Most of the PV installs people see in their community happen to be placed on top of a home’s roof. While this is aesthetically pleasing, space saving on the property and out of the way from people’s hands there are a few drawbacks to this method, which I have outlined below.
1. If the angle and orientation of the roof is not ideal then you will lose some percentage of your overall production. Ground mounting allows you to be more flexible in both regards.
2. Maintenance on your PV array may be required in the future and having to trouble shoot an array on the roof is much more time intensive and dangerous than a ground mount array which allows you you to walk right up to the array and easily navigate around it for maintenance and trouble shooting.
3. Performance with a ground mount system will always be better than roof top arrays since the system on the ground has more airflow to cool down the panels vs. having the array on the roof and within inches of a hot surface.
4. Your PV system is designed to run for many decades to come and the odds are your roof will need to be replaced before the end of the PV systems lifetime. Ground mounting will make sure you will not have to take down and reassemble your array when a new roof is needed.
5. For roof top installs the racking system has to penetrate the roof with lag bolts and as any roofer knows the more hole penetrations you have the more chances for failure in the future. Installing your PV array with a ground mount will guarantee not to interfere with your existing structure and will also allow you the space to mount the panels anyway you like vs. having to conform around roof vents, antennas, and other misc obstructions.
6. Room for expansion is usually not as much of an issue with a ground mounted array.
I do see the reasoning to put PV on roofs in locations that do not have adequate space or a good solar window on the ground within the property boundaries. Also, some properties have other problems such as tall trees or a heavily wooded property in general. I guess one could also say wiring runs are shorter for roof top than ground mounted arrays, but with high voltage grid tie inverters the wire size can be smaller even for longer distances. And for battery based systems there are now high voltage charge controller options such as the Midnight Classic’s or the new Xantrex XW MPPT 80A Solar Charge Controller (up to 600V DC input!) to keep wire size down. I’m sure others could come up with a host of “custom” situations where roof would be better. But that’s just it, most all installs are custom, but I’d say in general, a ground mounted array is the best choice if you have the option to do so.