String availability at string level show if a string is producing or not (it takes values 0 or 1).
String level availability at site level is calculated as the weighted average from the string availability values. It shows the percentage of strings nominal power that is not producing 0 current.
String Availability calculation at string level
In each string, if the irradiance exceeds the "Irradiance threshold for availability calculation" and if the string current is > 0, then string availability receives the value "1" for this quarter of hour.
You can find the "Irradiance threshold for availability calculation", in the tab "Settings" as shown below. You can edit the Settings by the "Edit" icon.
At string level apart from the parameter "String availability", there is also "Weighted string availability".
Weighted string availability=(String availability)*(pv panels number on this string). String is considered to be each input of the DC Box.
String Availability calculation at site level
String availability at site level is calculated as weighted average of the string availability values of all the strings.
Let's suppose that there k strings in the site. The formula is
String level availability=100%*(string1_AL*string1_nominalpower+...+stringk_AL*stringk_nominalpower)/(Site Nominal Power), as long as the condition "Irradiance PoA Average" > "Irradiance threshold for availability calculation" is satisfied.
So if all strings have exactly the same nominal power, then String level availability is just the average of all availability values of all the strings. In that case let's suppose a site with 100 strings, with 2 of them not producing. Then String level avalability is 98%.