# KW and kWh

1. Jun 9, 2010

### SuperMiguel

Can any of you guys explain what exactly kW is and its relationship to kWh

Like a generator does for example 6gWy but its rated at 2gW Whats exactly does it mean?

I know that kw is power and kwh is energy..

is there a online reference's that would explain this?

2. Jun 9, 2010

### Staff: Mentor

Energy is power multiplied by time.

kW is a unit of power, kilo Watt = 1000 Watts

kWh is a unit of energy, 1kWh = 1kW * 1 hour

3. Jun 9, 2010

### SuperMiguel

so if you have a 300 kW generator, how do you find the energy produced by it?

4. Jun 9, 2010

### Staff: Mentor

A 300kW generator produces 300kWh of energy each hour. How much energy does it produce in a year of continuous running?

5. Jun 9, 2010

### SuperMiguel

so that would be 300x24x356 = 2628000 kWh per year right?

6. Jun 9, 2010

### MRKN

Think of it this way, power is instantaneous, ie power occurs at the smallest imagineable fraction of time you can possibly consider. So to find energy, you take power and multiply it by some quantity of time. Energy is a way to explain what comes out of power applied for some duration of time.

7. Jun 9, 2010

### SuperMiguel

but then kW is always equal to kWh per 1 hour right?

8. Jun 9, 2010

### Staff: Mentor

That would be correct on a slightly different planet. Here on Earth, a year is 365 days. Unless you were counting on some downtime of the generator for periodic maintenance...

9. Jun 9, 2010

### Staff: Mentor

That's not a helpful way to think about it. Think of the power as the more fundamental thing. Especially if the power is varying with time, you would need to do an integration to calculate the energy over some time interval.

10. Jun 9, 2010

### Staff: Mentor

Another way to look at what a kWh is, is to calculate how big it is in more familiar units of energy. One watt is one J/s, so a kW is 1000 J/s. If you let that power "run" for one hour (3600 s), you get 1 kWh = (1000 J/s)(3600 s) = 3600000 J of energy.

11. Jun 9, 2010

### mheslep

Or think of energy (kWh, BTUs, joules, calories) as the fundamental concept, and power is the rate at which energy is used (Watts, Horsepower, etc)

12. Jun 9, 2010

### Staff: Mentor

If the power is constant, yes...

Typically, the electric company counts kWh in 15 minute intervals and multiplies by 4 to get the kW to use in their peak demand, but they could also do an instantaneous reading if they wanted to.