Register to reply 
Kilowatthour versus kilowatt per hourby rainstom07
Tags: kilowatthour 
Share this thread: 
#1
Mar412, 01:08 PM

P: 16

My professor asked the cost of running a hair dryer of 1500 W for 15.0 minutes for 5 days a week for 4 weeks. The utility company charged $0.60 per kWh.
Instinctively, i thought to combine 1500 W with the 15.0 minutes into my version of "kilowatt per hour". [tex]1500 W * \frac{1 KW}{1000 W} = 1.50 kW[/tex] [tex]1.50 min * \frac{1 hr}{60 minutes} = 0.25 hr[/tex] thus: [tex]\frac{1.50 kW}{0.25 hr} = 6 kW/hr[/tex] The dryer was used 5 times a week for 4 weeks... so it was used 20 times in the 4 weeks. [tex]6 kW/hr * 20 = 120 kW/hr[/tex] At... $0.60 per kWh [tex]120 kWh * 0.60 \frac{$}{kWh} = $72[/tex] The answer was $4.5 not $72. Therefore, my concept of kWh is clearly wrong. Can someone explain to me what kWh is intuitively? It clearly is not the amount of kW consumed in an hour. Thanks in advance! 


#2
Mar412, 01:22 PM

PF Gold
P: 2,031

when you pay your bills you pay for the energy you use.The Joule is an inconvenient unit to use for this purpose because it is too small*.Instead the energy unit we use is the kilowatt hour(not kW per hour).The kilowatt hour is literally the number of kilowatts times the number of hours.
(*one kilowatt hour is 3,600,000Joules.That costs $0.60) 


#3
Mar412, 01:24 PM

Sci Advisor
HW Helper
Thanks
P: 26,148

hi rainstom07!
if 5 men take 4 hours to dig a hole, that's 20 manhours of work done "man" in "manhours" means manpower (think of horsepower!) kWh is power times time, and the "energy company" charges for energy, ie for work done watts is power, power is work per time, so energy is work is power times time … joules = W times s = kW times h/3,600,000 


#4
Mar512, 01:59 AM

P: 777

Kilowatthour versus kilowatt per hour
A joule is simply an amount of energy. A (kilo) watt is power, or how fast you are using that energy. It is energy divided by time (energy/ time).
When you multiply this by an hour (to get kilowatt hours), you are multiplying it by a unit of time again. When you multiply (energy/ time) * time, the time actually ends up cancelling out, and all you are left with is energy again. So essentially a kilowatt hour is a unit of energy. 1 kilowatt hour = 3,600,000 joules. 


#5
Mar512, 06:32 AM

Sci Advisor
PF Gold
P: 3,759

Remember heat is energy, and so is mechanical work . The unit of energy is the "joule" and that's the work of one newton pushing through one meter. Rub your hands together and feel the warmth of Force X Distance...... When you're procucing that warmth at the rate of one joule per second you are making one watt  that's what a watt is, one joule per second. (I would guess that rubbing your hands together makes on the order of one watt.) So a KW is a thousand joules per second; ..and a KWSecond would be a thousand joules ....and a KWHour would be 3600 KWSeconds, 3.6 million joules. Thet's enough heat to evaporate a little more than three pounds water. Or to dry maybe four beautiful heads of hair? I hope that helps you 'feel' it. 


#6
Mar512, 09:54 AM

P: 261

Kilowatthour means running a device that uses kW of power for an hour, if you run the device for half an hour, you use 0,5 kW/h, for two hours 2 kW/h etc. A 2 kW device uses 2 kw/h per hour, only 1 kw/h if you run it for half an hour etc.
You made a mistake in this step: 


#7
Mar512, 10:21 AM

Sci Advisor
Thanks
PF Gold
P: 12,269

KW/hr has no meaning. It is just the way that the uninformed sometimes write kWh.
Watts is Joules per second. I.e. power is energy per unit time. Saying kW/hr is like saying "energy per second per second"  a rate of power Increase. Barmy. 


#8
Mar512, 10:46 AM

HW Helper
P: 7,180

As sophiecentaur suggested, it's more likely that someone meant kilowatt x hour, or 3.6 million Joules as already posted, as opposed to kilowatt per hour. 


#9
Mar512, 11:20 AM

Sci Advisor
Thanks
PF Gold
P: 12,269

A "kilowatt per hour" is not a 'unit of electricity' or an 'electrical unit'  it is just a rate of change of power.
There can be similar confusion when talking of battery capacity  Amp Hour Capacity is used, rather than Coulombs, because it is a useful 'size' for a unit. An Amp Second (Coulomb) is too small for most purposes. I have heard of people misusing the term "Amps per Hour" for capacity. 


Register to reply 
Related Discussions  
Do 2 Kilowatt Nuclear Reactor Still Exist  Nuclear Engineering  39  
Horsepower to kilowatthours?  Introductory Physics Homework  16  
Joules/kilowatt hour  Introductory Physics Homework  2  
Kilowatt hr  Calculus & Beyond Homework  12  
Ok im mad confused about generators and kilowatt/kw hours...  Electrical Engineering  23 