Understanding kW and kWh: A Primer

  • Context: High School 
  • Thread starter Thread starter SuperMiguel
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Discussion Overview

The discussion centers around the concepts of kilowatts (kW) and kilowatt-hours (kWh), exploring their definitions, relationships, and applications, particularly in the context of generators and energy production. Participants engage in clarifying the distinction between power and energy, as well as how to calculate energy output from power ratings.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants explain that kW is a unit of power (1000 Watts) and kWh is a unit of energy (1 kWh = 1 kW * 1 hour).
  • There is a question about how to find the energy produced by a generator rated at 300 kW.
  • One participant suggests that a 300 kW generator produces 300 kWh of energy each hour and asks how much energy it would produce in a year of continuous operation.
  • Another participant calculates the annual energy output as 2628000 kWh but is corrected regarding the number of days in a year.
  • Some participants discuss the instantaneous nature of power and the need for integration to calculate energy if power varies over time.
  • One participant provides a conversion of kWh into joules, explaining that 1 kWh equals 3600000 J of energy.
  • Another perspective suggests viewing energy as the fundamental concept and power as the rate at which energy is used.
  • There is a clarification that kW can be considered equal to kWh per hour only if the power is constant.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement, particularly regarding the relationship between kW and kWh, and the calculations related to energy output. Some points remain contested, especially around the interpretation of power and energy in varying contexts.

Contextual Notes

Some participants mention the need for assumptions regarding the constancy of power when relating kW to kWh, and there are unresolved details about the calculation of energy output over time, including considerations for generator downtime.

SuperMiguel
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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?
 
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SuperMiguel said:
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?

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

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
 
berkeman said:
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

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

A 300kW generator produces 300kWh of energy each hour. How much energy does it produce in a year of continuous running?
 
so that would be 300x24x356 = 2628000 kWh per year right?
 
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.
 
but then kW is always equal to kWh per 1 hour right?
 
SuperMiguel said:
so that would be 300x24x356 = 2628000 kWh per year right?

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... :biggrin:
 
SuperMiguel said:
but then kW is always equal to kWh per 1 hour right?

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
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
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
SuperMiguel said:
but then kW is always equal to kWh per 1 hour right?
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.
 

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