The National Trust has launched a 'Big Switch' project to change 40,000 light bulbs across its historic houses, offices, shops, restaurants and holiday cottages in England, Wales and Northern Ireland to low energy alternatives.

Once installed, the bulbs in Sussex alone will save 46.27 tonnes of CO2 each year – enough CO2 to fill 9 hot air balloons or equivalent to taking 16 cars off the road for a year – and will reduce annual energy and maintenance costs by £8,375.

The Trust will plough these savings back into reducing the environmental impact of its properties even further.

Changing light bulbs may sound simple, but there was a particular challenge for the Trust.

The first generation of energy efficient light bulbs, called compact fluorescents, produced higher levels of ultraviolet light than incandescents.

And whilst all light is damaging, UV is the most harmful for sensitive materials such as the delicate watercolour paintings, textiles and wallpaper found in many National Trust houses.

There was also the issue of how these bulbs look in a historic setting, an issue shared by many homeowners living in older houses throughout the UK.

"Lighting is really significant for the National Trust, perhaps more than many people appreciate," said Katy Lithgow, head conservator at the National Trust.

"Good lighting can help to bring collections alive and can help visitors to enjoy historic interiors. Too much exposure to light, however, is detrimental. Ultraviolet is the most damaging part of light, but happily ultraviolet is not necessary for us to be able to see. If this element is reduced or even eliminated from light sources, the benefits are significant.

"If the Trust can solve the problem of finding low energy lighting solutions in its houses, then the same is probably true for householders across the country."

As part of the Big Switch, the Trust will make use of hundreds of different types of low energy bulbs including a new bulb made by Philips Lighting called the Master Classic. This bulb uses a different technology which means it emits less UV.

Katy Lithgow added: "The developments that have taken place in energy saving light bulbs have enabled us to undertake our Big Switch. We have now been able to change bulbs in many locations without compromising the appearance of our collections for the visitor."

With nearly 500 properties in England, Wales and Northern Ireland and hundreds of different energy saving bulbs on the market, the problem of identifying the right bulb for the right location was never going to be a simple nut to crack.

The National Trust worked with Bristol based Lighting Services** to formulate an approach to ensure that the most appropriate bulb was chosen for each individual situation. An online ordering system helped each property to order the best bulb for each fitting.

This system will be available to the public shortly, and will help homeowners identify the most appropriate low energy bulb for the area they are trying to light.

In making decisions, the first choice for fittings has been compact fluorescents as they are the most energy efficient and their appearance has improved so they now look more historically appropriate.