A lot. The engine will probably never shut off it’s primary mission is emissions so it will warm up the cat converter first. At that temp I would block off the rad vents lots of cold Prius owners do that. Hopefully you have th3 right viscosity Oil in the engine and good anti freeze in the block. Good luck that’s crazy cold.
Too much, my volt was only getting 35mpg gas only and and 35 miles EV at that temp. It’s never that low otherwise
Temps were down in the -15F to -20F range for a few days here in sunny southern MN. The mileage on our 2010 went WAAAAAY down ( low 30s ). I have not done anything to mitigate the cold like grille blocking, etc... I'm happy to say that itemps are back up in the mid 30s and mileage is back up to the mid 40s or so.
