|
|
re: conserving electricity
26 oct 2000
prichter1 wrote:
>i have a question about electric hot water heater timers.
>it completely cuts off, right?
right.
>how long does it take for a 50 gal. hw heater to heat it back up?
5 kw is 17k btu/h, and 50 gallons is about 400 lb or 400 btu/f of water,
so heating it from 60 to 120 f (and risking the spores) would take about
400btu/f/17kbtu/h(120f-60f) = 1.4 hours, if the thermostat didn't cycle.
a typical water heater might have 25 ft^2 of 2" fiberglass insulation
(us r6) with a thermal conductance of 25ft^2/r6 = 4.2 btu/h-f, so keeping
it 120 f in a 70 f room for 24 hours takes about 24h(120f-70f)4.2btu/h-f
= 5040 btu or 1.48 kwh worth 15 cents at 10 cents/kwh.
if you turn it on every day exactly 1.4 hours before taking showers,
etc., and leave it on for say, 2 hours, before turning it off again
soon after using the last drop of hot water, it would only need about
2.7h(120f-70f)4.2btu/h = 567 btu or 0.17 kwh to keep the water warm
for that shorter time, with a potential savings of 13 cents per day.
most people are less precise in their timing.
nick
|
|
