We also have a surge guard and it's permanently installed, so it's always doing its job.
Finally this fall I broke down and bought an autoformer after dealing with one too many low voltage situations. I'm now sorry I didn't buy one a couple of years ago.
I won't drag out the autoformer unless I actually see that we have power problems since those situations are in the minority.
Recommend you always use the surge guard since it will be a little better at clamping voltage transients than the autoformer. When you need the autoformer, put it ahead of the surge guard so it is between the surge guard and power source.
2005 Horizon 40AD, 2005 Jeep Liberty CRD