By Damian Kahya
Business reporter, BBC News
The North Sea could offer a solution to much of Europe's energy needs
The power of the wind and waves is ever-present in the Danish winter.
The Copenhagen climate change talks will discuss how to capture the energy from such "renewable" sources.
The UK plans to get 15% of all its energy that way within just 10 years.
Some experts believe it could provide for all our needs.
But the government's adviser is sceptical of just how far we can go.
More than just our homes
Professor David MacKay is the author of a book on sustainable energy.
Optimists say the powers of nature alone could provide enough energy
He calculates that even if we covered every available plot, offshore location and tidal estuary with wind, wave and tidal power we may not meet all our current energy needs.
Total UK energy output is equivalent to 2.7 million GWhours of electricity or 310 GW of capacity.
The market value of all energy consumed in the UK is £130bn ($216bn) a year with the amount spent on energy by final users at about £75bn.
Only about a third of the energy we use comes in the form of electricity.
Transport and heating are also key.
The wind doesn't always blow
Renewable energy is often unreliable.
Wind turbines are being built across vast areas across the world
They include wind, hydro-electric, tidal, solar and wave and geothermal power along with energy from plant and waste material - so-called biomass and bio-fuels.
The wind does not always blow so most wind-farms average 20-30% of their capacity.
The governments suggested 33 GW of offshore wind will generate just a third of that.
Tidal and hydroelectric power are more reliable, but hard to locate.
Professor MacKay estimates there are enough good spots for hydro-electric to provide 0.8% of our current energy needs.
Tidal power has just one commercial turbine in the Strangford Lough, Northern Ireland.
Carbon Trust research suggests the technology could provide 0.6% of current energy use.
But the numbers are rough.
"We're roughly where aviation was in 1920s," says Peter Fraenkel, technical director for the company behind the Strangford project, Marine Currents Turbines.
Finding the space
Space is also an issue for wind, solar and biofuels.
The government says coal will remain in the energy mix
Installing just 33GW of offshore wind would need 10,000 square kilometres of coastal waters generating only 3% of our current energy supply, Professor MacKay estimates.
It means that we would struggle to exceed 50% of our current needs from renewable energy, he insists.
"People love renewable energy," according to Professor MacKay, "unless it is bigger than a fig-leaf".
Problems getting planning permission for wind power contributed to the decision by Danish wind turbine manufacturer Vestas to leave the UK.
Taking this into account Professor MacKay thinks the amount we could source goes down by half.
Others say renewable energy could generate far more power, per meter, in future.
A big investment
The economics are also debated.
The total costs of coal and gas power is estimated to be under 5p/kwh.
But the cost of buying electricity varies dramatically, partly because of the costs of fossil fuels.
The UK's Climate Change Committee estimates wind power would cost a more steady 7-9p/kwh.
Because it is unreliable, wind energy may need backup power costing an extra 1 -2p/kwh.
Hydroelectric and some biomass energy cost about the same.
Wave, tidal and solar power, though, are currently far more expensive.
Each extra pence per kw/h would be about £33 a year on current home electricity bill.
In total, the consultancy Ernst and Young calculates the investment needed to meet the governments targets at over £100bn.
Partner Ben Warren says raising the capital has become far harder recently.
The government accepts consumers are likely to pay more.
Heating bills are set to rise in the years ahead
It expects electricity bills to go up by 15% and gas bills by 23% within 10 years to fund its renewable strategy.
However, if oil prices rise to their 2008 levels again the additional cost of the measures is almost nothing.
Oil might push bills up anyway.
The government plan aims to tackle some of the problems raised by its advisor.
It hopes a raft of measures will speed up planning procedures.
Home energy efficiency measures may decrease demand and therefore cost to consumers.
Smart grids would help match energy demand to changeable supply, from wind for example.
Energy minister Lord Hunt says the government is also pursuing non-renewable energy.
"We are running with what Ed Miliband called the Trinity of low carbon energy; nuclear, renewables and clean coal".
In California, engineer Mark Jacobson and scientist Mark Delucchi say they have found a way to deliver 100% renewable energy.
They say it is cheaper than the alternatives, clean coal and nuclear power.
The plan calls for 3.8 million large wind turbines and 90,000 solar plants worldwide and envisage that production on such a scale would drive the cost down.
The key is sharing renewables across continents, using space where available.
"You want to connect resources," says Mr Jacobson. "Solar from the south. Wind from the north. You can solve multiple problems and provide a reliable supply of energy."
The plan raises concerns about energy security, but avoids the need to import or pay for oil.
They estimate transporting energy would add 20% to the cost.
Hydro-electric plants and millions of electric car batteries connected through a smart grid would be used to store and release electricity and increase reliability.
Energy efficiency is also emphasised.
Professor Mackay calculates that switching to an electrified economy and major efficiency savings in heating and transport could reduce energy demand by around half.
Mr Delucchi and Mr Jacobson say their scheme costs $100 trillion (£60 trillion) worldwide, spread over the life of the new equipment.
They admit it would need strong government and public support.
"You're just not going to luck into this kind of system," says Mr Delucchi.
It will not be an easy decision for the UK to take.