I fill my tank about every three mows (3 gal), and I have about 2/3 acre of actual grass to mow. So I guess it costs me about $3.50 per mow. Gas could be $10 a gallon and I'd still mow twice a week. Between tilling, grading, sodding, rolling, and fertilizing, aerating and thatching, I put a lot of money and effort into that lawn and I don't intend to neglect it when it comes to keeping it the proper length. I refuse to let gas prices alter my lifestyle. Since my wife stays home with the kids and I drive 25 minutes each way to work, it would make sense to leave the GMC in the garage and drive the VW to work, rather than the other way around. But I never claimed to have any sense. I guess I'm just a whipper-snapper here in this forum, but when I was old enough to mow but too young to work--1984 to 1989--I'd charge my grandfathers' friends $10 to mow yards which were less than 1/4 acre (pushed, no trimming, their mower). Gas was around a dollar then. Twenty years later, my neighbor's kid charges the other neighbor $20 to mow a 1/2 acre lawn (basically the same rate). And his parents even thought that was a bit steep. So even though fuel has tripled in price as of late, it wasn't until recently that inflation really started to affect so many other things. But I wouldn't say a homeowners' lawn-mowing frequency is one of them. Here is a link that might be useful: gas price index

For a natural gas fireplace that puts out 28,000 btu/hr: assuming 90% efficency and gas at $1.26 per 100 cubic feet: (28,000 btu/hr divided by (103,000 btu per 100 cubic feet)) x $1.26 x 1.1 efficiency = 38 cents/hour to run at rated output. For propane fireplace that puts out 28,000 btu/hr: assuming 90% efficiency and propane at $2 per gallon: (28,000 btu/hr divided by (86,000 btu/gallon)) x $2 x 1.1 efficiency = 72 cents/hour to run at rated output. Insert your actual price of gas and propane for more accurate price calculations. Note that propane may be as much as twice the cost for natural gas (don't believe every commercial that you see).

My gas meter also has a 2 cubic foot dial. I set a video cam in front of the gas meter in the morning knowing the furnace would be on for at least 20 minutes to bring the house up to temp from the night's setback. In 5 minutes the funace consumed 7.6 cubic feet of gas. In 10 minutes the funace consumed 15.2 cubic feet of gas. I also timed how long it took per revolution of the 2 cubic foot dial. That was determined to be 1 minute 19 seconds to 1 minute 20 seconds. So 15.2 cubic feet of gas in 10 minutes extrapolated out to 60 minutes would be 90.8 cubic feet gas per hour which is a little over 93000 BTUs.

I take it from all of that you are an engineer...... Thanks for the info.