I went down a few rabbit holes to figure this one out -- ultimately the search that got me going the right direction was "estimate savings on heating bills from adding insulation". The first few results there are helpful.
Ultimately, this calculator provides the answers you're looking for: "Home Heat Loss Calculator" from Build it Solar.
Designing for a typical minimum lowest temperature of -20F (-30C), and assuming 8,000 heating degree days in Montana (based on this map), for an 800sqft (75m2) house, using the DOE recommended insulation values for a home in Florida, your total electric usage on an annual basis would be 22,309 kWh. (To get this number I just put the electric cost as $1/kWh, then the value in the "fuel cost" column will be in kWh.)
Using this Degree Days Calculator, it looks like there are about 1,000 heating degree days per winter month in Billings, MT. Thus you can divide the total usage (for 8,000 heating degree days) by 8 to determine energy usage in an average winter month: 2,789 kWh.
I recommend using the tool yourself and adjusting the numbers to get a more accurate answer for your needs.