Hi John. We ran it at about 7000 feet in the Sierra Nevada’s this fall. About -15C. Ran great. Pumped out the heat. I would go for the same size I got as the power is perfect but I would not want much less. Mine came with the digital thermostat. Great as you can go out and program it to come on some hours later so your not heating while your not there. Very good on fuel. Let me know if you have any other questions. Good luck!