Solar panels are not currently functional at a quarter of Haringey Council sites, with some not switched on for two years.
Friends of Hornsey Library contacted Liberal Democrat councillors to tell them that the panels installed at the library had not been working since they were installed in 2020.
The panels were switched this month after the matter was escalated to senior officers
Liberal Democrat councillors performed an investigation on all council sites that have solar panels, discovering that nine out of 39 solar panels are not in operation.
Among the buildings reported were Rhodes Avenue School in Muswell Hill, where panels were faulty, and Shelley House, in Hornsey, which were not working.
The council has said a contractor will fix the problems "as soon as possible".
Highgate councillor Cllr Scott Emery, Lib Dem spokesperson for the environment,
said the council failed to provide details of the total financial cost or carbon cost from the panels not operating, or how long each panel has been off for.
He said the council has estimated that not having the panels switched on at Hornsey Library could have cost taxpayers £1,200 this year in paying for additional energy from the grid.
Cllr Emery added: “It is totally crazy that, in the midst of a cost of living crisis, in large part down to soaring energy costs, the council has failed to turn on the solar panels on Hornsey Library for two years, and that a quarter of council buildings with solar panels don’t actually have them running.
"Not only is this a failure in terms of our net zero carbon commitments, it is a major expense to taxpayers to have to pick up the tab for the more expensive energy the council is buying.
“We urge the council to look to get their full stock of panels in operation as soon as possible.”
Cllr Mike Hakata, Haringey's cabinet member for climate action, environment and transport said: "All solar panels were certified and working when fitted.
"A number have been installed at schools and leasehold buildings across the borough, but it was not brought to our attention that panels at eight of these sites weren’t functioning properly.
"A contractor will be getting these systems fully working as soon as possible so they can continue to cut the properties’ carbon footprint.“
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