Which of the following would decrease if a PV array is undersized?

When a photovoltaic (PV) array is undersized, the system may not generate enough energy to meet the demands placed upon it. This can lead to more frequent cycling of the batteries, as they are called upon to supply the additional energy needed. Over time, this increased cycling can degrade the battery's components and reduce its overall lifespan.

The other choices focus on different aspects. System efficiency relates to how effectively the array converts sunlight into usable energy and isn’t directly impacted by the size of the array in this context. Inverter output depends on the amount of energy produced by the PV array and its inverter capacity but it is not reduced purely by size alone—it is more about the conditions under which the inverter operates. Voltage stability involves the regulation of voltage levels within the electrical system, which is less direct than the effects on battery life when the array is undersized, as this might get impacted by overall system load and management rather than just the size of the PV array.

Thus, the connection between an undersized array and the negative impact on battery life makes the choice clear.