> But the _idea_ of using energy around the clock to keep 40 gallons
 > of water hot seems a bit screwy to me when you can just heat what
 > you need when you need it.
 > 
 > But educate me,
Ian,
 You're not "using energy around the clock", just kicking on the 
burner/element for a short time every now and again. The actual wastage is 
dependant on several factors: how much and how often you use hot water, and 
how well-insulated the tank is, along with the length of the pipes between 
the heater and the point of use. If your heater is fairly close to the point 
of use, and you use water several times during the day, you'll save pretty 
much nothing by going to a demand-type unit. OTOH:, if you often go 24 hours 
without using any hot water and the tank would be 30+ feet from the point of 
use, a demand heater would save "some" money.
 
When you consider the other costs and increased complexity of the demand 
heater, you'll find that it's not the be-all/end-all that many folks think.
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