scope 1 and 2 (location-based emissions) and whether they were able to provide scope 3 emissions data.

Other sustainable features of energy companies

Besides how they source and sell electricity and gas, companies have other approaches to improving their sustainability. These include:

• Sustainable accreditation, such as Bcorp or Vegan Society
• Encouraging sustainable behaviour through app features, schemes or including smart thermostats in their tariffs
• Making charitable donations
• Participating in community or charity energy programmes (such as involvement with solar panel community projects)
• Tree planting.

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