Australia is on course to surpass its renewable energy target by deriving 22.5% of its power from sources such as solar and wind by 2020, according to a new study.

The analysis of government agency data, by consultancy Green Energy Markets, forecast a long-term decline in fossil fuels, with the use of coal for electricity falling by a third over the next 20 years.

Brown coal, the most carbon dioxide-heavy of all coal varieties, is set to be phased out as an energy source completely by 2050, as is oil.

Meanwhile, renewables are set to grow from 13% of the energy mix to 51% by 2050, trumping the target of 20% by 2020 along the way.

Green Energy Markets says that changing consumption will also aid the bipartisan renewable energy target, predicting an average annual electricity generation growth rate of 1.47% until 2020 – a figure lower than previously thought.

The report states that electricity consumption fell by 5.5% from 2008 to 2012, with half of this reduction driven by solar and energy efficiency schemes. A drop-off in manufacturing activity and energy bill price rises have also played a role.

However, clean energy groups have warned that political uncertainty over the future of the RET is undermining investment in the sector, potentially compromising the 20% target.

“There’s no reason why Australia won’t hit its renewable target unless the RET is changed,” said Ric Brazzale, director of Green Energy Markets and author of the report.

“Even if the carbon price remains in place after the election, it will be low because of its link to the European market. It’s really the RET that is driving growth of renewables.”

Last year, the Climate Change Authority recommendedthat the RET should retain its key component of requiring energy companies to produce 41,000 gigawatt hours of renewable energy by 2020, citing “damaging investor confidence” if this were to change.

It also recommended that the RET was reviewed every four years, rather than the current two years – the next review is due in 2014. However, while the federal government accepted this idea, the Coalition says it will keep the two-year review cycle if it comes to power in September.

“In many ways, these forecasts are academic until we know what the policy settings are,” said Erwin Jackson, deputy chief executive of the Climate Institute.

“No investor will want to put money into clean energy when there will be a review straight away. It takes 18 months to hold a review, so nothing really happens in between reviews.”

“We have a stop-start renewables industry because we have stop-start policies. With no target, there is no push towards renewables. It makes it far more difficult. It seems that we’ll be reviewing every two years and holding off until we get to 2016 and then the Coalition can say, ‘We can’t meet the target. Let’s change it.’”

A spokeswoman for Greg Hunt, the shadow climate action minister, denied that the Coalition was creating uncertainty and said it planned no change to the 20% target and the 41,000 gigawatt hour requirement.

“We’ve been absolutely consistent,” she said. “The Coalition brought in the renewable energy target and we’ve supported it ever since.”

“We’ve been very consistent with our opposition of the carbon tax too, which the government promised not to do and then brought it in. That is a classic example of creating uncertainty.”

Kane Thornton, deputy chief executive of the Clean Energy Council, said that the RET policies needed to remain in place to ensure investment.

“The clean energy industry is tracking along well and the 20% target means there are lots of projects ready to go,” he said. “There are lots of investors looking to Australia and the key question is ‘will the RET stay in place?’ This investment will come to Australia as long as those policy settings are stable.”