So as promised I did a brew yesterday and was pretty methodical about collecting what I thought was relevant data.
I have tried to post a pic of the excel spreadsheet below. In summary I used approximately 74,633 BTU's with my set-up. The set-up being a 10.5 gallon Polarware Kettle and a Blichmann Floor Burner. Couple of key points:
Time to heat 8 gallons of water from 59* to 160* = 29 min
Time to Heat 8 gallons from 152* to 166* = 11 min
Time to Heat and Boil ~6.75 gal for 60 min, took 80 min, raised temp from 162*
A pound of propane = 21,951 BTU's. If I leave out the boil portion for just a moment and simply calculate energy used to raise temp from point A to point B, I think I can come up with some efficiency numbers. A quick refresher. 1 BTU = energy used to raise 1 gallon of water 1* F. 1 gallon = 8.34#. Therefore, 8 gallons of water x 8.34 = 66.7#. If I do the math for the heating of strike water and mash out water, in a 100% efficient system you would only use 7,670 BTU's. Based on the weight of the tank before and after, I used 1.6# = 35,122 BTU's. Thus, heating with Propane is 21.8% efficient, right? I am pretty sure my math is correct.
So here is the comparison:
Propane @ 3.57/gallon = $2.93 for this brew
NG = $0.90 (assuming same efficiency + 15% for difference between LP and NG)
Electricity = $0.80 (using some very loose estimations)
In the end, assuming some of my estimations are correct, due to heat loss as many suggested, going all electric can save (in my case) almost $2.10 per 5 gallon batch. Needless, to say, breakeven is a ways off. However, as some suggested the bling factor alone will more than cover the cost.
Thanks to all who chimed in on this. I hope it was a bit of fun to think of our brew days in yet another way.
Admittedly in order to do a apples to apples comparison, I would need to conduct same experiment with an electric element.