I keep my arena at 40 I guess ( 5 C) and the barn at 15 C (59 F). We ride and turnout not matter what the temperature. We will shorten turnout when the windchill is -30 and below. The horses are all clipped and wear blankets with hoods when they’re outside. The days they get cold are spring days with rain, rarely in dead of winter. The young horses that live outside don’t get blanketed and they are fine! (with shelters full of straw). Riding horses that live out are clipped with with legs and heads left on. They don’t get ridden when it’s super cold if it’s at night.
It is very expensive! We are lucky to have natural gas so less than some places but still a lot. Electricity is almost as much in winter as gas since we have to plug in all the machines and the waterers outside are heated. Plus it gets dark here at 4:30 in the afternoon and doesn’t get light until 8am in December. Thankfully it’s light out until 11pm in summer!