# Power measurements?

1. Nov 18, 2009

### gloo

I am a little confused on the difference between statements of electrical output when the talk of gigawatts or gigawatt hours?

for the formula P=ghrk
where P is power in kilowatts
g is gravity (9.8) constant
r is flow of say water per second
k is efficiency coefficient

say g=9.8 , r = 10, k =.75

thus P=1470 kilowatts , that is 1.47 megawatts.... what is the variable needed to get the kilowatt hours or mega watt hours?? Can someone give me a website?

2. Nov 18, 2009

### tiny-tim

Last edited by a moderator: Apr 24, 2017
3. Nov 18, 2009

### gloo

oh really? that simple? i feel stupid.

so i take the 1470 kw and multiply it by 60seconds * 60 minutes? = 5292000 kwH? that's it?

4. Nov 18, 2009

### Staff: Mentor

No, you multiply it by 1 hour. 1.47 megawatts for one hour is 1.47 megawatt-hours.

5. Nov 18, 2009

### tiny-tim

Nooo … that's 1470 kwH, isn't it?

it's 5292000 kJ

6. Nov 19, 2009

### Archosaur

Personally, I've always hated the term "Kilowatt-hour".

It's no different from measuring distance in "Miles per hour-hours".

7. Nov 20, 2009

### gmax137

Hate is a pretty strong emotion for a unit . Kw-hr, Btu, joules, ergs, MeV, take your pick. The 'nice' thing about kW-hr is that it gets across the idea of power applied for a specified duration (I think that's why it seems applicable or natural to electrical distribution uses.

8. Nov 20, 2009

### Archosaur

Okay. I will.

I [STRIKE]hate[/STRIKE] dislike Kw-hr for the reason I already stated, though I admit it could be useful in industrial applications.

I dislike Btu because people use it as a measure of power and energy. Minus points in my book.

I dislike ergs because it doesn't do anything that scientific notation can't do.

I like Joules and eV. Normally, I try to speak exclusively in terms of SI units, but I like eV because its definition is simple, it's useful, and the conversion to joules isn't 'pretty'.

For some reason, I've always been really picky about units. I realize I'm hard on them :)

You should have seen the grief I gave my chemistry prof when he wrote a conversion from pounds to kilograms on the board...

9. Nov 20, 2009

### gmax137

I think when people use Btu for power they are really using Btu/hr; it's just that the 'per hour' is silent. But that's not the unit's fault.

I'm stuck using Btu and kW-hr in my work. We also use 'pounds' (both kinds) and have to keep our factors of 32 straight. If you don't know what that means, count yourself lucky and move on.

I dislike Btu because there's more than one definition! You will see conversion factors of 3412 Btu/kW-hr, 3413 Btu/kW-hr even 3415 ! It took me awhile to figure out that like the 'calorie,' there's more than one 'Btu.' Recall that a Btu is the heat to raise one pound of water one degree Fahrenheit; the problem is, water at, say, 32 deg F takes a different amount of heat than water at 68 F, etc. (not a constant specific heat value).

I hope this isn't too far afield of the original post.

10. Nov 20, 2009

### Archosaur

Unfortunately... yea, I follow you... I work for a company that designs temperature-controlled showcases for grocery stores.

11. Nov 20, 2009

### Staff: Mentor

More fun:

MBH is 1000 BTU per hour, but
kWh is 1000 Watts for an hour!