The government's plan assumes that projects with a total installed capacity of 10.6GW will be operating by 2020, but the report points to a figure of 5.3GW as more realistic.
This also skews the predictions for the levels of employment in the sector, with the report saying that 19,000 are likely to work in offshore wind, compared with the government's 28,000 figure.
Wind is the fastest-growing renewable energy source in Scotland, with 4.3GW installed, but the report calls for "additional steps" to be taken to accelerate the development of infrastructure to support the offshore wind industry.
Investment in ports, which is seen as key to the growth of the sector, is singled out as "much slower than expected".
The report warns that the government will find it "challenging" to meet its overall 2020 renewable energy target of generating 30% of its energy from renewable sources, and that doing so will depend on private-sector investment.
It warns that renewable energy projects are progressing more slowly than expected, due to problems in the economy and changes in UK energy policy.
As a result, public bodies have experienced delays in spending some of the money available for initiating and supporting projects.
However, it also said that Scotland is making "steady progress" towards meeting its goals, with public-sector spending rising sharply over the next two years, with a total budget of £264 million (EUR 313 million) available, compared with £209 million over the last ten years.
"While there are aspects the Scottish government and other public bodies should improve, the main challenge is that private-sector investment has been slower than expected, reflecting the state of the economy and the uncertainty of developments in the wider UK energy sector," said auditor general for Scotland Caroline Gardner.
The report also calls on the government to start considering what its longer-term ambitions for renewable energy are beyond 2020.