Nuclear power will remain the cheapest way for the UK to grow its low-carbon energy supply for at least a decade, according to government advisers.
But renewables should provide 30-45% of the nation's energy by 2030, says the Committee on Climate Change (CCC).
Its new report suggests ministers may want to temper ambitions for offshore wind, which is still fairly expensive.
The coalition asked the CCC to advise on options for a low-carbon future shortly after taking office a year ago.
"People argue that offshore wind is very expensive - and it's true, it is more expensive at the moment than some other technologies, so nuclear at the moment looks like the lowest cost low-carbon option," said CCC chief executive David Kennedy.
"But we can expect significant cost reductions over the next two decades across a range of technologies, whether wind, marine or solar, and that's why these technologies are promising."
Wind could replace nuclear as the cheapest option within about 15-20 years, he indicated.
By 2030, the cost of using these low-carbon technologies rather than fossil fuels would put about £50 onto the average household's energy bill.
However, bills could actually go down if plans to improve energy efficiency, such as boosting home insulation, come to fruition.
The committee's advice comes against a number of different targets and constraints.
First, there are European Union targets under which the UK has to achieve a 15% share of renewables by 2020, and a 34% cut in greenhouse gas emissions from 1990 levels.
It is already most of the way to the second of those targets, thanks mainly to the "dash for gas", measures to clean up methane emissions from landfill sites and the recession; but the renewables target is likely to prove tougher.
The government's main strategy is to encourage the installation of offshore wind farms - committee calculations suggest that even if 10MW turbines come into the market, at least 3,600 would be needed.
Here, the committee has two concerns. Some of that electricity could be generated more cheaply through onshore wind or buying renewable electricity from overseas; and currently, financial incentives end in 2020.
"There isn't anything in the way of government support after 2020 - it falls off a cliff - so we have to ask, 'why would you expect anybody to build an offshore wind turbine factory in the UK?'" said Mr Kennedy.
"So we're saying the government should commit to renewables support through the 2020s, and we've got offshore wind and marine technologies in mind here."
The committee is also looking at the government's long-term target of a cut of at least 80% in greenhouse gas emissions by 2050, and asking what policies are needed in order to get on the right road.
They say that by 2030, it would mean generating virtually all electricity through low-carbon technologies - nuclear, renewables, and perhaps fossil-fuel stations that capture and store the carbon dioxide they produce.
Nuclear and renewables would each have about a 40% share, the report envisages.
This would require an additional two or three nuclear reactors on top of those developers are already planning to build.
The total amount of electricity generated would be about 20-25% larger than today, with the additional energy going to replace some of the heating currently done through burning gas and coal, and to power a burgeoning fleet of electric vehicles - 10 million by 2030.
"Smart batteries" in those vehicles would be among the resources used to store electricity at times of low demand and release it during peaks, helping to "smooth out" the supply across the day.
About 30-35% of the remaining heat requirement could be supplied through renewable technologies such as heat pumps and biogas, the committee says.
Only in transport would there be an enduring requirement for fossil fuels, with biofuels constrained by issues such as the increasing need for land to grow food crops.
Environmental groups have given the report a mixed reception.
"It's great that the committee has recognised the huge role renewable energy could and should be playing in taking Britain towards a clean, prosperous future - and is right to call for a dramatic increase in investment to make this happen," said Craig Bennett, director of policy and campaigns with Friends of the Earth.
"But nuclear power can't be part of the answer - our analysis shows it will divert vital money and effort away from developing renewable energy, and the jobs and industries it could bring to the UK.
"We've had 50 years of successive governments pandering to the nuclear lobby. If their promises of cheap, low-carbon energy were true, they would have been delivered by now."
EDF, one of Europe's largest energy companies that is aiming to build new reactors in the UK, welcomed the nuclear emphasis.
"The CCC has said that safe nuclear power, the lowest cost, large scale, low-carbon electricity source, is a key element; we agree," it said in a statement.
"EDF Energy has already taken steps to respond to early lessons from Fukushima.
"The designs we propose for the future already build in the lessons from previous extreme events, inside and outside our industry [and] we will take account of new lessons from Japan."