City Hall now has a custom-designed £540,000 photovoltaic system - but it only provides 1.5 per cent of the building's daily electricity requirement even on the brightest day.
The solar panels have been operational since last year and were officially unveiled at a ceremony held last month.
The photovoltaic panels have a peak capacity of 67kW and are expected to generate about 50,000kWh of renewable electricity each year – saving up to 33 tonnes of CO2.
A written answer by Ken Livingstone to a question from London Assembly member Roger Evans revealed that the peak capacity of 67kW – on the brightest day – represents just 1.5 per cent of City Hall's electricity requirement.
The complex geometry of the roof of the Norman Foster-designed building required an innovative design.
Normal photovoltaic panels are rectangular and aluminium framed whereas City Hall's panels are made-to-measure trapezoidal (four-sided shape of which two sides are parallel and two are non-parallel), bare unframed glass in a black colour.
This allowed the 617 panels to be arranged in concentric circles around the roof.
To develop the glass-glass laminate array for the 'eyelash' all 46 photovoltaic panels were of different size and cell layout to adjust to the curved design of the building and changing pitch.
The array includes some of the largest glass-glass laminated photovoltaic panels manufactured in the UK to date.
"Reducing carbon emissions in order to tackle climate change is the biggest challenge facing this planet," says Mayor of London Ken Livingstone.
"This renewable energy scheme is an example of City Hall leading by example."
But not everyone is so impressed.
"Solar panels are not cost-effective, especially in northern climes like London," says Damian Hockney, London Assembly member and One London Party mayoral candidate.
"There are plenty of ways to save energy that cost nothing or even save money. Wasting taxpayers' money like this brings the environmental movement into disrepute."
"This folly, combined with the expensive flop at Palestra, home of the pointless LDA, prove conclusively that solar and wind schemes are not economically viable. Sadly, Londoners have already paid a million pounds for these appalling schemes."