A higher lines charge for solar power users has prompted a legal complaint.
Solarcity is asking the Electricity Authority to stop Unison's solar tax, on the grounds that it wrongfully disadvantages customers and breaches the Electricity Industry Participation Code.
Last month, Unison became the first lines company in New Zealand to increase line charges for households generating their own electricity.
It announced higher fees for customers who use a combination of solar energy and the electricity grid in Hawke's Bay, Taupo and Rotorua.
The lines company said solar electricity users connected to its network had been subsidised by other power consumers so a lift in charges was needed.
Home electricity generators paid about $300 less than other customers but received the same level of service. Under the new fee structure home generators would still save about $150 to $190 a year in lines charges. The increased charge was for new installations while existing customers will stay on their current rate until March 2019.
The complaint filed with the Electricity Authority contends that Unison has violated two sections of the Electricity Industry Participation Code.
Unison's solar charge was likely to see fewer people installing solar panels, leading to less take-up of renewable energy and continued reliance on fossil fuels such as coal and gas, Solarcity chief executive Andrew Booth said. Unison business assurance general manager Nathan Strong was surprised by the suggestion the company's Distributed Generation price category was in breach of Part 6 of the Code. "We informed the Electricity Authority of our intentions to introduce a new Distributed Generation price category prior to its introduction. We will of course work with them during their review of this alleged breach.
"Also, while we understand that solar vendors are upset their customers will receive a lower subsidy from Unison, it is worth reminding our customers that a typical residential customer with solar will continue to pay lower annual charges under our new approach than a non-solar consumer.
"Our approach promotes fairness between all our 110,000 customers who continue to rely on our network to provide electricity at all times, especially during the cold, dark winter peaks," Mr Strong said.
Solarcity argues that Unison's decision to increase solar charges was a discriminatory penalty. It would add hundreds per year for customers, and is out of step with New Zealand's obligations under the recently signed Paris Climate Agreement, Mr Booth said.
"What Unison - and indeed other companies - are doing is unacceptable, unlawful and a clear abuse of monopoly power."
Mr Booth said internationally, both in the United States and Australia, regulators were recommending solar because of the value they brought to the grid and consumers.
But this year the Parliamentary Commissioner for the Environment, Dr Jan Wright, highlighted a report that claimed solar panels did little to help New Zealand's carbon footprint.
The Concept Consulting report said carbon dioxide emissions from generating electricity were at their highest in winter. But solar panels are most effective in summer.