Units of AC annual output

Hello, 

I just want some clarification as to how the AC annual output is calculated from the AC hourly output in PVWatts. I've been working with PySAM and I wanted to calculate the Annual AC output from the hourly data. I added all the values in System power generated (which is hourly data in kW) and then multiplied by the number of hours in a year (8760 hours) to get the kWh generated in the year, but I got a different result than the Annual AC output that PySAM gives. I noticed that I just needed to add all the values in System power generated to get the Annual output that PySAM gives. But System power generated is in kW and Annual AC output is in kWh, so why is the time not included in the calculation? How do we get kWh from kW? 

I checked the  manual  but it only says, "The hourly outputs DC and AC power are also aggregated into monthly and annual energy totals that are reported to the user", so I'm not sure how the values are aggregated in order for the units to be in kWh.

Thanks!