Bloomberg

Amid a more general makeover, the owners of the Empire State Building have spent the last couple of years burnishing the 80-year-old landmark’s green bona fides as well — chiefly though a series of efficiency upgrades and other energy retrofits.

On Thursday, the building’s caretakers announced another milestone: it has become the largest commercial purchaser of renewable power in the state.

In a two-year deal brokered with Green Mountain Energy, a renewable power and carbon offset retailer recently acquired by NRG Energy of New Jersey, the Empire State Building will purchase 55 million kilowatt-hours worth of renewable energy certificates annually — enough to cover its yearly electricity consumption.

The certificates are sourced through NRG’s wind power facilities.

In a statement, Anthony E. Malkin, the president of Malkin Holdings, which supervises the Empire State Building, suggested that the move, along with the building’s other greening efforts, “gives us a competitive advantage in attracting the best credit tenants at the best rents.”

To be sure, trumpeting large-scale purchases of renewable energy certificates, or R.E.C.’s, has become increasingly common among businesses and institutions seeking to improve their green reputations.

The Environmental Protection Agency, which “supports the organizational procurement of green power” through its Green Power Partnership Program, is a prime mover along these lines. The program’s Green Power Leadership Awards for 2010 recognize a number of businesses and organizations — including the Indianapolis Zoo, which now covers 100 percent of its electricity needs (14 million kilowatt-hours annually) through R.E.C. purchases.

The Intel Corporation, which buys 1.4 billion kilowatt-hours worth of R.E.C.’s — about half of its annual consumption in the United States — has been the national leader since 2008, according to the agency.

Of course, as popular as renewable energy certificates have become, their real-world impact in helping to advance green energy is notoriously difficult to measure — and a subject of much debate.

The E.P.A. provides a fair amount of explanation on just what R.E.C.’s are — essentially nontangible assets representing “the environmental, social, and other nonpower qualities of renewable electricity generation.” One R.E.C. is created for every 1,000 kilowatt-hours of renewable power generated, and the certificates can be sold with, or in some cases separately, from the power.

Ideally, the whole thing would work as Green Mountain Energy elegantly lays out in its “bathtub” analogy: Power from all sources, clean and dirty, is dumped and mixed into the grid, which might be thought of as a tub. Customers draw power from that tub as needed, but they can’t really separate the clean wind power, say, from sooty coal-based power.

Still, the company explains that demand for renewable energy — as expressed through voluntary purchases of certificates — will slowly increase the proportion of clean energy going into the tub.

Some skeptics caution that this is only true if the certificates help create new clean-energy facilities, rather than simply provide added profit to existing ones.

Matthew Freedman, a staff lawyer with the Utility Reform Network, a California consumer advocacy organization, suggested that there are two scenarios where R.E.C. sales do make a difference:

(1) The R.E.C.’s are sold under a long-term contract and this long-term contract allows a new renewable generation facility to receive financing and achieve commercial operations. In this case, the long-term R.E.C. deal provides critical revenues that enable the project to be financed. (2) The R.E.C.’s are sold by a facility with high operational costs and the facility would shut down without future R.E.C. revenues. It is possible that some high-cost biomass plants could meet this criteria if their fuel costs are significant and they lack a sufficiently lucrative energy off-take agreement. The key question is whether the facility is likely to continue to operate based only on energy market revenues (without any R.E.C. sales). In the case of an existing wind project, there is no chance that a R.E.C. deal makes a difference because the operational costs are low and most wind projects receive lucrative federal tax credits based on production over the first 10 years. Once a wind project is online, the facility will continue to operate with or without a R.E.C. deal.

“The bottom line,” Mr. Freedman added, “is that you need to investigate whether the R.E.C. transaction satisfies the principle of additionality. Otherwise, the transacting of R.E.C.’s serves only to provide the buyer with positive public relations value and increases profits for the seller.”

Whether or not the Empire State Building’s R.E.C. outlay will prompt NRG to build new wind farms — or will help shore up one of its existing facilities — is unclear. That aside, it still seems a notable milestone for Manhattan’s most famous landmark. The purchase, according to Thursday’s announcement, is “more than double the amount of renewable power that any other commercial customer in New York City is currently buying.”