Air conditioning (AC) of large commercial buildings represents an attractive target for many different forms of demand response (DR) including DR for ancillary services (AS) such as frequency regulation. The operating cost of such DR is typically discussed in terms of occupant discomfort. However at fast timescales, perturbations to well-functioning building controls may increase the total energy consumption relative to a baseline that does not provide DR ancillary services (DR-AS). The extra energy is a cost of control to the asset owner and should be factored into the cost of providing DR-AS. We performed DR experiments on a similar to 30,000 m(2) office building, and at the 15-min time scale of these experiments, we find the extra energy consumption to be significant. Similar to battery energy storage, we express the energy cost in terms of a round-trip efficiency and use this metric in a simple economic analysis of the cost of frequency regulation from these resources and discuss potential impacts on advanced load management methods. (C) 2015 Elsevier B.V. All rights reserved.