Non-car related electric question


#1

Sorry this has nothing to do with cars,



But this forum doesn’t have an “other” board, I digress…



If my washer and dryer draw a current of 36A combined, how can I compute the cost of running these appliances?



i.e. Washer drawing 8A for 70 minutes, followed by the dryer drawing 28A for 45 min?



Are electricity rates $x per Kilowatt hr? How do you go amps to Kilowatt hours?










#2

You can’t compute it. Neither will draw the full amps while they are running.

The dryer will cycle the heater to maintain a specific temperature. It will also have a cool down cycle when it is tumbling, but not providing heat. The washer likewise will have different amps when it is spinning vs tumbling or rotating.

You need a recording amp meter. Even then there will be some differences depending on the load, so you would need to record several loads to get an average.

Kilowatt = 1,000 * volts * amps

Now back to cars (I hope you are washing rags used on your car.)


#3

The proper calculation is easy:

The washer uses a 110V line. Watts (power) is V * I, so 110V * 8A = 880W. Run this for 70 minutes, 880W * 1.1667 hours = 1,026 Watt-hours or 1.026 KW-h.

The dryer uses a 220V line. 220V * 28A * .75 hours = 4,620 Watt-hours or 4.62 KW-h


#4

To further complicate matters, you can not calculate the power an AC motor draws by measuring the amps. That’s because the ampere sine wave is rarely perfectly in sync with the voltage sine wave. In fact the current usually lags behind the voltage by about 20 degrees at full load and nearly 90 degrees at no load and even more than 90 degrees when the motor is holding the load back, such as on a down-escalator.

An unloaded motor draws the least amps but nowhere close to zero amps. A negative load (i.e. down escalator) also increases the amp draw but instead of making the electric meter go faster, it makes the electric meter go backwards.


#5

As others point out, it’s not simple. A few notes.

.In many parts of the US, the single line voltage is close to 117v AC, not 110 volts. In some places it may be 120 volts. The “220 volt line” will almost certainly be the voltage across a second 110/117/120 volt line and the 110/117/120 volt line used for the washer. Thus the drier voltage may be 220 volts or 234 volts or 240 volts or something else in that ballpark.

.In addition to the actual power used, there are reactive “losses” caused by the phase shift B.L.E mentions. These aren’t exactly real as they don’t represent useful power. But you pay for them anyway.

.In any case, the power usage is going to be dominated by the heating elements in the drier – which should be pretty much a pure resistive loss without reactive elements and therefore without reactive losses.

Power = voltage times current (P=EI). Assuming 234 volts and 28 Amperes for 45 minutes and no cool down cycle, the drier might use nearly 2342845/60/1000 = 4.9kw hours. Add 20 percent for the washer and other losses = about 5.9kw hours for a wash-dry cycle.


#6

“Now back to cars (I hope you are washing rags used on your car.)”

I forgot to throw those in with my socks. xD


#7

Ok, so interpolating between Busted and xtc’s results, I get 5.75 kw-hours.

And where I live electricity is $0.07068 per kWh

So in theory it costs $0.41 to run these (in electric expense, only)?

I wonder how much water the washer uses? I’m a nerd when it comes to accounting so I wish there was a way to amortize the wear & tear expense over the life of the machines.

I ask because the person I live with b****ed at me last night for not including his socks with mine, frankly i’m too lazy to sort them out so I’m trying to come up with a fair amount to compensate him for the “unnecessary” cycles.


#8

You really need to know the “duty cycle” of the dryer heating element in order to determine the actual electricity usage. The heating element will be “on” a larger percentage of the time in the beginning of the run (when the clothes are damper) and less toward the end when the clothes are dryer.

And the rating for the washer is for the worst case, eg when the motor is beginning to spin. The washer only pulls that load momentarily. Its current draw is much less during the steady spin cycle or wash cycle.


#9

Hey, don’t forget to include the costs for heating the water!
Water bill, sewage bill (if city sewers), Wear and tear on capital goods, extra maintenance on the lint screens :wink:

This is why they have a selectable size on the wash cycle and load (humidity) sensing on the dryer- for small, individual loads!


#10

If you have the brochure of the washer, it will tell you how much a wash cycle consumes. Our Maytag top loader uses 35.8 gallons for a full load. If you wash with cold you can forget about the water cost; if you wash hot there will be some heating cost involved. Water here is very expenssive, and costs $0.0133 per gallon, so to fillthe tub will cost 0.0133X35.8=$0.48. In your area it will likely be much less.

Busted Knucles did a good job in getting an approximate cost for the electricity. The energy needed to heat the water would be about 120 degX32.8X8.33 =32,789 BTUs. The gas company will tell you how much they charge per 100,000 BTU or Therm. The gas water heater is about 80 efficient, so you can divide the answer by 0.8.

For an electric water heater the efficiency is about 90%, so divide th cost by 0.9. One Kilowatt Hour is equal to about 3500 BTUs. So to heat a load with elecricty costs $0.07068X32,789/3500X1/.8=$0.7357 or 74 cents. So a complete load to wash and dry would be 0.41 + 0.74= $1.15 if you heat water with elctricity, and wash with HOT!

If you heat water with gas, divide the water heating costs by about 4, since gas heating is much cheaper. So 0.71/4=17.75 cents per large load.

The person you “live” with seems preoccupied with trivia. Although I’m trained as an engineer, there are more important things in life to worry about than the cost of a load of washing. We only calculate this stuff when we go shopping for a new appliance. A front loader uses less water, so if you wash with hot water they provide some savings.

From the above you will have learned that washing with cold water saves a great deal of money; my wife has been doing it for years. If you have a backyard or balcony you can also dry your clothes outside, the way your grandmother did it.

P.S. A washing machine typicallly last about 15 years, and does 5-7 loads per week. If we take 15X52X5= 3900 loads (5 loads/week) over the life of the machine and an average cost of $500 for a basic washer, you end up with $500/3900= $0.128 per load amortization cost. If the dryer cost the same, add another 12 cents. So the depreciation per load will be about $0.25 for the set.


#11

Thanks man.


#12

#13

I have something similar; it will measure voltage, current, and KW hours. if you plug in the cents/kwh, it will also give you the total cost figure.

Used it extensively where all the energy goes; the 50" plasma TV gobbles up the most, believe it or not.

Also, a fridge in the basement, which we call a “beer fridge” uses only half the rated energy on the Energuide label, mostly becaue of the even cool temperature, and the fact that it does not get opened very much.

OP should buy one to monitor energy use.


#14

.In addition to the actual power used, there are reactive “losses” caused by the phase shift B.L.E mentions. These aren’t exactly real as they don’t represent useful power. But you pay for them anyway.

Actually, electric meters are designed to only measure real power, not reactive volt*amps, however, utilities often add a surcharge to industrial users for using power at a low power factor. Large capacitor banks are frequently used to correct low power factor in an industrial plant.

Watch the electric meter supplying power to an oil well pump jack and you will see the little wheel go forward for a few turns, then stop and go backwards for a fraction of a turn as the pump jack nears the end of its stroke and the inertia of the reciprocating mass coming to a stop causes the motor to be overdriven, and then the cycle repeats. The current never goes to zero but the meter “knows” when the phase angle results in zero or regenerative power.


#15

To get a rough estimate you could monitor the meter before and after you use the appliances. That is what the electric company is going by for the billing. Just try not to use any high current electrical devices while the washer and dryer are running.


#16

for an even better estimate, turn off all other appliances, run a full load, and then dry it, after first recording the meter reading, and then when done, record the meter reading again. the meter is in watts, so it will tell you exactly what you used.