Another option to cosider, particularly for a guest house situation, is
a tankless heater. Available in both electric and gas and in various
capacities they provide a continuous supply of hot water when running
and use no energy when the water stops flowing. I have two gas fired
tankless unit, one for each side of the house. We can run mutiple
showers simulteaneously an both and we never run out of hot water. When
turn off the water the heater goes to sleep til next time. No waste.

From:Chuck Yerkes


How much power does it take to go from a "cold" start to heated?
Or even from partially cool back to "running"?

For a guest house the notion is straightforward - turn it off when
nobody's there.

But I see these "energy saving" devices that turn off the heater in
the day. My understanding is you use up that energy getting the
water back to its running temp. That you'd do better to insulate
the heater if it's losing that much.

But when my heater needs replacing, we're going for an
'insta-hot'/on-demand type one. Lower costs in the long run
and infinite hot water.

Phil Sherrod wrote:

On 29-Mar-2004, Edward Cheung wrote:


Nice work on the investigation. My minor comment is that you used a
standard line voltage in your
calculations. How important is this? Probably not much. In my case,
I just checked the voltage change
over one day. It peaked at 123.4V, and the min was 118.6V. If one
assumed 120V, there would be
only about 2% error


The line voltage will make no difference in the total energy usage,
it just changes the duty cycle.

The water temperature is set by the thermostat. When the
temperature drops below the setpoint, the thermostat turns on the
heating element which runs until the temperature reaches the
thermostat's setpoint. If the voltage is below the standard, less
heat will be generated per minute of operating time, so it will take
longer to heat the water and the heater will run longer -- but it
will draw less power while it is running. So as long as you have
enough voltage to generate sufficient heat to balance the energy
loss (70 watts in my case) the same net energy will be used.

What it boils down to (no pun intended) is the law of conservation
of energy: to maintain an average temperature in the water heater,
the heat going in has to equal the heat being lost. The duty cycle
changes, but the total energy used does not.