You can define the BESS duration by editing the component in the Library. HOMER calculates the power of the battery by multiplying the Max. Discharge Current (A) by the Nominal Voltage (V) to get power in Watts. So it doesn't really matter what you set the Max. Discharge Current or Nominal Voltage values to be, you just need the product of the two numbers to equal the power you want in Watts. So this is how a 1MW BESS could be set up:


Then to set the duration, multiply the Max. Discharge Current (A) by the duration in hours (h) and enter that value as the Maximum Capacity (Ah):

You can confirm you did it correctly by opening up the "Complete Catalog" of storage components and checking the power & duration in that table: