The Parliamentary Commissioner for the Environment is casting doubt on the environmental benefits of solar water heating.
Solar water panels are generally lauded by the green movement because they use the rays of the sun, rather than electricity or gas, to heat water.
But in a new report, the commissioner says solar panels do little to reduce the demand for new power stations or new transmission lines.
Jan Wright's report says solar panels work best in summer, or on sunny days, when renewable electricity is already plentiful.
They are less effective in winter, or on cloudy days, because the water they heat starts off colder and the rays of the sun are weaker.
That coincides with times when fossil fuel plants are used most and the environmental costs of electricity are greatest, but solar panels are least able to help out.
The report quotes figures showing solar panels use one kilowatt of back-up electricity a day in summer, and seven kilowatts of electricity a day in winter.
The Green Party strongly rejects the report, saying it actually shows that even in the middle of winter, solar water heating reduces electricity use for heating water by about 50%.
The party's energy spokesperson, Gareth Hughes, says over a year solar heaing reduces electricity use by 70% and is good for power bills and the environment.
Solar power company SolarCity's chief executive Andrew Booth says the commissioner's view does not align with the international energy agency's opinion.
He says the commissioner's view contradicts previous commissioners' opinion and up until the last Budget, the Government was promoting solar power by giving grants of up to $1000 for people to install systems in their homes.