"Donna Ohl, Grady Volunteer Coordinator" wrote in
message t...
On Sun, 10 Feb 2008 21:49:53 -0700, Rick Blaine wrote:
What's the easiest way to compare heat leakage between two water
heaters?

Look at the yellow energy label on the heaters. The annual cost to
operate
printed there can be compared on different models.

Actually, since my cost per therm is far from the average used for those
stickers, the sticker numbers are innacurate in my situation.

The labels show the estimated annual energy usage AND the estimated annual
cost, based on an average cost of fuel. Take the energy usage and multiply
by your own fuel cost assumptions.

The sticker numbers might work to roughly compare two models in
performance
but they don't work to calculate the true payback period for two models.

For that, the math I've posted today should be sufficient (assuming the
calculations hold water under scrutiny of this esteemed group).

I doubt that your calculations are "true" - in the way that you initially
didn't account for the time value of money, I didn't see that you took into
account the changing cost of fuel. What is today a $25.00 annual saving
could be a $50.00, $75.00, $100.00 (make your own assumptions) annual saving
a year or three down the line. Nor did you account for general inflation,
or even the normal tendency for people's income to rise over time - today's
annual payback might mean an hour or two of work a year, but assuming
constant fuel costs, it'll probably be less work time a year or two down the
line.

You can estimate payback periods only by making a cat's cradle of
assumptions.