Just to add to what others have said, the only really useful purpose of a circulator is to keep ice off the dock structure during the winter, when ice moves and heaves from the expansion as it freezes. Remember those pressure ridges out on the open lake? That ice movement is exceedingly slow, so that the circulator can keep up with it. But as ice-out approaches, and the pack melts out at the shore and becomes free to move with the wind, a circulator won't help. A pack of ice a half mile or more across, set in motion (however slowly) by the wind is an enormous mass that your dock can't halt.
As mentioned, a circulator ought to be on a 24-hour timer that you can set to run for an hour or more a few times around the clock, typically adjustable in 15 minute increments. There is no good reason to have more than a few feet of open water around the dock. More than that is a waste of power ($$) and may open up the ice too far along the shore, possibly annoying your neighbors who may want access to the ice for fishing or snowmobiling; there have been threads on this forum addressing this issue.
To really optimize the use of power, the ON run times will have to be more frequent and longer during the extremely cold parts of winter, but less frequent and of shorter duration as the end of winter approaches. That means having someone make at least a few changes to timer settings over the winter.
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