IDG News Service — Google has paid US$94 million for a stake in four photovoltaic power generation projects around Sacramento, California, bringing its total investment in renewable energy generation this year to $880 million, it announced Tuesday.

The photovoltaic projects were set up by Recurrent Energy, and Google teamed with private equity fund Kohlberg Kravis Roberts (KKR) to buy them.

The projects will be among the first to profit from a special feed-in tariff offered by the Sacramento Municipal Utility District (SMUD) to encourage renewable electricity generation. It guarantees a minimum payment of 6.14 cents per kWh during the spring off-peak period, rising to a summer peak of 24.73 cents per kWh when demand for air conditioning is at its highest. The utility resells that same electricity to consumers for 9.67 cents in winter, and up to 18.59 cents in summer. Other businesses hoping to profit from the tariff are out of luck: the utility has now filled its quota for the tariff and has a waiting list.

Together, the four projects are expected to generate almost 160GWh in their first year of operation, about the consumption of 13,000 homes, and will have a peak generating capacity of 88MW when they come online next year, Google said. SMUD put the generating capacity of the four Recurrent Energy projects at nearer 70MW.

Google has already made investments aimed at putting photovoltaic panels on 10,000 homes, but this is its first investment in utility-scale generation, it said. It does, though, already have its own massive photovoltaic generation system on its Mountain View, California, campus.

The company is keen to promote its green credentials. It announced earlier this year that it has been carbon neutral since 2007, and appears to be competing with another Web giant, Facebook, to see who can build the greenest data centers. A common measure of this is the PUE (power usage effectiveness) of the data center, the ratio of total power consumption (including lighting, ventilation and cooling) to power consumed by the IT equipment. A data center with a PUE of 1.0 would waste no power on ancillary functions.

In a report on third-quarter performance of its data centers, Google said its best data center had a PUE of 1.12 for the third quarter, pushed up by the effect of warm summer weather on cooling efficiency, and 1.11 over the 12 months to Sept. 30. Its worst-performing data center had a PUE of 1.24 for the quarter and 1.20 for the 12-month period.

In November, Facebook began construction of a new data center in Sweden that should have a PUE of 1.07, about the same as that of its existing data center in Prineville, Oregon, it said.

Beyond the bragging rights, such competition does have a purpose, lowering the companies' overall utility bills. Typical data centers have a PUE of around 2.0, wasting half the electricity they consume, according to Google.

Peter Sayer covers open source software, European intellectual property legislation and general technology breaking news for IDG News Service. Send comments and news tips to Peter at peter_sayer@idg.com.