The installation of residential fuel cell systems (FCs) with photovoltaic generation systems (PVs) in residences has begun in Japan. Because of the reverse power flow from PVs, the line voltage may exceed the proper range during daytime operation. In addition, because residential PVs are connected to a single-phase distribution system, the voltage imbalance rate may increase. Here, we focus on an FC with a large available capacity during the daytime, so as to adjust the voltage by reactive power control with the available capacity of the FC. We propose a method of reactive power control that uses the available capacity of FCs to improve the voltage magnitude and voltage imbalance rate in a distribution network.