Is there any particular criteria for sizing of battery storage in HOMER ? How do we size a converter ?
We have found that there is no reliable rule of thumb for determining optimal value of battery autonomy. It depends on many factors, including the daily and seasonal patterns of the load, the daily and seasonal patterns and the intermittency of the relevant renewable resources, the correlation between the load and renewable resources, the fuel price, the size of the backup generator relative to the peak and average load, etc.
So we always rely on HOMER's ability to optimize the size of the battery bank, rather than using some pre-defined size. One possibility is to calculate the number of batteries corresponding to a comfortably long autonomy such as 4 days, and specify a range of possible values from that value down to zero. In truth, I usually rely on trial and error, using HOMER's feedback to revise the battery search space over several runs. This process is much easier in version 2.1 that it was previously because in version 2.1 the information on "winning sizes" appears in the Optimization Inputs window. So you can specify some battery sizes, hit Calculate, then check the bottom half of the Optimization Inputs window to see which battery size was optimal, and use the top half of the window to revise the battery search space if necessary. You can do the same for any other component as well.
Sizing the converter is somewhat easier that the battery bank because there is a clear upper limit to the converter size, namely the peak load. For example, if the load is AC and its peak value is 6 kW, it is very unlikely that the optimal converter size will exceed 6 kW. I might try a size range such as 6, 4, 3, 2, 1,and 0 kW.