You might want to revise your numbers a bit.
You won't be running your heater at 100% capacity 24 hours a day.
This. If you ran a 60K BTU heater 24/7 in a well insulated house, the internal temp of your house would probably get hot enough to literally cook you.
You currently have heat right? You paid for it thus you can compute the average daily energy use you spend on heating. That is the amount of energy you would need to replace.