Eric Jansseune, a Belgian solar engineer, said people had come to him for advice having spent thousands on under-performing installations.

Mr Jansseune, who now lives in New Zealand, said he spent 15 years in the solar industry in Europe. "We are making the mistakes they made there 20 years ago. There is a lack of expertise in the field."

Mr Jansseune cited a shortage of trained installers, problems with overheating or freezing, and product failures like corrosion or discoloration of solar panels.

Consumers were not given reliable data on how much energy they would actually save and many were left disappointed, he said.

Some systems were installed without good insulation, which led to heat loss, and Mr Jansseune was also critical of systems where the water cylinders were installed on top of roofs, where they lost energy at nigh