I would not recommended RECS (Renewable Energy certificates). Even though they are endorsed by the USEPA and several other NGOS, it's been empirically proven to have no impact on the additionality of renewable energy investment or the dispatching of renewable energy generators. For more information I would read studies from Michael Gillenwater and the GHGinstitute. Having the servers powered directly from a renewable energy source (renewable energy generators physically connected to the electricity transmitted to the servers) is the best way to ensure your using renewable energy. However this may not be the easiest or most inexpensive method. Renewable generation is also variable generation which I would assume may be an issue for servers that need to run 24/7.
To elaborate further, Renewable Energy Credits are commodity instruments used to represent the ownership of an environmental attribute for 1 MWH of renewable energy generated. I don't want to go too deep into it, because I'm basically say that I would not recommend it as a metric for determining how sustainable a company or organization is (in this case a hosting service). Many organizations endorse RECS, but if you would like to further understand why they do not impact renewable energy generation, please check out the links below.
Brander, M. 2014. “The Enron of carbon accounting?” Isonomia Blog Post, March 7th, 2014.
Gillenwater, M., Lu, X. & Fischlein, M., 2014. “Additionality of wind energy investments in the U.S. voluntary green power market.” Renewable Energy, 63, pp.452–457.
Gillenwater, M., 2013. “Probabilistic decision model of wind power investment and influence of green power market.” Energy Policy, 63, pp.1111–1125.
Gillenwater, M. 2013. “Redefining RECS: Additionality in the Voluntary Renewable Energy Certificate Market“, Doctoral dissertation, Princeton University.
Offset Quality Initiative, 2009. “Maintaining Carbon Market Integrity: Why Renewable Energy Certificates Are Not Offsets”, June.
Gillenwater, M., 2008a. “Taking green power into account,” Environmental Finance, October.
Gillenwater, M., 2008b. “Redefining RECs (Part 1): Untangling attributes and offsets,” Energy Policy, Volume 36, Issue 6, Pages 2109-2119.
Gillenwater, M., 2008c. “Redefining RECs (Part 2): Untangling certificates and emission markets,” Energy Policy, Volume 36, Issue 6, June 2008, Pages 2120-2129.