Solarcity challenged the dismissal, taking the matter to the independent Electricity Rulings Panel.
"For months, we have been fighting for our right to challenge the legality of Unison's tax which wrongfully disadvantages solar users," Solarcity CEO Andrew Booth said.
"Unison is discriminating against its smartest, most energy-aware customers and is charging up to an extra $239 per year without providing any extra services.
"That effectively makes it a tax on solar."
At the end of last year, Greenpeace delivered a petition signed by 45,000 people calling on the Electricity Authority to support solar energy and prohibit electricity providers from penalising solar users.
Unison challenged Greenpeace's description of the extra fee as a tax but the Advertising Standards Authority ruled the word "tax" was appropriate because the charges were compulsory and a demand on people's resources.
Unison business assurance general manager Nathan Strong said the increased solar fee was about fairness.
He said a relevant analogy was electric cars and the petrol tax that funded roads. If the number of electric cars increased markedly then roads would be underfunded.
He said solar-electricity generators connected to Unison's power lines were subsidised by other power consumers because, while they used the lines less, they typically needed external electricity at peak demand times.
"Meeting peak demand drives the majority of infrastructure costs," Mr Strong said.
Without the increased charge, non-solar users paid about $300 less for external electricity than those without solar, he said.
Even with the increased solar charge the average residential consumer with solar panels would pay $150 to $190 less, despite receiving the same service from Unison.
A date for the Electricity Rulings Panel hearing is yet to be set.
"We are not expecting any different outcome then what the Electricity Authority ruled," Mr Strong said.
Mr Booth said it was likely Unison's increased solar charge would be consigned to history "as a bad idea".
"These kinds of approaches by monopolies to try and stop solar won't work. Similar moves in Spain, Sweden and south Australia have all failed.
"Here in New Zealand, Vector, Orion, Network Tasman and Power co have told us they have no intention of introducing a solar tax.
"With climate change being the major threat facing the world, it's vital all countries do everything possible to encourage the shift to clean energy."
