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A guide to green energy

By Nicole Wilson

Let’s take a look at renewable energy from source to supply. Where does it come from? How is it sold? And how does it end up powering our homes?

Illustration of a solar farm and some happy sheep

What is renewable energy?

Illustration of a lightning bolt with renewable arrows around it
Energy is a catch-all term for the gas and electricity we use at home. Renewable energy comes from resources that are constantly replenished, like sunlight, wind, water and - less glamorously - waste.

It’s also known as ‘green’ energy, because generating it doesn’t rely on burning fossil fuels. Generating energy from fossil fuels uses finite resources like coal and releases harmful greenhouse gases into the atmosphere, which contribute to warming our planet.

How is green electricity generated?

The energy regulator Ofgem is responsible for defining the types of electricity generation that are considered renewable in the UK. And there are a few ways to generate green electricity at scale. Wind, bioenergy, solar and hydro power all contribute to the UK’s fuel mix. In fact, around 40% of the UK’s electricity comes from these sources.

1. Wind

Wind power makes up a whopping 26% of the electricity generated in the UK. Wind turbines harness the movement of the wind to turn their blades and spin a rotor. This rotor is attached to a generator, which creates electricity. You might see a turbine out on its own, but you’re more likely to see one as part of a wind farm, either on shore or at sea. Electricity generated by turbines can be used to power local properties, or it can be directed straight into the grid. All in all, it’s a good reason not to complain about the weather.

2. Bioenergy

Electricity from bioenergy accounts for around 12% of the UK’s electricity (but 0% of our electricity at Bulb). It can be generated in three ways:

a) Biomass

Biomass is an umbrella term for any living material. It can be used as a fuel to produce green electricity through combustion. Burning organic fuels – either purpose-grown crops like sugarcane and wood pellets, or waste products from farming, food and people – creates heat and steam, which turns a turbine and produces electricity. Burning biomass does release carbon dioxide, but is considered renewable by Ofgem because it releases the same amount of CO2 that the organic matter absorbed during its lifetime.

b) Anaerobic digestion

Green electricity can also be produced by burning green gas (or ‘biogas’). This gas is created by fermenting the same kinds of organic material in a process called anaerobic digestion. More on that later.

c) Energy from waste and landfill gas

This involves burning any waste from homes and businesses that can’t be recycled. This creates steam, drives a turbine, and generates electricity. Landfill waste can also be left to decompose, which releases biogas that can be captured and burned. Just how green the energy from waste is depends on the efficiency of the power plant, as well as the ‘net calorific value’ of the waste used. This kind of combustion does produce some CO2 emissions, but it's burning waste, not fossil fuels.

Illustration of a happy pink sun looking cool by leaning on a cloud
3. Solar

Solar power uses sunlight to generate electricity, and it makes up nearly 2% of the UK’s electricity mix. Solar panels (also called photovoltaic cells, if you’re feeling fancy) are capable of converting light into electricity directly. In sunnier parts of the world, Concentrated Solar Power (CSP) is a system which directs and magnifies sunlight using mirrors and lenses. This uses thermal energy from the sun to make steam, and generates electricity through a turbine.

4. Hydro

Hydropower harnesses the kinetic energy of falling or flowing water, and makes up a small fraction of the UK’s electricity generation (around 2%). Hydroelectricity can be generated in a few ways, but the idea is to flow or force water through a turbine. Large-scale sites do this with a system of pumps, dams and reservoirs, while small-scale plants rely on diverting the natural flow of a river. Even the smallest hydro projects can have a big impact, just ask our friend Tegwyn at the Tyn Y Cornel hydro plant in Snowdonia.

At Bulb, we supply 100% renewable electricity to all of our members from solar, wind and small-scale hydro sites. We choose not to purchase power generated from burning biomass or waste, or from large scale hydro sites. Find out why in our electricity sourcing policy.

How do suppliers buy green electricity?

Some energy providers generate their own renewable power and sell it on to customers. For suppliers without their own renewable generators, there are two main ways to buy green electricity.

1. Using Power Purchase Agreements

Also known as PPAs, these are contracts where suppliers agree to buy electricity directly from generators. Alongside each unit of power, suppliers buy a Renewable Energy Guarantee of Origin (REGO) certificate to verify it was from a renewable source.

Illustration of a lightning bolt with a sold sign

2. On the wholesale market with Renewable Energy Guarantee of Origin certificates (REGOs)

Suppliers can also buy electricity on the wholesale market. For every unit of power bought wholesale, green suppliers buy REGOs to match. Renewable generators are awarded REGO certificates by Ofgem, the energy regulator. They receive one certificate per megawatt hour (MWh) of renewable electricity generated. By matching with REGOs, suppliers can ensure that every unit of electricity they supply can be traced back to a renewable source.

How does renewable electricity get into our homes?

Green suppliers can’t deliver green electricity to your property directly from a wind or solar farm. It’s all delivered through the National Grid, a melting pot of different types of energy. The National Grid takes all the power produced by generators and moves it around the UK according to demand.

We often get asked how someone can switch their home to green energy without changing all the wires and meters. The truth is that the energy coming into your home won’t change when you switch supplier. It’ll come from the grid, just like your neighbours’. The green bit happens at the other end, when suppliers are buying your energy.

No supplier can guarantee the origin of the electricity that’s powering your particular kettle during Eastenders. But what they can do is buy green energy on your behalf through Power Purchase Agreements or on the wholesale market with REGOs.

The higher the demand for green energy from customers, the more money can be invested into the UK’s renewable energy marketplace as a whole. It’s all part of the bigger picture to green up the grid and reach Net Zero emissions by 2050, if not before.

What about green gas?

You might have noticed that we’ve been talking about renewable electricity a lot. That’s because the amount of green gas generated in the UK is still very small. In fact, there’s only enough available to meet around 1% of the UK’s demand. The only viable way to produce green gas at the moment is through anaerobic digestion, though there are hopes for new green gas technologies in the future.

An illustration of a little ear of corn breaking wind
Anaerobic digestion

This is a type of generation inspired by a cow’s stomach. It starts by feeding organic matter, either purpose-grown crops or manure, into a large air-tight container (the word ‘anaerobic’ means ‘without oxygen’). The contents are stirred around at warm temperatures so that bacteria can thrive. These bacteria break down the material and produce a green gas, also known as ‘biogas’ – primarily biomethane. Green gas can either be put onto the grid to use in cooking and heating at home, or burned in a generator to create electricity.

Lots of us rely on gas to heat our homes. We’d love to supply 100% green gas to our members at Bulb, but the industry is still very small.

We source a small percentage of our gas from green generation plants like Huggin farm, and we offset the gas that we can’t buy green by supporting verified carbon reduction projects across the world. You can read more about why we think this is a good thing to do in our guide to carbon offsetting.

We also help our members to offset their own carbon footprint - take a look at our Carbon Calculator to work out your impact.