What my gas bill tells me

Actually, NStar (now Eversource) tells me some interesting things on my gas bill.

It tells me:
– avg therms per day
– avg temp over the billing period.

So for the most recent bill it was 5.1 therms/day and 19F avg
Last bill: 4.0 therms/day and 29F avg.

Does it make sense? Yes! If we make the assumptions that:

1) 99% of our gas use is for heating and not
– hot water heater
– stove and oven
– clothes dryer (gas)

2) An average inside temp over the course of day and night is roughly 65F

Then
DeltaT= 65F-19F=46F with 5.1 therms per day
DeltaT= 65F-29F=36F with 4.0 therms per day

46/36 = 1.28
5.1/4.0 = 1.28

Or another way:

46/5.1 == 36/4.0 => 9.0 dT for each 1 therm

The ratios are the same! This is a little too good to be true. Probably some luck in my assumptions and guess of 65F. But in any case, that’s roughly what you would expect.

So IOW, if you want to keep your gas bill constant when it gets much colder outside, just keep it colder inside by an equal amount (to keep your deltaT constant) and you should do ok!

The only problem with this (besides not being able to wear more sweaters and hats at some point) is that you should be careful (if your house is like mine and colder in some spots) that you don’t cause some pipes to freeze in that already cold bathroom. Unlikely for most houses, but possible.