In 2011, enough solar panels were produced worldwide to generate 27 gigawatts, compared with 7.7 GW in 2009. Solar power is now cheaper than diesel anywhere that has reasonable sunshine. That means vast areas of Latin America, Africa and Asia could start adopting solar power.

In the US, the solar panel maker Solyndra received a $535 million federal loan guarantee which came from the stimulus package in 2009. The company was supposed to become an American success story of greentech innovation and manufacturing. Instead, it became a high-profile casualty when it was evident that it couldn't compete against solar panels built by Chinese competitors with big government subsidies.

Despite the collapse of Solyndra, early investments in clean energy have begun to pay off. The US solar industry grew 69% in the last year, still representing the fastest growing industry in the nation. More than 100,000 Americans in all 50 states are employed by solar, more than US coal mining or steel production.

The price of solar panels fell by almost 50% in 2011. This is now just 25% of 2008 prices, making solar power a cost-effective option for many people in developing countries.

25% of India's people don't have access to electricity, and those who are connected to the national grid have frequent blackouts. To cope, many homes and factories install diesel generators. But burning diesel has been linked to health problems, plus it contributes to climate change.

In India, solar electricity is now cheaper than diesel generators. This boosts India's "Solar Mission" to install 20,000 megawatts of solar power by 2022, and has implications for other developing nations.