kW and kWh Explained
A lot of people, energy professionals included, don’t fully understand the difference between kW and kWh. If you are one of them, fear not, this article should set you straight!
Energy calculations, and energy saving, become much easier when you understand the difference between a kW and a kWh.
If you’re working with energy on a regular basis, and you don’t fully understand the difference between a kW and a kWh, we promise you that taking 20 minutes or so to fully understand the concepts explained in this article will save you many headaches in the future. Quite likely it will save you some embarrassment at some point too, as you’ll be much less likely to make embarrassing calculation errors.
(If at any point you’d like to thank us for our help in reducing headaches and embarrassment, please point your colleagues and website visitors towards this article so that it can help them too. Or, if you find it useful, you could buy or recommend our Energy Lens software - we really appreciate the customers that keep us in business.)
Anyway, that’s more than enough preamble… Let’s get to it…
What is the difference between a kW and a kWh?
Well, the difference is really very simple. Though it only seems simple after you understand it.
kWh is a measure of energy, whilst kW is a measure of power…
OK, but a lot of people don’t really understand the difference between energy and power either… So let’s start at the beginning:
What is energy?
Energy is a measure of how much fuel is contained within something, or used by something over a specific period of time.
The kWh is a unit of energy.
(A physicist might throw their arms up in disgust at how we’ve over-simplified one of the fundamentals of the universe. But fortunately we’re not writing this for physicists…)
The kilowatt hour (kWh) is a unit of energy… The calorie is a unit of energy… And the joule (J) is a unit of energy… And these aren’t the only units of energy - there’s the BTU, the watt hour (Wh), the therm, and plenty of obscure units that you’re unlikely to have heard of.
It’s a bit like how you can measure distance in units of feet, metres, miles, km and so on. The distance between New York and London is fixed, but you can express that distance as 3,459 miles, or 5,567 km, or 18,265,315 feet etc. Similarly, you can express a measure of energy in joules, or calories, or kWh, or BTU etc.
When people talk about a particular biscuit containing 172 calories, they’re talking about the amount of energy contained within that biscuit. 172 calories is equivalent to around 0.0002 kWh.
Energy can change form. We could eat the biscuit to provide us with energy. Or we could burn the biscuit and turn it into heat energy. Given the right equipment we could turn the heat energy from the burning biscuit into electrical energy to run lights and fans and so on. Some energy would be wasted in the conversion process, but it should be possible to get that burning biscuit to run a light bulb for a few seconds.
Probably the best option would be to eat the biscuit, but hopefully you get the general idea - the biscuit contains energy that can be converted into different forms…
Electricity and other fuels supply energy in a form that we can use to run the equipment in our buildings.
Our biscuits contain a certain amount of energy - 172 calories or 0.0002 kWh per biscuit. But biscuit energy is not in a form that we can easily use to run the equipment in our buildings…
However, we can easily make use of electricity. And, provided we’ve got a gas or oil burner, we can easily make use of gas or oil. One form of energy comes through wires (isn’t electricity clever?!), and others come as gases, liquids, or solids that we burn (to turn into heat). At the end of the day it’s all just usable energy in different forms. We can express quantities of these forms of energy in terms of kWh. We buy or generate the kWh of energy, and we use it to fuel the equipment in our buildings.
The relationship between energy consumption (kWh) and time
A typical building uses more energy over long periods of time than it does over short periods of time:
- On February 16th 2010 a building might have used 95 kWh.
- Over the week starting April 12th 2010 it might have used 550 kWh.
- From January 1st 2009 to December 31st 2009 it might have used 31,250 kWh.
Given the three figures above, we can easily see that the building used more energy over the course of 2009 than it did on February 16th 2010. No surprises there.
However, we can’t immediately compare the efficiency of the building over each of those periods. If a kWh figure covers a day, we can only compare it fairly with other kWh figures that cover a day. If a kWh figure covers a week, we can only fairly compare it with other kWh figures that cover a week.
If we have the kWh from February and the kWh from March, we can’t really compare the two figures fairly, because February is typically 28 days long, whilst March is 31 days long. This article explains more about the problems that arise if you compare the kWh used in one month with the kWh used in the next.
Energy consumption expressed in terms of kWh doesn’t often mean much unless you also know the length of the period that the kWh were measured over. And it’s difficult to make fair comparisons between kWh figures unless they are all from periods of exactly the same length. Figures expressed in terms of power (e.g. kW) make many things more straightforward…
What is power?
Power is the rate at which energy is generated or used.
The kW is a unit of power.
(Strictly speaking energy isn’t actually generated or used, it’s converted from one form into another. Like how the energy stored in oil is converted into heat when you burn it. And like how the electricity that runs a fan is converted into the motion of the fan blade (kinetic energy). But this is a distinction that people generally don’t worry about when they’re staring at an excessive energy bill and wondering how they can “use” less energy.)
So power is a measure of how fast something is generating or using energy. The higher a building’s kW, the faster that building is using energy.
Joules per second (J/s) is a nice, clear unit of power. Joules per second makes it obvious that power is the rate at which energy is being generated or used. It’s like how miles per hour makes it obvious that speed is the rate at which distance is being travelled.
The watt (W) is another unit of power. It doesn’t make it quite so obvious what power means. But the watt is actually just another name for Joules per second. J/s and W are the same thing. Just some bright spark decided that equations and whatnot would be simpler if power had its own unit (instead of being expressed using units of energy and time together). And they named this unit after James Watt, the Scottish inventor who had an important hand in the development of the steam engine.
So, joules per second (J/s) is a measure of power… The watt (W) is a measure of power… And the kilowatt (kW) is a measure of power too (one kW being 1000 watts).
Things that “generate power”
Items of equipment like boilers, electricity generators, and wind turbines, take energy in one form (e.g. gas or oil or wind) and turn it into another (e.g. heat or electricity).
There’s a limit to how much useful stuff these things can generate, and that is expressed as the rate at which they can generate energy. Which is, by definition, their power.
Consider a 10 kW wind turbine… Provided it has the optimum level of wind (which probably doesn’t happen nearly as often as its owner would like), it can generate 10 kW of power.
How long does it take to generate 10 kW…? Bzzz! No! Wrong question! That’s a question that would only be asked by somebody that didn’t understand what power was. It’s a bit like asking “how long does it take to travel 10 miles per hour?” It makes no sense.
10 kW is the rate at which the wind turbine can generate energy, not the amount of energy that it can generate in a certain period of time. The two are closely connected, but we’ll get to that shortly.
Things that “use power”
Items of electrical equipment like light bulbs, computers, and fans, take energy in the form of electricity, and use it to do useful things for us. Really they’re converting the energy into other forms (heat, motion etc.), but we say that they’re “using” it because we don’t really care about what exactly is happening to it, we just want our equipment to work when we switch it on and stop when we switch it off.
The rate at which these things use energy is their power. Or, depending on the thing, and the person you’re talking to, you might hear it called their “load” or their “demand”, or you might just hear it referred to in terms of a W or kW value.
Light bulbs are a simple example: if you have a 100 W light bulb you know that it will use 100 W of power when it’s running (100 W of power being the same as 0.1 kW of power). The watts aren’t affected by how long the 100 W light bulb is running for… A second, an hour, a day - no difference - so long as it’s switched on it will be using 100 W of power. If it’s not switched on it won’t be using any power (i.e. 0 W).
Some equipment is more complicated. Consider a laptop: at any one instant it might be using 50 W of power, or 30 W of power, or 43 W of power, or any similar such value. It depends on what it’s doing - if it’s sitting there doing nothing it’ll probably use less power than if you’re hammering away on an Excel spreadsheet, listening to some music, and burning a DVD, all at the same time.
Read more: http://www.energylens.com/articles/kw-and-kwh
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