It’s an energy manager’s worst nightmare: making an ancient, draughty building like Dinefwr Castle in Wales (pictured) more energy efficient. But that’s exactly what the National Trust has been doing – and saving itself £280,000 a year in the process.

The conservation charity has cut its energy use in its Welsh properties by 41% in two years – which is why it has been nominated as a finalist in the Ashden Awards, a prize which rewards sustainable energy.

The National Trust faces competition from the UK’s greenest hospital, a European cycling city, a micro-hydro programme in Afghanistan and a global solar company.

Keith Jones the Environmental Practices Advisor for the National Trust in Wales said the charity made sure energy monitoring only took 30 seconds a day. The efficiency measures also mean the Trust has cut back on 1700 tonnes of CO2 emissions a year.