Energy firms have been accused of potentially misleading customers with claims about their 'green tariffs'.

Some firms that claim their electricity is '100 per cent renewable' do not run any wind farms or even pay for a specific amount of power from green sources to match customer demand.

Instead, they can get away with making this claim by simply paying a small annual fee – of only £1.55 per customer per year – to a firm that does generate renewable energy.

Energy firms have been accused of potentially misleading customers with claims about their 'green tariffs'

An investigation by Which? found a long list of firms that all claim to sell '100 per cent renewable' electricity tariffs without generating renewable energy themselves or having contracts to buy it direct from generators.

Instead, they rely on what are effectively accountancy measures that involve paying small fees to buy Renewable Energy Guarantees of Origin (REGO) certificates.

The consumer group said: 'REGO certificates can be purchased by suppliers from renewable energy generators for as little as 30p to 50p per megawatt-hour (MWh).

'With the average customer using 3.1 MWh of electricity a year, a supplier could buy REGO certificates to match this usage for as little as £1.55 and state their customer's tariff is 100 per cent renewable.

We are concerned the system allows suppliers who rely exclusively on REGOs to "greenwash" their tariffs while seemingly doing very little to support new renewable electricity generation.'

Which? said the firms at risk of misleading customers include Green Star Energy, Foxglove Energy, Shell Energy, Ovo Energy, Pure Planet, So Energy, Tonik Energy, and Yorkshire Energy.

For example, Green Star Energy claims to source all electricity from renewable generators, but neither owns renewable generation nor has any contracts with renewable generators.

An investigation by Which? found a long list of firms that all claim to sell '100 per cent renewable' electricity tariffs without generating renewable energy themselves or having contracts to buy it direct from generators

It buys REGOs to match customers' use.

Which? found that some providers – including Ecotricity and Good Energy – do generate or have contracts with generators to buy enough renewable electricity to match their customers' usage.

Green Star Energy challenged the attack from Which? saying: 'Purchase of REGOs to substantiate supplying green energy is not a greenwash.'

Energy UK, the trade body that speaks for suppliers, said: 'Ofgem rules allow suppliers to label their tariffs as renewable if they either generate renewable power themselves, buy directly from generators or purchase renewable certificates equivalent to the amount supplied to their customers.

'The purpose of these certificates is to incentivise renewable generation, so all customers are increasingly using renewable energy.'

Shell Energy said it does buy renewable energy from generators under power purchase agreements (PPAs) as well as relying on REGO certificates. The firm said it also makes donations to UN approved conservation and reforestation schemes. It said: ‘All our Shell Energy tariffs feature 100per cent renewable electricity, as standard’.