I do not understand the usefulness of the Maximum annual capacity shortage. Which are the common values for this parameter for a small turbine of 300W?
The most common value for the maximum annual capacity shortage is zero. If you set that value to zero, you are saying that the power system must meet 100% of the electric load, plus the required operating reserve. If your load is an important one that you want the system to supply at all times, just leave the maximum annual capacity shortage at zero.
On the other hand, if you are willing to accept some unmet load in order to allow a smaller, less expensive power system, you might want to set the maximum annual capacity shortage to 1%-5%. If you set it to 5%, you are saying that you will accept a system that meets 95% of the annual electric load plus the operating reserve. In an all-renewable system, that might save a lot of money. For example, with a wind-battery system there is probably some part of the year, maybe one week or two, with a very low wind speed. For the system to get through that low-wind period without experiencing any unmet load, it might be necessary to have two wind turbines or a very large battery bank, which are not necessary to meet the load in the rest of the year. If you allow some unmet load, HOMER might find that a single wind turbine or a smaller battery bank can supply the load at all times except for part of that low-wind period.
You can read more about unmet load, capacity shortage, and operating reserve in the help file.