Lines company Unison says solar electricity users connected to its network were subsidised by other power consumers, necessitating a lift in charges.
On Friday it announced higher fees for customers who use a combination of solar energy and the electricity grid in Hawke's Bay, Taupo and Rotorua. It is the first lines company in New Zealand to levy the extra charge, typically from solar generation.
The Green Party called on the company "to join the 21st century". Napier MP Stuart Nash said it was "a backwards approach" that set a bad precedent.
"If consumers move their appliances from electricity to gas, there is the same effect of falling electricity usage, but power companies don't charge them more for their electricity," he said.
"Unison should be working with customers who go solar, rather than against them. The risk is that treating customers like this will only encourage them to move onto emerging technologies like batteries that will move them off-grid.
That would increase lines charges for everyone else."
Unison's business assurance general manager, Nathan Strong, said other customers, including struggling families, should not have to pay higher lines charges because solar users needed electricity on chilly evenings.
"We have to build our network to meet peak demands on the coldest winter night, when solar customers need the network as much as any other customer," he said.
"Solar reduces total annual consumption and therefore under old prices these customers reduce their contribution to the cost of providing the service they require to keep the lights on during winter, with other customers bearing this cost or subsidising these customers in the long-term."
He said the new Distributed Generation price category for solar connections "ensures that these customers make a contribution to the cost of providing them the service they require - although for a typical solar customer they still receive a saving of around $150 to $190 per year".
Solar hot water and gas systems were not targeted because they lessened the cost of maintaining the network by lessening peak demand.
"Solar hot water heating systems differ in their impact on the network, by removing consumption from evening peaks, and can still be subject to load control. They do have a different consumption profile and are therefore not subject to the new price category."
The same applied to customers with gas heating, LED lighting or extra insulation, he said.
"Gas is used for heating and cooking, activities that predominantly occur during winter, and in the evening. The associated reduction in consumption therefore directly contributes to a reduced demand on the network and reduced network costs, which is the same for energy efficient LED lighting - this is passed through to the consumer through lower power bills."