How do I calculate what my gas fireplace costs to run from its BTU rating?
Divide the BTU-per-hour input rating by 100,000 to get therms of natural gas burned per hour, or by 91,500 to get gallons of propane — then multiply by the price on your fuel bill. That's the entire formula. A 26,500 BTU fireplace is 0.265 therms an hour; from there, your per-hour cost is one multiplication away.
The formula, worked
Find the input rating on the unit's rating plate or in its manual. Natural gas: input divided by 100,000 (the BTU in a therm) gives therms per hour; therms times your utility's per-therm rate gives cost per hour. Propane: input divided by 91,500 (the BTU in a gallon) gives gallons per hour; gallons times your supplier's per-gallon price gives cost per hour. From our own floor: the Napoleon Ascent 36 draws 26,500 BTU, so 0.265 therms each hour at full fire, while the Kozy Heat Springfield 36 on propane draws 30,000 BTU — about a third of a gallon an hour wide open.
Input, not output — and why the difference matters
The rating plate lists two kinds of numbers. Input is gas consumed — that's the one for cost math. Output is heat delivered after efficiency, and it's where fireplaces separate: a sealed direct-vent unit commonly turns 70% or more of its input into room heat, while vented gas logs send most of theirs up the chimney. Two appliances with identical inputs can cost the same per hour and deliver wildly different warmth — so when you're comparing, run the math on cost per useful heat, not just cost per hour.
Correct for how you'll actually burn
Nameplate input is maximum flame, and most evenings aren't maximum. Modulating burners change the picture substantially — the Valor H5 spans 24,000 BTU down to 9,000, so the same fireplace on low burns about a third of the gas. Thermostatic controls cycle the burner as the room warms. For a realistic monthly estimate, run the formula at a mid-flame number and your expected hours, then add a little for a standing pilot if you have one. Want help reading a rating plate — or picking a unit whose math you'll like living with? Call (828) 545-0525.
Related questions
- How much does it cost to run a gas fireplace per hour?
- How much will a gas fireplace add to my monthly gas bill?
- How much propane does a gas fireplace use per hour?
- Is propane more expensive than natural gas for running a fireplace?
- How much money does electronic (intermittent pilot) ignition save compared to a standing pilot?
- How much does a gas fireplace cost installed?
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