By Richard Black
Environment correspondent, BBC News website
If the European Commission's proposals on energy amount to a "revolution", as president Jose Manuel Barroso claimed when he announced them on Wednesday, what does the revolution look like?
Offshore wind farms may become a common sight around Britain
How will it change the face of Britain over the next 12 years?
The targets our island nation is supposed to achieve, remember, are several: a 20% jump in energy efficiency, 10% of vehicle traffic powered by biofuels, and 15% of energy derived from renewable sources.
How EU states achieve the targets is up to them; the UK already has a raft of policies in place to stimulate the growth of renewables, and may have to draw up new ones if and when the commission's proposals are endorsed by the European Parliament and Council of Ministers.
The renewables target is particularly challenging because it refers to overall energy use, rather than just electricity.
To get an idea of the scale of the challenge, look across to Sweden.
Because Sweden already derives 40% of its energy from renewable sources, it received a much higher target than Britain for 2020 - 49% against Britain's 15%.
It looks steep. But in relative terms, going from 40% to 49% means building just under a quarter more than you already have.
Britain only derives 2% of its energy from renewables, and so must build six and a half times what it has now - all within the next 12 years.
As they say in sporting circles, it is a big ask.
So where will all this energy come from?
The biggest supplier, virtually all experts agree, will be wind.
"I'd say that by 2020 we would have about 6,000 or 7,000 turbines offshore and about 5,000 onshore," says Charles Anglin from the British Wind Energy Association (BWEA).
"That alone could provide about 25%, possibly 28%, of the country's electricity."
Rob Gross, head of policy and technology assessment at the UK Energy Research Centre (UKERC), agrees.
"I would envisage seeing a number of large offshore wind farms, particularly along the east coast," he says.
Last month, Business Secretary John Hutton announced that the government wanted to see enough offshore wind farms come into operation to power all the UK's homes - though he admitted this would bring changes to the coastline which might not be to everybody's taste.
Mr Barroso described the proposals as a "revolution"
The BWEA says there are two big issues holding back the expansion of wind power - planning, and national grid connections.
"In England and Wales, with schemes considered 'substantial' such as a housing development or a supermarket, the government sets a 16-week time limit for local authorities to make a planning decision," says Mr Anglin.
"The overall proportion of these that get decided within 16 weeks is 70%. With wind farms, it's 5%."
The government is pushing through planning legislation aimed at drastically reducing these delays, though this will not affect Scotland.
With offshore farms, the BWEA says, the time interval between a company deciding it wants to build a wind farm and the first electricity flowing into the grid can be seven years. The main issue is the procedure for getting connected to the grid, which companies see as time-consuming and laborious.
Still, a lot of wind can clearly take Britain a long way. There are technical issues with ramping up, particularly concerning the National Grid's capacity to compensate for intermittent supply - but analysis done at UKERC suggests it is less of a problem than providing cover in case a single large power station goes off line.
Barrage of ideas
The scheme that on its own could contribute more than any other would be a tidal barrage on the Severn estuary - an idea much discussed down the years, emotionally charged, and about to be evaluated again at government level.
With the capacity to generate about 5% of the nation's electricity, it would at a stroke take Britain about one-tenth of the way towards its 15% renewable energy target.
HOW TIDAL POWER WORKS
As tide comes in, sea water passes through barrage to landward side
At high tide, sluice gates shut, trapping water in estuary or basin
When tide recedes on sea-side of barrage, sluice gates open
Water flows through barrage, driving turbines and generating power
Power can be generated in both directions, but this can affect efficiency and economics of project
Do the commission's proposals make it any more likely?
"The problem is, it's such a huge capital project that financing it is a huge headache," comments Rob Gross.
"The installed capacity is equivalent to six or seven nuclear reactors; also it would take a long time to build."
Even if it makes sense in policy terms - and that is still a big "if" when all the social, economic and environmental factors are taken into account - the project may still contain too many uncertainties to make sense as an investment, without significantly more government support.
The other renewable electricity options also may need more of a push.
Solar electricity is still expensive - the commission's proposals, even if the government rushes to activate them with new initiatives, will not produce a sudden rash of photovoltaic panels on Britain's roofs - while wave and small-scale tidal technologies are years away from commercialisation.
Away from electricity, areas for introducing renewable technologies include heat for homes and industrial buildings, and transport.
Biofuels - that increasingly difficult technology - are the only real option at the moment for cars and lorries.
In an attempt to eliminate the environmental and social problems that could stem from a rush to biofuels, the commission has set "sustainability criteria" which would, for example, prevent the clearance of virgin forest for biofuel crops, and mandate that biofuels must bring carbon savings of at least 35% compared with petrol and diesel.
Whether Britain's landscape now gets covered by biofuel crops depends on a whole range of factors outside the gift of the European Commission or the British government.
Guidelines aim to stop biofuel production ravaging biodiversity
How economic will the nascent British growers and refiners prove to be? How quickly will scientists develop "second generation" fuels that use plant mass much more efficiently?
"It's not clear yet what proportion of biofuels will be provided by use of land in the UK, as opposed to what's brought in from overseas," observes Mark Williamson, director of innovations at the Carbon Trust.
"We need to see how that will play out. But in any case, existing biofuels are not very efficient - in the UK, we need to think about how we support the transition to second generation fuels."
Which leaves the option of heat. In a way, it is the most obvious; my grandparents heated their house and water with a wood boiler, and they were far from alone in the Britain of yesteryear.
Parts of Europe - Austria, Finland, Sweden - progressively brought in more efficient ways of using the same renewable resource.
Britain switched to natural gas.
Yet Mark Williamson believes wood burning could perhaps provide 5-10% of our heating needs by 2020, principally in commercial properties which already have some of the infrastructure they would need, and which can provide a reliable market for suppliers.
"The analysis we've done suggests that where there's demand, a supply will come in," he says.
"It's a combination of waste wood and new wood; managed forestry is an option, where you grow and harvest it cyclically, so it becomes a nice sustainable source."
Does Britain yet have the policies in place to ensure the 15% target is met?
"If we don't do anything in addition to what we have now, I would bet that we won't get there," says Rob Gross.
He believes the government needs to look at introducing feed-in tariffs for renewable generators, as Germany has done to good effect.
Mr Hutton recently ruled that option out; though his department is looking at restructuring the existing Renewables Obligation scheme so it gives more support to "experimental" electricity generating technologies such as wave power.
Energy efficiency gains would help Britain achieve its target
On transport, the government is introducing later this year a Renewable Transport Fuel Obligation (RTFO) that will oblige fuel sellers to source a given proportion of what they sell from renewable sources - in other words, biofuels.
Experts bemoan the lack of support for microgeneration; but the big policy gap, many say, is on heat, where there is no real incentive to introduce sustainable wood burning on a large scale.
Without it, there seems little prospect of new forests springing up all over the country to produce wood for a million boilers.
It is pretty certain that some new policies will come in, especially if the targets look liked being missed; the embarrassment of claiming world leadership on climate change and then failing to transform our energy sector would surely be too much.
It would all be so much easier if we could improve energy efficiency - the least sexy topic in the whole energy book, something that Britain is spectacularly bad at, but most experts' choice as the single most effective measure any country can take to curb its energy use and carbon emissions.