Each ballast change can incur 3 costs, the cost of a new ballast, the cost of disposing the old ballast, and the cost to pay some one to change out the ballast. The calculation does not allow for partial quantities of replacement, ie .2 ballast a year, is rounded up to 1. Ballast replacement is done using a spot replacement strategy. An annual quantity of ballast to be replaced is determined from the ballast life and the annual hours of operation. Ballast changes are only calculated if the ballast type is selected as something other than 'none'.

When controls are used they can reduce the hours of operation which reduce the ballast changes.

The default ballast lifetimes are based on the selected lamp type. The default ballast lifetime is 100,000 hours. HID sources have a auto filled ballast life of 60,000 hours. The links below will take you to pdfs produced by a ballast manufacturer that document their expected ballast lifetimes. These ballast lifespans may not be applicable for your luminaire.
Fluorescent Ballast Life
HID Ballast Life
LED Driver Life