WA consumers will pay almost $1 billion extra in energy costs a year under Bill Shorten’s plans for a 50 per cent renewable energy target, previously secret modelling has found.

Research by the State Government’s chief energy adviser shows lifting the amount of green energy in WA to 50 per cent by 2030 would cost $946 million compared with a business-as-usual approach.

The finding is in a report by the Public Utilities Office in late 2016, when Mr Shorten had already committed to doubling Australia’s renewable energy target.

But shadow energy minister Mark Butler stood by Federal Labor’s plans, saying renewable energy combined with storage such as batteries was already the cheapest form of new power.

The PUO found that under a baseline scenario the share of WA’s power coming from renewable sources was likely to be 33 per cent — 3124MW — by 2030.

Most of the investment in new capacity, however, would happen before the 2020 deadline for the current green energy target. The PUO said a 50 per cent green energy share by 2030 would amount to more than 4600MW, an extra 1500MW over and above the baseline.

According to the PUO, about $560 million of the extra annual cost to WA would come from the wind farms and solar projects that would need to be built to ensure the State met its share.

It also estimated an additional $116 million a year would be needed for “flexible ramping services” such as fast-response gas turbines that could spring into action when the Sun was not shining or the wind not blowing.

And there would be a cost of more than $200 million a year needed for upgrades to accommodate the new green power projects on the networks of poles and wires operated by companies such as Western Power.

Despite the capital costs needed for the new plants and upgrades, the PUO noted the operating costs of renewable projects were “negligible”.

The report also failed to consider the effects of batteries on the grid, even though the PUO concluded “it is reasonable to assume that batteries would only reduce the cost of meeting the target”.

Mr Butler said attacks by opponents of the ALP’s policy, led by the Morrison Government, ignored the commercial reality that renewable energy and storage were now the cheapest new source of power.

“No matter how much Scott Morrison and the Liberals try to rubbish renewable energy, the proof is in the irrefutable data,” Mr Butler said. “That’s why a Shorten Labor government’s plan for more renewables, with our 50 per cent renewable energy target, will deliver cheaper power and lower pollution.”

State shadow energy minister Dean Nalder said WA households grappling with big power price hikes faced a further $1000-a-year impost under the target.

“The people of Western Australia have a right to know that, if elected, Bill Shorten's 50 per cent renewable energy target will cost each household approximately an additional $1000,” Mr Nalder said.

“West Australians are already experiencing bill shock from the $700 per year household increases by the McGowan Labor Government.

“The Treasurer, when asked in parliament what the financial impact of a 50 per cent renewable energy target will be on Western Australia, he said no work had been done.

“We now know that Treasury has done the work and Treasurer Ben Wyatt knowingly covered up the $1 billion cost of the 50 per cent renewable energy target.

“Treasurer Ben Wyatt needs to come clean and tell the people of Western Australia the true cost of a Bill Shorten Labor Government before the Federal election which will be held in a few short months.”