If there is a golden rule for data center managers, it is this: Don't mess with your power source. For just about all data centers, that source has been AC power, which has worked quite well for everything from a lone PC in a cubicle all the way up to the most massive server farms.

Yet, as data center power usage soars -- the EPA estimates that data center energy requirements will double in the next five years -- some companies are turning to alternative power sources, including solar arrays, natural gas turbines, wind power, fuel cells and hydro power.

ROI is what is driving interest in alternative power. Ted Ritter, analyst, The Nemertes Research Group

The big question is whether these power sources will deliver returns on the investments they require, and how long that ROI will take to materialize. "ROI is what is driving interest in alternative power," says Ted Ritter, an analyst who studies green IT at The Nemertes Research Group Inc.

Ritter says many companies have a hard time justifying an investment in alternative power sources right now -- especially if it means completely replacing the reliable AC power already coming into the building, as opposed to merely supplementing AC. He says another factor to consider if you're mulling a move to alternative power is that you may also need to look for new reliable backup power source -- and that could involve additional costs and a range of other challenges.

The organizations we interviewed for this article -- and others profiled in Computerworld's latest "Top Green-IT Organizations" report -- have worked out those issues. In many cases, they expect full payback within 15 years.

Solar power at a California governmental agency

For the North County Transit District (NCTD) in San Diego, Calif., solar was the most obvious choice for alternative power. (Even though the NCTD is located near the coast, the wind in the area is not steady enough to be a viable power option.) The organization's data center is relatively small, but it's big enough to enable the agency to serve about 12 million public transportation users per year, processing all ticketing systems and video from the security cameras in transit stations.

The North County Transit District sells power generated by solar means back to the local utility to earn credits on AC power usage.

Angela Miller, the NCTD's CIO, says her group felt the need to go green and become a better environmental citizen. As part of a data center redesign, the agency spent about $600,000 on a 30-panel solar array, plus virtualization technology for existing server and storage systems, and new pods that pull hot air out and help cool equipment inside the racks. The NCTD sells power generated by solar means back to the local utility to earn credits on AC power usage (which is allowed under California law), meaning the solar power initiative has become a profit center. The solar panels do not generate power for the building directly.

It works like this: The local utility sells the NCTD AC power, then the agency sells the utility the solar energy for a 100% credit on data center usage. The agency has a five-year plan in place to offset all AC power in the building. The NCTD is now generating 15 kilowatt-hours of solar power, but this month it is scaling up to 450 kilowatt-hours, and it plans to reach 1 megawatt-hour in five years. (A kilowatt-hour represents usage of 1,000 watts for an hour.)

"Solar is what has made the [data center redesign] project even have an ROI," says Miller.