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For photovoltaic (PV) systems suffering energy loss due to an accumulation of soil, there is an optimum cleaning interval that balances the economic cost of lost revenues due to soiling loss with the cost of cleaning operations and, thereby, minimizes the total cost per unit energy delivered by the system and, thus, the levelized cost of electricity. This optimum interval may vary, depending on cleaning costs, as well as seasonal variations in soiling loss and energy production. In this work, a general expression has been derived for the optimal cleaning interval and then applied to losses actually observed over a one-year interval at a test site in central Saudi Arabia and fitted to a simple exponential loss model. The results show that costs attributable to cleaning to an optimum schedule are a small fraction of typical operation and maintenance costs even for completely manual cleaning, and even more so with machine-assisted cleaning.