The maximum grid demand parameter on the Advanced tab of the Grid Inputs window will force the generator on whenever the net demand (Load - renewables) is greater than that value.  HOMER may choose to run the generator more than that if it is cost-effective to do so.  Another option would be to schedule the generator to run for certain hours using the Schedule tab of the Generator Input page.  That would be more work and less elegant.

You can add noise to the load when you have the radio button labeled "Enter daily load profile(s)" checked.  You would then have to enter or cut and paste the 24 hour profiles for each month from your spreadsheet into HOMER.