The battle of what's being touted as a tax on the sun is heating up as an independent appeals authority prepares to review an Electricity Authority decision.

The Electricity Rulings Panel has announced it will hold a full hearing to determine if Unison's added fee on its solar customers is legal.

A fee was introduced in April last year for customers using renewable energy of the sun -- the country's first tariff on solar energy.

In May Solar City made a formal complaint to the Electricity Authority alleging the new tariff breached the Electricity Industry Participation Code because it charges over and above the cost of the services provided by distributed generators.

But the EA decided not to take any further action because it was not charging over and above the distributed generators, it was charging a retail consumer tariff to steady the market.

The Authority was concerned about the stability of the market, and that every consumer should pay their fair share of the electricity grid.

SEE ALSO: Authority rules electricity company allowed to introduce solar tax

But Solar City hasn't backed down, and laid a formal complaint with the Electricity Rulings Panel on September 6 last year.

Today, the panel has decided to go ahead with that hearing to 'fully test' the tariffs. The panel also rejected a request from Unison to disallow a hearing.

Greenpeace campaigner Amanda Larsson said they're pleased the Rulings Panel has decided the case deserves a full hearing.

"We're pleased with that result because it's good news for all the households and communities using solar panels to cut power bills and pollution."

Ms Larsson said they're very hopeful that the Rulings Panel will rule Unison cannot implement the tax.

"We're hopeful that that message will also come through to all the other lines companies out there that this kind of unfair taxation of solar households is unjustified."

Ms Larsson said families should be able to produce their own electricity and it's not their fault power companies haven't prepared for the technology.

“The electricity industry needs to adapt fast. For some companies, like Unison, the knee-jerk reaction has been to slap a fee on solar while they work out how to deal with it."

In a statement Greenpeace said there's an increasing power struggle between households wanting more independence over their electricity and industry players wanting to maintain control.

"Solar is rapidly becoming the cheapest form of new electricity generation globally, and uptake in New Zealand is growing by the day”.

It says the Solar City complaint is one of three lodged with the EA and a petition asking the EA to stop Unison and other lines companies from doing the same has now reached over 79,000 signatures.

“As we sit on the cusp of a solar boom, the outcome of this Electricity Rulings Panel hearing is important because it will shed light on whether the electricity system exists fundamentally to help households get clean, affordable energy or simply to make big profits for energy company CEOs.”

A date for the Electricity Rulings Panel hearing is yet to be set.