Accurate estimation of leakage power at runtime requires post-silicon power measurements across a wide range of temperature and voltage conditions. Testing individual chips, especially at high-temperature corner conditions, is expensive in cost and time. We examine this problem in an industrial context and introduce FreqLeak, a frequency step based method for inexpensive and efficient leakage power characterization in a system. It enables a more thorough characterization than can be accomplished on a wafer prober alone due to time and equipment costs. Experimental evaluation on IBM POWER8 based systems demonstrates the efficiency of the proposed method, within an error of 5%. Further, we discuss the application of FreqLeak in system level power management.