I have a squirrel guard on my system - IF it is well secured, then it will help, but in my case I've had to have it fixed twice in 5 years. The first one was put on with zip ties - the squirrels use those as dental floss and quickly got in. This was upgraded to metal fasteners. The 2nd time they found a way in at the top of the roof.
Once under the panels, they made a nice nest under the lowest row of panels up against the mesh of the guard. Perfect spot - nice and warm, protected from the weather. They filled it with leaves and bits of plastic bags. If I didn't have a guard they may not have been able to do that as the nest material would have just fallen off the roof.