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The head of energy at retail giant John Lewis has claimed that a lack of consistent benchmarks and data is holding back his firm from introducing display energy certificates.

Bill Wright, coporate energy and environment manager at the firm, said that it would be some time before retailers would show DECs. Shopping centres do not as yet have to show DECs by law but the government is keen for buildings to display them on a voluntary basis.

Speaking at the Retrofitting and Energy Management conference yesterday, Wright said: "DECs are a nightmare for us. There is no consistency as yet between different retailers as to how they measure our stores. We measure them in different formats." Wright said there were moves to rectify the problem. "We do talk about energy matters (with other supermarket chains)."

Wright, whose firm owns the Waitrose supermarket chain, revealed that his group's energy costs had doubled in the past few years, from £20m annually to £40m. "They will be £60m in a few years," he said.

The group is targetting a drop of energy use by 2010 but Wright said making initial energy saving was relatively easy. "The first 10-15% is a doddle, but it's a law of diminishing returns. There is a need for greater sharing of best practice to tackle further reductions."

Wright said the structure of the company, John Lewis is a partnership, made persuading staff to reduce energy use easier. "Any profit in the busines is shared. By using too much energy you are effectively burning your bonus."

Wright also discussed the use of CO2 controls, natural ventilation and the Merton Rule at the event, which took place at the Design Museum in London. other speakers at the conference included Arup director Chris Jofeh, who discussed his research on existing buildings, Darren Ball from low energy consultant Novarama and Faber Maunsell associate Malcolm Hannah.