Two possible operation modes of various pumps with redundancy in electric power generating units during low load periods (night) were analyzed. The first mode - two of three pumps work at night with 25% of nominal capacity, the third pump is a cold (passive) reserve. The second mode - one pump works at night with 50% of nominal capacity, two pumps are in cold reserve. A Markov reward model was built for the comparison analysis of these possible operation modes. The model takes into account all important factors - pumps power consumption, pumps failure rate, pumps starting availability, cost of alternative energy, and penalty cost of energy not supplied. It was shown that under current operation conditions the second operation mode is more effective one.