The ACT Greens have asked the consumer watchdog to investigate ActewAGL's Green Choice premium.
They have written to the Australian Competition Consumer Commission (ACCC) with concerns that the Green Choice premium has remained unchanged since 2002, despite green energy and solar prices dropping.
Greens MLA Shane Rattenbury says solar prices have fallen by 80 per cent in the past five years and wind is nearly grid parity.
Mr Rattenbury says he expects that the premium price for green power would have fallen to reflect the change in the market.
"Recently Origin Energy has cut their premium by 44 per cent, so at the least ActewAGL should explain to its customers that why, given the reduced price of renewable energy, the premium they're paying to purchase green power is not falling at the same time," he said.
The ACCC is already investigating complaints from ActewAGL customers about the carbon price being added to Green Choice energy.
Chairman Rod Sims says green products are sold as an add-on to the average price, which covers the additional cost of supporting green energy products.
He says the foundation price has increased under the carbon tax.
"If you buy green energy you have helped create additional projects that wouldn't have happened and you can feel good about that," he said.
"But the way the price is constructed, the way the scheme is constructed by the policy makers, is that additional amount you pay is added on to the average price." ActewAGL agrees, and says the consumer watchdog has no objections to the way it is calculating its charges for green energy customers.
The ACCC says some other retailers have reduced their premiums for green energy as compensation for the carbon tax, but spread the cost to other customers.
Chief executive Michael Costello says ActewAGL has taken a different approach.
"We charge more than some others," he said.
"As Mr Simms, the Chairman of the ACCC pointed out, [others] have done that by raising their price of the black energy that underlies this to all their customers, we have done it by offering it in a very generous bundle offering."