All these amenities exist without the benefit of power lines or fossil fuels, or a freshwater supply, so there are no utility bills. But cellphone reception can be a problem.

Recently, Mr. Soley adapted a signal extender made for Humvees, which seems to be helping. Before that, he was dependent on an elaborate system of pulleys rigged to the tallest tree on the island. If he sent his phone up there for 20 minutes, he found, he could usually get a few emails through. Then one weekend, he paddled all the way back to the mainland before realizing his phone was still on the island, dangling from a tree.

Problems like this seem to preoccupy him, but not in an unpleasant way. And electricity — what the home’s system is capable of, how much he can add without overtaxing it — is always on his mind. That’s not because he’s worried about it, he said: “I think about it all the time mostly because I like to.”

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Not that there aren’t plenty of other distractions on the island. Mr. Soley recently put in a zip line, and there is a climbing wall on one side of the house. There is also plenty of entertainment of the natural variety, which appeals to Mr. Soley, who grew up in Camden, Me., and “was the kid always out playing in the woods,” he said.

More than once, he and Luna have watched a mink battle a lobster along the shore, wrestling it into submission. (The mink always wins, he said. “They’re tough little suckers.”) They have detected the presence of a deer passing through, as well as a moose that stopped by while they were away, devouring a black walnut sapling they had brought over the from the mainland. And they have amassed a collection of sand dollars and sun-bleached sea urchins that now lines the trail to their favorite low-tide swimming spot.

As Luna said, “It feels like another universe surrounded by water.”

Still, it is not the ideal home for aging in place. But then, Mr. Soley doesn’t seem interested in aging just yet. In many ways, he is still that boy playing in the great outdoors. He once kayaked from Newfoundland to Labrador, across the notoriously difficult Strait of Belle Isle, and thinks nothing of donning a double layer of wet suits and swimming from one island to another in the frigid water. Every year, he trains for a marathon solo swim; this year’s was seven miles, from the Isles of Shoals to Kittery, Me., which he finished easily.

Does Luna ever worry about him?

“Nah, I have faith in him,” she said. “He’s careful.”