I keep my place at 15.5c in winter because it’s super drafty. (I’m getting the siding redone soon, I really hope that helps, but ultimately we have the same climate as Siberia so there’s only so much to be done) even at 15.5, it’s still about $200 USD/mth to heat, but at 18c it more than doubles in cost.

I’m like your wife; made for warmer climates. My ideal temp is around 30c, and I’m cold at 23, but I have heated mattress pads on my bed and couch (much much much cheaper to run than furnace) so it’s not too bad overall. They are a bit pricy up front, but definitely worth the spend.

Perhaps that sort of thing would be a good compromise for you two; a couple heated chair covers or couch cover or something to bring her temp up while keeping the overall temp lower.