Based on what I've read here, this is what I've come up with. What am I missing?

1 btu 1 hour 1 lb 1 degree

5000 gals

8 lbs

40000 lbs H2O

5 degrees

200000 BTU Needed

400000 BTU Avail

0.5 Time to heat (hr)

0.45 per therm

100000 BTU per Therm

2 Therm to heat

0.9 cost to heat