This paper reports on a trial of flywheel energy storage technology on a High Speed Two railway construction site in London, UK. Originally designed for Formula 1 racing cars, the system has the potential to improve the efficiency of electric-powered plant and reduce greenhouse gas emissions. The trial application on a tower crane achieved savings of 44 t of carbon dioxide equivalent a year. It proved the technology works with mains power, cutting-edge generators and batteries. With improvements to telemetry, the system could now have wider use across the whole construction industry.