European Union leaders have agreed to adopt a binding target on the use of renewable energy, such as wind and solar power, officials say.
EU states will have to embrace wind, solar and hydroelectric power
European Commission President Jose Manuel Barroso said Europe was now able to lead the way on climate change.
The 27 EU states will each decide how they contribute to meeting a 20% boost overall in renewable fuel use by 2020.
The measures could include a ban on filament light bulbs by 2010, forcing people to switch to fluorescent bulbs.
The bulbs last longer but more are more expensive to buy.
In another key measure, agreed on Thursday, EU leaders said they would cut carbon dioxide emissions by 20% from 1990 levels by 2020.
BBC world affairs correspondent Nick Childs says there is an air of real achievement in Brussels.
But, he says, the compromises over each nation's share of the burden in reaching the targets have yet to be negotiated, meaning the hard decisions may still lie ahead.
Mr Barroso described the agreement as historic, saying it was the most significant in which he had played a part.
"We can say to the rest of the world, Europe is taking the lead, you should join us in fighting climate change," he said.
UK Prime Minister Tony Blair said: "These are a set of groundbreaking, bold, ambitious targets for the European Union.
"It gives Europe a clear leadership position on this crucial issue facing the world."
Looking ahead to the G8 summit of industrialised nations in June, Mr Blair said the European deal would give "a good chance" of getting the US, China and India on board too.
German Chancellor Angela Merkel, who chaired the two-day summit, also welcomed the package of binding measures.
"I personally am very satisfied and happy that it has been possible to open the door into a whole new dimension of European co-operation in the years to come in the area of energy and combating climate change," she said.
"We can avoid what could well be a human calamity."
The EU plan involves:
- A 10% minimum target on the use of bio-fuels in transport by 2020
- A commitment to increase use of solar, wind and hydroelectric power
- A possible ban on incandescent bulbs - with filaments - in offices, street lights and private homes by the end of the decade
EU officials are working on a directive that would compel the use of modern low-energy fluorescent light bulbs. It could come into force as early as next year.
The Australian government announced similar plans to phase out old-style filament bulbs last month.
The statement on renewable energy sources allows flexibility in how each country contributes to the overall target for the EU.
Poorer Eastern European countries, which are more dependent on heavy industry and carbon-heavy coal, had argued they would struggle to make the investment in wind farms and solar power necessary to meet binding targets.
EU leaders will negotiate on how the overall target is reached
The final text allayed their fears by stating that "differentiated national overall targets" for renewables would be set, "with due regard to a fair and adequate allocation taking account of different national starting points".
In what is viewed as a concession to France, the text recognises the contribution of nuclear energy in "meeting the growing concerns about safety of energy supply and carbon dioxide emissions reductions".
However, it also highlights safety concerns, stating that "nuclear safety and security" should be "paramount in the decision-making process".
It is thought the EU could offer to extend its 20% target for emissions cuts to 30% if other heavy polluters like the US, China and India come on board.