A Friends of the Earth (FoE) group in Kent has raised concerns over hundreds of street lamps which have remained on during daytime since at least July.
The Tunbridge Wells branch of the society said the energy used by 250 day-burning lights in the town meant "a considerable amount of power wasted".
The environmental campaigners fear unnecessary carbon dioxide emissions.
Kent County Council (KCC) said a fault made the lights stay on and it was being repaired as quickly as possible.
Trevor Humphreys, from the Tunbridge Wells FoE, said: "We spotted the lights purely by chance in July.
"It was a really hot, sunny day and there were three street lamps on in my road."
Mr Humphreys said KCC had fixed some of the 250 day-burning lights, but there was still a "large percentage burning all day, every day".
Greg Clark, the MP for Tunbridge Wells, added: "We've been having a big debate about climate change [with the publication of the Stern Review]... and if we are using electricity that doesn't need to be used then we are generating emissions that are pure waste."
KCC said a third of the lights had been fixed and that the faulty ones represented just 0.25% of all those in Tunbridge Wells.
A statement said: "The priority is to repair lights which are not working at night, however we have a programme of work with extra staff taken on to repair the day-burning lights."