Institution of Mechanical Engineers joins Citigroup in criticising aim to provide all electricity from renewables within nine years

Alex Salmond's goal of meeting all Scotland's electricity needs with green sources by 2020 has been attacked for the second time in a week for lacking credibility.

In a highly critical report, the Institution of Mechanical Engineers (Imeche) said the first minister's target was poorly worked out, uncosted and unrealistic.

Salmond had failed to properly explain how it could be achieved, said Tim Fox, the institution's head of energy and the environment. The target was "very problematic", Fox added, because it failed to explain how it would be met, paid for and supplied using new grid connections.

"The Scottish government needs to lay out a very clear, engineering-based plan on how they're going to have an energy mix which is secure and affordable for Scotland," he said. "At the moment, there is no clear vision for the engineering of this infrastructure. Without a clear vision of what it's going to take to deliver it, it's impossible to put any credible cost against that."

This week Citigroup said investing in Scottish renewables was extremely risky because Salmond's drive to make Scotland independent by 2015 raised far too much uncertainty about public funding.

The bank said hitting the 100% target would need about £46bn of subsidy in total, an annual subsidy of £4bn and a doubling of green generating capacity in Scotland alone to 26Gw. An independent Scotland would put that subsidy "at grave risk".

The two reports echo widespread misgivings in parts of the energy industry and banking sector about whether Salmond's plan to turn Scotland into the "Saudi Arabia of renewables" within the decade is achievable. He has won applause from green campaigners and renewables firms for vigorously championing green energy; industry sources argue that his ambition has built up confidence and excitement for green power.

Al Gore, the climate campaigner and former US vice-president, heaped praise on Salmond in his speech to a Scottish government-sponsored conference on low-carbon investment in September. Several major green energy companies, including Mitsubishi and Doosan, have moved to Scotland.

Imeche said it supported Salmond's goal of 100% renewables but warned there was a significant lack of credible detail and planning. Fox said there were significant doubts over whether the 2020 target could be met.

Imeche said the Scottish government had failed to provide stable sources of renewable energy, such as biomass or geothermal power, to give baseload electricity supply when wind farms or solar energy failed to provide enough electricity during unfavourable weather. Scotland would instead have to import power from England or abroad, Imeche said.

The report said infrastructure and market subsidies would "quickly cost billions", so pushing up fuel prices for ordinary consumers and increasing fuel poverty. The 100% target would also mean green energy sources needing to be built five times faster than the rate achieved in the past decade.

Imeche said the most intelligent starting point was to cut demand and increase efficiency, before finding new energy sources.

A Scottish government spokesman dismissed the criticisms, insisting it had published a very clear "renewables routemap" in June. He stressed the target was to generate the equivalent of 100% of its electricity from green sources, not provide that round the clock.

"We have always been clear about the need for baseload," he said. "Our ambitious and achievable target is to generate the equivalent of 100% of Scotland's electricity needs from renewables by 2020, supported by cleaner fossil fuel baseload electricity, such as the recently consented gas power station at Cockenzie."