London's entire Tube network could be powered by renewable electricity directly purchased from wind and solar farms within the next decade, under plans unveiled today by the city's Mayor Sadiq Khan.

Transport for London (TfL) is one of the UK's largest consumers of electricity, with its 1.6TWh annual power demand equivalent to that used by more than 437,000 average homes or roughly 12 per cent of all households in the capital.

But City Hall said the publication to market today of TfL's renewable electricity requirements marked a significant step towards its ambition to operate a zero emission underground train system in the UK capital by 2030.

The move is aimed at testing the market for securing TfL's electricity through Power Purchase Agreements (PPAs) with renewables generators, which it said would allow Tube trains to be supplied with power directly from wind and solar projects, rather than from the mix of clean and fossil fuel generators offered via the National Grid.

At present TfL sources its electricity directly from National Grid via the Crown Commercial Service, but it is aiming to secure deals from the growing PPA market to eventually meet all the Tube network's electricity with renewables.

Taking a phased approach, TfL said its ambition was to consider purchasing up to 10 per cent of the network's power demand via renewable PPAs by spring 2022 - subject to market approval from its Finance Committee later this year - with a view to contracting 100 per cent by the end of the decade.

"As one of the single biggest purchasers of energy in London, it is important that TfL leads the way on green energy," said the Mayor of London, Sadiq Khan. "This is a vital step towards my ambitions for TfL - and London - to be zero-carbon by 2030."

City Hall also said it would look at the potential for securing PPAs to supply its entire estate with renewable electricity in future, which would cover all TfL's requirements as well as those of the London Fire Commissioner, the Mayor's Office for Policing and Crime, the London Legacy Development Corporation, and the Old Oak Common and Park Royal Development Corporation.

Altogether, the Greater London Authority's public body's and services account for 143GWh of annual power demand, it said.

Moreover, the Mayor's office said it was looking at opportunities to meet London's wider public sector electricity demand with renewables, too, including local authority offices, schools, and leisure centres. In total these organisations along with the GLA group consume around 3TWh of electricity every year, or the equivalent to over 820,000 homes, which is almost a quarter of all London households.

Dr Nina Skorupska, chief executive of renewable energy trade body the REA, welcomed TfL's move to scope out the PPA market, which she said would help both decarbonise the Tube network and reduce its energy costs.

"This is a major step forward for both the low-carbon transport and power sectors, and we hope to see other London boroughs also getting involved," she said.

It is the latest in a string of efforts to decarbonise the capital's energy and transport use. Since earlier this year, the London Underground network has been providing heating and hot water to more than 1,350 homes, a school, and two leisure centres in Islington from a new energy centre in Bunhill.