I found a calculator that coverts gallons of heating oil to therms. (Therms are apparently the unit you're billed for in natural gas).

http://www.convertunits.com/from/gallon+[U.S.]+of+residual+fuel+oil/to/therm+[U.S.]

450 Gallons of heating oil = 674.2295 therms

Here is the price of gas from a provider local to me:

https://www.nationalgridus.com/niagaramohawk/home/rates/4_gas.aspThere is a minimum charge of $17.85 for the 1st 3 therms. I assume one would get hit with that monthly. So 12 months x 17.85 = $214.20 for 36 of your 674 therms.

The next tier is .4189 for the next 47 therms. That's $19.68 per month. 19.68 x 12 = $236.26 and 564 therms. So we're up to 590 therms. 84 are left @ .06385 = $5.36.

214.20 + 236.26 + 5.36 = 455.82

This seems crazy cheap to me vs what I paid for oil. Maybe I'm doing something wrong or maybe there are fees and taxes not shown on the rate table. Maybe someone with a gas bill or better math skills can illuminate me.

This month's average for my oil provider was $3.777. 450 gal x 3.777 = $1,699.65.

Using my weirdly low gas cost and pretending prices are stable = $1244 annual savings. So if 4K is the total cost of the switch, it would be 3.2 years to break even. After that, your 4K pays you $1244 a year. I'm stuck on figuring a rate of return for that. It takes 3.2 years to get your investment back, so after another 11.8 years you earn $14,679 over a total of 15 years.

I honestly don't know how big a rate of return that is. Anybody want to fix my f'd up math or assumptions here?

Other important considerations, what is the lifetime of the new equipment before it needs replacement? 15 years total? So, invest 4K to earn $14,679 over 15 years. What happens if you put that yearly $1244 to work for you @ 5% (over the last 11 of 15 years), vs putting $4000 lump sum to work for you @ 5% for 15 years? I'm way over my head here. Somebody save me.