The diesel O&M cost is a critical factor in HOMER Pro analyses, and we know frustratingly little about it.  For diesel generators, we typically assume an O&M cost of about 2 US cents per kWh at rated output.  That would be $1/hr for a 50-kW diesel generator.  HOMER assumes that the O&M cost per hour is independent of the power output, so that 50 kW diesel generator would cost $1 per hour of operation whether idling or running at maximum capacity.

HOMER Pro calculates the annual O&M cost of the diesel by multiplying the hourly O&M cost by the operating hours per year.  In a diesel-only system, that the 50-kW generator would run full time and the O&M cost would be $8,760/yr.  In a wind-diesel-battery system if it ran only 3,000 hours per year, the O&M cost would drop to $3,000/yr.