The US Department of Energy (DoE) finalised almost $5bn of loan guarantees to four solar projects before the programme deadline expired at the end of last week.

SunPower, NRG Energy, Exelon and Desert Sunlight all managed to secure loans on Friday as officials rushed to complete the necessary paperwork.

Desert Sunlight was awarded the largest sum, $1.46bn, to install around 8.8 million cadmium telluride thin film solar photovoltaic (PV) modules in Riverside County, California. The project will be built in two phases, eventually reaching 550MW capacity, making it one of the world's largest solar PV plants.

Another California plant, SunPower's California Valley Solar Ranch Project in San Luis Obispo County, finalised a $1.237bn loan guarantee. Panels at the 250MW facility will be fitted on single-axis trackers to follow the sun, a technology which the DoE said increases annual output by approximately 25 per cent compared with traditional fixed PV installations.

The 230MW Antelope Valley Solar Ranch, also in California, sealed the $646m loan guarantee necessary to finalise its $1.36bn acquisition by Exelon Corporation from original developers First Solar. First Solar will build, operate and maintain the project, which will use innovative inverters with voltage regulation and monitoring technologies new to the US market to provide more stable and continuous power when it comes online in 2013.

Finally, Project Amp, a scheme to fit 752MW of solar panels across approximately 750 rooftops owned and managed by Prologis, the world's largest warehouse manager, finalised a $1.4bn loan guarantee. The nationwide scheme will install the equivalent of around 80 per cent of the total amount of PV fitted in the US in 2010 in up to 28 states and the District of Columbia.

"This is a remarkable project that will end up being the largest rooftop solar deployment in US history," said Energy Secretary Steven Chu in a statement accompanying the announcement.

"The project not only funds jobs across the country and makes solar power more cost competitive, it can also be a model for other rooftop solar projects of this magnitude and help the US restore its leadership in the solar industry."

The latest awards bring the controversial loan guarantee scheme to a close, having handed out 28 loan guarantees worth a total of $16bn.

The programme has been much criticised by Republicans for a lack of financial oversight after the collapse of Solyndra, a solar panel manufacturer that received a $535m loan guarantee. Reports in The New York Times over the weekend suggested that Nevada Geothermal Power, which was awarded a loan guarantee of $79m plus at least $66m in grants, is also struggling.

But, despite the criticism, President Obama has repeatedly defended the scheme's effectiveness and its necessity in achieving his target of producing 80 per cent of the country's electricity from 'clean sources' by 2035.

Under his leadership, and that of former president George W Bush, who initiated the three loan programmes administered by the DoE, nearly $40bn has been awarded to over 40 clean energy projects across the US.

These have included several of the world's largest solar generation facilities, three geothermal projects, the world's largest wind farm and the nation's first new nuclear power plant in three decades.