I wanted to find out how much energy in kilowatts was required to bring my water heater from dead cold up to cycling and to see how much energy was needed to keep the water hot over a twenty-four hour period. No water was drawn off. Ambient temperature out of doors was 8 C (46.4 F). The starting temperature of the water in the tank was 8.5 C (47.3 F) and the cycle temperature is 54.4 C (130 F) The heater draws 1131 watts (~3862 btu) on the electric setting.
As the water heater is a ten gallon (83.6 pounds) Atwood propane/electric, theoretically the energy needed to get from 47 F to 130 F is 83 X 83.6 = 6938 btus. That may represent a run time of 1.8 hours or 108 minutes. The owners manual suggests a recovery time of 90 minutes on the electric setting, but they also quote a higher number of watts than the actual measured value in my unit.
I am providing power for this test using my 2500 watt Cobra MSW inverter and 875 amp-hour battery bank. Wattage measurements are being done with a kill-a-watt unit and the watt numbers are a running total.
Once I turned on the hot water tank heater, 12.1 volts were in the Blue Sky 3024di display. Unfortunately I was not able to monitor that as I was called away to work. It is a grey day so, little in the way of charging from the solar panels, is happening
12:45 pm 0000 watts consumed ambient temperature 8 C (46.4 F)
15:30 pm 2360 watts consumed ambient temperature 9.2 C (48.6)
17:45 pm 2570 watts consumed ambient temperature 9.2 C (48.6)
20:00 pm 2870 watts consumed ambient temperature 9.2 C (48.6)
22:00 pm 2870 watts consumed ambient temperature 8 C (46.4 F)
24:00 pm 2870 watts consumed ambient temperature 5.9 C (42.6 F)
08:00 am 3990 watts consumed ambient temperature 4.6 C (40.2 F)
11:00 am 4530 watts consumed ambient temperature15 C (59 F)
12:45 pm 4530 watts consumed ambient temperature 11.8 C (53.2 F)
Voltage was 11.9 under load at the end of the test.
Total watts consumed from a “standing start” were 4350 watts or about 53 cents (@ 12 cents/kwh). That represents about 435 amp-hours drawn from the battery bank. This represents 14,847 btu's. Translated to Propane = 0.163 of a gallon or about 57 cents (@ 3.45 per gallon). An assumption is made that the propane heating of the water heater is 100% efficient. Of course, that is not so. At best one might expect 50% efficiency, so that 57 cents probably is really more like $1.14.
It took 2360 watts before the first cycle happened or a little over 2 hours of run time. Total consumption was 4350 watts leaving 2170 for recycling. That means that, after the initial warm up, it takes about 99 watts per hour to keep the water "up to full temperature".
It appears that once the original cycle happens that it recycles about every four hours and takes approximately 500 watts to do so. I do not know what temperature the water drops to before the heater cycles.
Of course, in the real world, water would be used so energy consumption would be higher, but this at least gives a base line.
It suggests to me that if you plan to be away from your RV for four hours that it is more efficient, from an energy use point of view, to turn the water heater off.
Clearly the hot rod and lightening rod “add on” units are able to "keep up" to static losses and may, over time, "pay for themselves" in propane cost savings if you are camped where electricity is included in the site rental fee. Their recovery times would be measured in hours, but the tortoise did, after all, beat out the hare. They could certainly be used for preheating in the same manner as many of us precool the fridge.
It is cheaper to heat water with propane than to run a generator to do so—but if the generator is being run for battery charging anyway, then the excess electricity may as well be used for the water heater.
I'm a propane miser and last purchased in May of 2009 (Today is May 30, 2010). I use my inverter, battery bank, solar, and alternator for energy, as well as opportunity plugging in at 15 amp circuits. I rarely am in a paid campground.
Is it possible to safely “beef up” the insulation around an Atwood Gas/Electric unit?
Has anyone bothered to do so?
The latest solar charge controllers are capable of diversion loads. i.e. when the batteries are fully charged the energy from the panels may be used for other purposes such as water heating. Has anyone done so? It does require the installation of a 12 volt heating element in the water heater. (or you can be “cheap” like me and just flip on the 115 volt, if the battery bank is bursting full).