Unison's increased lines charge for households generating their own electricity has unleashed a solar storm of protest.
It is the first lines company in New Zealand to levy the extra charge, typically from solar generation.
Unison manager of business assurance Nathan Strong said it was to make sure grid users paid their fair share - they paid less because of lower electricity volumes yet they still used the network.
"Unison must build and maintain a network to support the delivery of electricity at peak demand, which is during the winter evenings - a time when most distributed generation systems, such as solar panels, are not helping to reduce the network peak," he said.
Home electricity generators paid about $300 less than other customers but received the same level of service, which he said was not fair.
Under the new fee structure home generators would still save about $150 to $190 a year.
The increased charge is for new installations, while existing customers will remain on their current rate until March 2019.
"Unison supports people choosing to install their own generation - our aim is simply to ensure other customers without distributed generation don't pay higher prices as a result."
Cellpower director Kevin Hunter said solar customers had already suffered from power companies slashing the rate they paid for solar energy fed back into the grid.
"People are saving money and energy by installing solar power, doing their bit for the climate," he said.
"Isn't this what we should all be working towards to limit climate change and reduce electricity costs?"
The Green Party called on Unison "to join the 21st century".
"Given that today Genesis Energy, Mercury Energy and Contact Energy are all raising their normal residential prices too, no wonder people are looking to solar as an alternative," energy spokesman Gareth Hughes said.
Greenpeace executive director Russel Norman said the April 1 announcement was no joke.
"That, in 2016, only months after the world agreed in Paris to reduce emissions to stop climate change, a New Zealand company is specifically taxing solar energy users is extraordinary - and wrong," he said.
Sustainable Energy Association chairman Brendan Winitana said Unison's charge was "a clear abuse of monopoly power against ordinary Kiwis who are trying to reduce power bills and help stop climate change".
Energy analyst Molly Melhuish said Hawke's Bay was one of the sunniest places in New Zealand and the electricity industry should co-operate with alternative energy.
"Hawke's Bay makes New Zealand's most efficient wood burners, but air quality regulation is suppressing use of even approved wood burners," she said.
"Wood and solar together could integrate with electricity supply to reduce costs year round."
Mr Strong said currently about 20 households a month were going solar and the charge did not apply to non-electric hot-water heating.