Can anyone recommend any good solar calculating software they have used for off-grid commercial applications ?
Provided you know how much energy your system needs to deliver, then there is a global solar resource data map available free on line. This will give you a kWh/kW resource figure for your intended location. This makes the size of the solar array a simple (?) calculation. You then need to allow any margin for shading, if the array is not optimally aligned etc..
Usually a margin is also allowed for aging of the solar modules and system inefficiency. Calculating these margins are where the professionals earn their money.