The clean tech sector is "exploding, especially in the US", says Torsten Merkel, European director of Cleantech Group, which advises investors.

California attracted $307m of venture capital in the first quarter of 2007, compared with $172m in the whole of Europe, he says.

California is putting $3bn into a 10-year programme to put solar panels on a million roofs, while the federal government chips in 30% of the installation costs.

The state has mandated that 20% of its electricity must come from renewable sources by 2010.

WASHINGTON (AP) - The boost in troop levels in Iraq has increased the cost of war there and in Afghanistan to $12 billion a month, and the total for Iraq alone is nearing a half-trillion dollars, congressional analysts say.

All told, Congress has appropriated $610 billion in war-related money since the Sept. 11, 2001, terror assaults, roughly the same as the war in Vietnam. Iraq alone has cost $450 billion.

The Vietnam War, after accounting for inflation, cost taxpayers $650 billion, according to separate CRS estimates.

The $12 billion a month "burn rate" includes $10 billion for Iraq and almost $2 billion for Afghanistan, plus other minor costs. That's higher than Pentagon estimates earlier this year of $10 billion a month for both operations. Two years ago, the average monthly cost was about $8 billion.