Need help from physicists .. electrical engineers? haha

[desiplaya4life]

stupid question, but i need help so i thought i ask my fellow ATOT's

we have a dishwasher at home that I want to use but my mom insists not to because she complains it uses raises the monthly electrical bill sky high. For i disagree with her statement. So i am pondering now how much electricity juice does our dishwasher actually consume with the following stats:


Motor draws: 5.2 Amps
Heater: 4.4 Amps
TOTAL CURRENT: 8.6 Amps

(According to its label)

Electricity current rate: .15805cents KwH


We use it once a nightx7daysxfor and it runs for 1hours. in that one hour, only 30mins it is, that the heater turns on and does its job, offcourse.

so how much extra would that churn out into our monthly electricity bill?

Thanks guys.

 

[Mark R]

The amount of energy a dishwasher uses varies with model and program.

But a fairly typical European model, running a standard wash program uses about 1 kWh of energy. So that works out at $0.16 per wash.

 

[magomago]

i'm not an EE but,

Energy = integral (v*i) dt

we see that the i_motor and i_heater is constant

to figure out the power you need to know the voltages they run on...would it be 110V out of the plug? I have no idea

but in any case the voltage and current will be constant

then v*i=constant and draw it out and power is simply

Energy=v*i (T2-T1)

where T2 and T1 are in seconds

so you'd plug [30 minutes *60 seconds] else you'll have unit dimensional issues. You can take T1 as 0 in this instance since we can take the time of reference as when you turn it on

I also divide by 1000 to give us Kilojoules instead of just Joules
sooo

Energy_motor= v*i_motor *30*60/1000

and likewise

Energy_heater=v*i*30*60/1000

edit for stupidy told you i wansn't an EE, but atleast it caught it b4 u guys yelled at me

===========

although actually you just need to find the power (again i'm assuming that junk is all constant)

Power_x= V_x*I_x/1000

where "x" is either motor or heater

and then multiply by .5 since its running for 30 minutes

and then do

.15805/Power_x and that is the cost for each portion

 

[desiplaya4life]

mannnn that just went over my head..


=(

 

[magomago]

Originally posted by: desiplaya4life
mannnn that just went over my head..


=(

focus on this part

although actually you just need to find the power (again i'm assuming that junk is all constant)

Power_x= V_x*I_x

where "x" is either motor or heater

and then multiply by .5 since its running for 30 minutes

and then do

.15805/Power_x and that is the cost for each portion

So

Power_Motor= Voltage_motor * Current_motor (units here should be in J)

Then Energy Motor uses = Power Motor *.5 /1000

then total cost: .15805/ The Energy Motor Uses

 

[iamwiz82]

Just look here: http://www.energystar.gov/inde...ash.search_dishwashers

 

[TankGuys]

Assuming your dishwasher runs @ 110v (I'm tired, and can't remember if they are 220v standard off the top of my head)

Power = Voltage x current, so the power you use is 110v x 8.6 amps = 946 watts.

Assuming you run the dishwasher for on hour per day, thats 7 days x 1 hour x 946 = 6622 watt/hours = 6.622 Kwh

Total price for a week of operation = cost / kwh * kwh used = 15.8 cents * 6.622 kwh = $1.04 per week.


So, there you go. Unless I'm too tired to think straight, it would cost you ~$1.04 each week to use the dishwasher.

 

[Special K]

Originally posted by: TankGuys
Assuming your dishwasher runs @ 110v (I'm tired, and can't remember if they are 220v standard off the top of my head)

Power = Voltage x current, so the power you use is 110v x 8.6 amps = 946 watts.

Assuming you run the dishwasher for on hour per day, thats 7 days x 1 hour x 946 = 6622 watt/hours = 6.622 Kwh

Total price for a week of operation = cost / kwh * kwh used = 15.8 cents * 6.622 kwh = $1.04 per week.


So, there you go. Unless I'm too tired to think straight, it would cost you ~$1.04 each week to use the dishwasher.

First of all, that assumes the current draw is constant throughout the entire cycle.

Second, what about the power factor of the dishwasher? Unless the current and voltage are exactly in phase, you can't just do P = VI.

Maybe I'm overcomplicating this, but I've seen applicances with power factors listed, so I figured it might be an issue here.

 

[Fenixgoon]

don't forget that you have AC current, not DC, unless the washer has an AC ---> DC transformer
also depends on how the motor and heater are arranged the circuit - your conditions assumes they are in series, and you'd have to account for the voltage drop, no?

assuming voltage and current are in phase, can you use the Vrms? (which is roughly 77V for 110VAC)

<--- engineer, just not EE, so circuit analysis is still a bit of a mystery to me

 

[WisMan]

Originally posted by: Special K

Originally posted by: TankGuys
Assuming your dishwasher runs @ 110v (I'm tired, and can't remember if they are 220v standard off the top of my head)

Power = Voltage x current, so the power you use is 110v x 8.6 amps = 946 watts.

Assuming you run the dishwasher for on hour per day, thats 7 days x 1 hour x 946 = 6622 watt/hours = 6.622 Kwh

Total price for a week of operation = cost / kwh * kwh used = 15.8 cents * 6.622 kwh = $1.04 per week.


So, there you go. Unless I'm too tired to think straight, it would cost you ~$1.04 each week to use the dishwasher.

First of all, that assumes the current draw is constant throughout the entire cycle.

Second, what about the power factor of the dishwasher? Unless the current and voltage are exactly in phase, you can't just do P = VI.

Maybe I'm overcomplicating this, but I've seen applicances with power factors listed, so I figured it might be an issue here.

Wouldn't a power factor of say .8 or whatever a dishwasher would be just lower the amount of watts it uses? Making it cheaper?

I seem to remember Actual Power (Watts) = Power Factor x Volt Amps

 

[AlienCraft]

Originally posted by: Special K

Originally posted by: TankGuys
Assuming your dishwasher runs @ 110v (I'm tired, and can't remember if they are 220v standard off the top of my head)

Power = Voltage x current, so the power you use is 110v x 8.6 amps = 946 watts.

Assuming you run the dishwasher for on hour per day, thats 7 days x 1 hour x 946 = 6622 watt/hours = 6.622 Kwh

Total price for a week of operation = cost / kwh * kwh used = 15.8 cents * 6.622 kwh = $1.04 per week.


So, there you go. Unless I'm too tired to think straight, it would cost you ~$1.04 each week to use the dishwasher.

First of all, that assumes the current draw is constant throughout the entire cycle.

Second, what about the power factor of the dishwasher? Unless the current and voltage are exactly in phase, you can't just do P = VI.

Maybe I'm overcomplicating this, but I've seen applicances with power factors listed, so I figured it might be an issue here.

some heating elements come on to heat the water pre wash too. Ours does.
There are logging meters one can use to actually document the load draw under use. I am familiar with old school analog models (think loong strips of paper) but I'm sure there is one than can be accessed via a network IP address.
Maybe see if there is someone the power company can refer you to that does energy audits for their commercial customers.

 

[Special K]

Originally posted by: WisMan

Originally posted by: Special K

Originally posted by: TankGuys
Assuming your dishwasher runs @ 110v (I'm tired, and can't remember if they are 220v standard off the top of my head)

Power = Voltage x current, so the power you use is 110v x 8.6 amps = 946 watts.

Assuming you run the dishwasher for on hour per day, thats 7 days x 1 hour x 946 = 6622 watt/hours = 6.622 Kwh

Total price for a week of operation = cost / kwh * kwh used = 15.8 cents * 6.622 kwh = $1.04 per week.


So, there you go. Unless I'm too tired to think straight, it would cost you ~$1.04 each week to use the dishwasher.

First of all, that assumes the current draw is constant throughout the entire cycle.

Second, what about the power factor of the dishwasher? Unless the current and voltage are exactly in phase, you can't just do P = VI.

Maybe I'm overcomplicating this, but I've seen applicances with power factors listed, so I figured it might be an issue here.

Wouldn't a power factor of say .8 or whatever a dishwasher would be just lower the amount of watts it uses? Making it cheaper?

I seem to remember Actual Power (Watts) = Power Factor x Volt Amps

No, the lower the power factor, the more reactive power the appliance uses. Reactive power doesn't actually do any work, so consuming lots of reactive power is not a good thing in this case.

For anyone who has any EE background, I find the following discussion of complex power to be pretty good:

link

 

[LS20]

Go to Sears, check out the energy consumption tag (every home appliance has one) that has estimated yearly operating cost. probably somewehre in the neighborhood of 50-100$ a YEAR. so less than 10$ a month

 

[Dedpuhl]

heh. This is why I'm a Civil Engineer.

 

[Colt45]

Originally posted by: Fenixgoon
don't forget that you have AC current, not DC, unless the washer has an AC ---> DC transformer
also depends on how the motor and heater are arranged the circuit - your conditions assumes they are in series, and you'd have to account for the voltage drop, no?

assuming voltage and current are in phase, can you use the Vrms? (which is roughly 77V for 110VAC)

<--- engineer, just not EE, so circuit analysis is still a bit of a mystery to me

110/120v AC is stated as RMS already

peak is like 165 - 170v

 

[hanoverphist]

we know its going to be less than a couple bucks a week, so if thats worth arguing over, take the thing out and tell her to burn some old fashioned calories doing the dishes. the easy route would be to look up the dishwasher on the manufaccturers website and get the energy info there, or open it up and get the fla of the motor itself and the draw of the heater element. youll also have to take into consideration any moving parts (mine has an extending arm that sprays the top rack too) when figuring out the draw. but again, none of this is really worth it when youre talking a buck and a half a week.