Fun with a Kill-A-Watt (and a Water heater on a timer question)

[TT]

I recently bought a kill-a-watt thingy off ebay, it's quite interesting to see what electronics use what as far as power consumption.

This assumes average power cost of 8.5 cents per kwh (winter is $.075 and summer $.09)

My washing machine pulls .38kw/h per 18 minute load = ~3 cents per load

My christmas tree costs ~3 cents per hour that it's on.

My laptop pulls .02kw/h which means it only costs 2 cents per hour to run it, or $15/yr

Surprisingly my humidifer would cost me $5.60 per week if I ran it

24/hrs a day.

My printscanfaxmachine costs me $9.50/yr to keep it plugged in and on round the clock.

I haven't yet tested my dishwasher to see what a 55min heated wash cycle costs, nor my refridgerator or microwave (essentially an unneeded clock when not reheating my food).

I recently switched my entire household to compact fluor. lights, so I probably won't bother measuring individual light fixtures. I wish P3 made a 240v meter. Curious what my dryer pulls.

After that I'll be measuring my landscaping lighting in the backyard, curious about that one.

HAS ANYONE DONE EXPERIEMENTS WITH THE FOLLOWING: I'm trying to figure out if it would be cheaper to put my water heater on a timer to turn it off at night (from midnight to 8:30AM), or if it would use more electricity reheating the water in the AM than it would keeping it warm all night. I'm in AZ so the coldest it gets in the garage is probably

50 in the winter, and during the summer, well, probably 100. My water heater is a brand new GE 8yr warranty 50 gallon dual element that I just installed.

Also, wonder if it would be more cost effective to turn it up to 140 (from 125) and not use the dishwasher heating element for the wash. Would also effectively lessen the amount of hot waster used for other purposes, but to what end?

Thanks

-Tom

[Lou]

I don't know the answer from any practical experience, but in a theoretical way I can inch toward an answer.

The water heater runs on electricity, and the dishwasher heating element runs on electricity. As a first approximation, there's no saving using one electrical heating element over another.

The dishwasher probably uses on the order of 10 gallons or less of hot water to do a load. If you turn up the temperature on the water heater, you'll end up heating 50 gallons of water to the higher temperature when you only need to heat 10.

Heat loss from a hot object to cooler surroundings, in this case from the water heater itself as well as from the run of pipe from the heater to the diswasher, is directly proportional to the difference in temperature between the hot object(s) and the surroundings. In other words, the hotter the temperature, the more heat the appliance will lose in a given amount of time.

So turning up the heater temperature will likely mean you heat more water than you need to a higher temperature than you need and increase heat losses.

The non-economic consideration to bear in mind is the possibility of scalding. According to the Consumer Product Safety Commission, each year there are about 3800 injuries and 34 deaths in US homes due to scalding from excessively hot tap water. Burns will occur in adults with a six second exposure to 140 degree water, with children it's even quicker. This outfit recommends that water heaters be set to a maximum of 120 degrees F. Even at

120 degrees, an adult can suffer third degree burns after a 5 minute exposure. This setting will also be more economical because of the reasons given above.

[nicksanspam]

Turning the tank heater down might save energy, with low-power tankless electric heaters in the kitchen and bathroom. Boosting 1.25 gpm from T to 110 F with 5 kW for a shower makes 1.25x8x(110-T)x60 = 3.412x5000, so T = 82 F. Lots of Caribbean people are happy taking showers at 85 F.

Nick