The next step that Phil Nail, the IT manager of Internet services provider ASIO.net, intends to take to improve the energy efficiency of his data centre is to put a layer of dirt on the roof.

It's called a "green roof" and it involves putting about three to four inches of earth on the roof and planting bush-type, drought-resistant plants. It will cost the firm about $30,000 to cover 2,000 square feet over a data centre that houses about 300 servers. But Nail believes his green roof may cut data centre cooling needs by half.

For those who need convincing of the effectiveness of dirt to cool data centre servers, Nail's advice is to stick their hands in some. "It's cool down there," he said.

Nail is ahead of most in adopting green technologies. His firm already uses solar panels that generate 12 kilowatts electricity, enough to power his servers and lighting. But the US state of California wants to see many more companies add solar power to the mix and began offering this month a $2.50-per-watt tax rebate for systems of up to 1 megawatt in size. Its California initiative has budgeted $2.9 billion for incentives over 10 years.

Globally, solar energy use is an approximately $15 billion industry. But research firm IDC estimates that US spending accounts for only about 10 per cent of the global market.

"For most companies to invest in what is perceived as novel energy technologies is a tough sell," said IDC analyst Nicholas Lenssen. "Most companies want a two- to three-year payback. But, by and large, there isn't a whole lot of interest in on-site generation."

The costs are often high, making incentives critical. For instance, a solar-power-based 500-kilowatt system would cost about $3.5 million based on an estimated cost of about $7 per watt to buy and install it. California's credits can cut that cost by about $1.5 million, and federal tax credits would reduce it by another $1 million or so, according to Rhone Resch, president of industry group Solar Energy Industries Association.

Resch estimates that the bottom-line cost of system would be about $1.3 million.

Any return on investment depends on how much a user is paying for electricity, although Resch said a six- to eight-year payback for a large user would not be unusual. And since a business would likely finance the project, it could generate a positive cash flow from the investment immediately, he said.

Another green approach was taken by Alan Hirsh, executive vice president of Sea Gull Lighting. Last autumn, he completed a 500-kilowatt system that provides 20 per cent of the power needs for a 500,000-square-foot warehouse during the week. At the weekend the panels can provide all of the power needed. The solar panels take up about 60,000 square feet on his warehouse roof.

Hirsh's company is using SunEdison of Baltimore, a solar services company that installs the system and continues to own it. Sea Gull, however, pays a rate for the electricity it uses that's below what it would pay if it bought from a local power provider, he said.

As long as the price of electricity doesn't go down, Hirsh said, the agreement with SunEdison makes sense. "I don't know of anybody who thinks electricity is going to be cheaper," he said.