The "maximum grid demand" input (on the Advanced page of the Grid Inputs window) does allow you to simulate peak-shaving systems.  For example, if you are simulating a grid-connected microturbine serving a load that peaks at 100 kW and you set the maximum grid demand to 75 kW, the microturbine will operate any time the load exceeds 75 kW because HOMER will not allow more than 75 kW of grid purchases.  The maximum grid demand is a decision variable so you can enter multiple values and let HOMER find the optimal one, but you can enter only one value if you wish.  It applies throughout the year, which might not be quite what you want.