You can't calculate a system if it's undersized

In fact, you can allow the system to supply less than 100% of the electric load.  The principal constraint in HOMER is the maximum capacity shortage constraint, which by default is 0% meaning the system must fully supply the electric load plus and required operating reserve.  But you can set that input to any value you want.  For example, if you can accept the system serving only 95% of the electric load plus operating reserve, you can set the maximum capacity shortage constraint to 5%.  You enter that value on the Constraints window.