degreedays dot net will assist in determining
1. how many weeks natural gas would be needed.
2. if last year was atypical or not.
How much gas did you use last year?
Conversion to electric should be investigated.
100 therms =~ $100. natural gas.
100,000 BTU per hr/ therm
3412 BTU per Hr/ kW
$0.09 $ / kw or YOUR Rate
$263.77 electric strip backup
$163.77 extra over Natural Gas
$ _50 N. Gas
$ 131 electric
$ _81 extra for electric strips over N. Gas
Getting rid of the $20/mo average (use your numbers) for the gas meter = $240/yr savings.
Also keep in mind HP and strips can run at same time, giving a slightly higher COP.