A first look at longer-term data trends for the fridge...
At the start of this period, it was well chilled down, note that the water temp was just above 0C. Then a week with an unusual amount of rain and clouds, nearly worst-case situation. It was only able to run for 1-2 hours most days that week.
Looks like 2 hours of running per day is enough to keep the water temperature from rising, while with 1 hour of running, the water temperature rises by around 1.5C per day. (Given exterior temps in the mid-20's.)
The water is a good couple of days buffer to the temperature. I was satisfied with the overall temps in the fridge during that period; while in places they rose to the 8.5C shown in the graph, the place I keep meat and milk etc was below 5C except for a few hours. No problems with any food.
Once the weather improved, it took 2 days of long running (3 and 4 hours) to push the water temperature back down. As long as there's a good day now and then, the fridge seems like it will do well.