Solar system for GSM sites

Dear all,

Please give your comments on solar system for GSM sites. We are going to implement some of our sites, but still don't know physical how much will be benefited. Just as per our cost benefit analysis it will take almost 3 years to get investment back and after that we will get power almost free of cost (only for those sites where we are using DG sets only, no city power). Wind is not too good here (Afghanistan) for wind power as average wind speed is not more than 4m/s. Looking for some comments from them on solar system who are already installed and getting power from solar system.