Well, admittedly I didn;t bother to look at your calculations but it certainly doesn't make sense that keeping 60-80 gallons of water hot constantly is more efficient that only heating a small amount of water when you need it. Liek I said, if you don't have gas my understanding is the savings are not as great and the water takes longer to heat, which may be what Tom is looking at.
Being on Nat. Gas and living out in the county I have easily seem the saving. Mine has paid for itself already. I won;t even go do down the road about having unlimited hot water - like I said you can run a bath and a shower and still get hot water from the kitchen sink. Wife being able to take a hot shower for as long as she wants scores a big bonus on the happy meter.
regardless, for brewery usage having one is amazing in so many ways. Having 185 degree water on tap almost instantly (it does take a few moments for it to get that hot, probably 3 gallons or so worth, maybe 5 - I've never measured it. But 5 gallons of water in a brewery is a drop). You save so much time over having to heat the entire volume, it's not even a question whether it was worth it or not. Also, you can crank it down to 140-160 for pasteurization - I flush all my hoses and lines with 150 degree water for a minute or two and rinse my fermenters with 140 degree water via spray ball.
Phil, my understanding is that the Rinnai HWH are all the rage in Europe so you might look into it for your brewery application.