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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!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 headachesin the future. Quite likely it will save you some embarrassment at some point too, as you'll be much less likely to makeembarrassing calculation errors.(If at any point you'd like to thank us for our help in reducing headaches and embarrassment, please point your colleagues andwebsite visitors towards this article so that it can help them too. Or, if you ﬁnd it useful, you could buy or recommend our EnergyLens 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.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?
(A physicist might throw their arms up in disgust at how we've over-simpliﬁed one of the fundamentals of the universe. Butfortunately 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 thesearen'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 tohave 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 andLondon 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 expressa 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 withinthat biscuit. 172 Calories is equivalent to around 0.2 kWh. (That's with 172 Calories meaning 172 kilocalories, which is the unitthat is almost always meant when people talk about the calories in food.)Energy can change form. We could eat the biscuit to provide us with energy. Or we could burn the biscuit and turn it into heatenergy. Given the right equipment we could turn the heat energy from the burning biscuit into electrical energy to run lights andfans and so on. Some energy would be wasted in the conversion process, but it should be possible to get that burning biscuit torun a light bulb for at least 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 canbe converted into different forms...
Energy calculations, and energy saving, become much easier when you understand the differencebetween a kW and a kWh.kWh is a measure of energy, whilst kW is a measure of power... E nergy 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.
Our biscuits contain a certain amount of energy - 172 Calories or 0.2 kWh per biscuit. But biscuit energy is not in a form that wecan 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 (toturn into heat). At the end of the day it's all just usable energy in different forms. We can express quantities of these forms ofenergy 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, 2018 a building might have used 95 kWh.Over the week starting April 9th, 2018 it might have used 550 kWh.From January 1st, 2017 to December 31st, 2017 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 2017 than it did onFebruary 16th, 2018. 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, wecan only compare it fairly with other kWh ﬁgures that cover a day. If a kWh ﬁgure covers a week, we can only fairly compare itwith 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 istypically 28 days long, whilst March is 31 days long. This article explains more about the problems that arise if you compare thekWh 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 thekWh were measured over. And it's difﬁcult to make fair comparisons between kWh ﬁgures unless they are all from periods ofexactly the same length. Figures expressed in terms of power (e.g. kW) make many things more straightforward...
What is power?
(Strictly speaking energy isn't actually generated or used, it's converted from one form into another. Like how the energy stored inoil is converted into heat when you burn it. And like how the electricity that runs a fan is converted into the motion of the fanblade (kinetic energy). But this is a distinction that people generally don't worry about when they're staring at an excessiveenergy 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 isusing 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 energyis being generated or used. It's like how miles per hour makes it
obvious
that speed is the rate at which distance is beingtravelled.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 anothername for Joules per second. J/s and W are the same thing. Just some bright spark decided that equations and whatnot wouldbe simpler if power had its own unit (instead of being expressed using units of energy and time together). And they named thisunit after James Watt, the Scottish inventor who had an important hand in the development of the steam engine.
Electricity and other fuels supply energy in a form that we can use to run the equipment in our buildings. Power is the
rate
at which energy is generated or used.The kW is a unit of power.
James Watt
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. gasor 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
atwhich 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 happennearly 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 somebodythat 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 nosense.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 periodof 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 usefulthings for us. Really they're
converting
the energy into other forms (heat, motion etc.), but we say that they're using it becausewe don't really care about what exactly is happening to it, we just want our equipment to work when we switch it on and stopwhen 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 mighthear 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 (100W 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... Asecond, 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'tbe 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 lesspower than if you're hammering away on an Excel spreadsheet, listening to some music, and burning a DVD, all at the same time.We make a distinction between
instantaneous power
and
average power
:
Instantaneous power
The instantaneous power (or instantaneous demand, or instantaneous load) is the power that something is using (or generating)at any one moment in time. Put your laptop on standby and its instantaneous power will drop immediately. Bring it back to lifeand its instantaneous power will rise immediately.If, at any particular moment, everything in an ofﬁce building is switched on, that ofﬁce building might be using 42 kW of power.That's 42 kW of instantaneous power. If, at any particular moment, everything in the ofﬁce building is switched off, that buildingshould be using 0 kW of power. That's 0 kW of instantaneous power.The instantaneous power of most buildings varies constantly. People are constantly switching things on and off, and many itemsof equipment within the building have instantaneous power that is constantly changing too.
verage power
The average power represents the power that something uses or generates, on average:over a speciﬁc period of time (e.g. yesterday); orover multiple periods of time (e.g. across all the weekends on record); or
throughout a certain type of operation (e.g. typical laptop usage, or typical building usage - Monday to Friday 09:00 to 17:00,or typical efﬁciency for something that's generating power).Remember our example of an ofﬁce building that uses 42 kW of power when everything's switched on, and 0 kW of power wheneverything's switched off? If, on average, half the things in the ofﬁce building are switched on, and half are switched off, then theaverage power will be around 21 kW overall (21 kW being half of 42 kW).Or maybe that's just the average power of the ofﬁce building on
weekdays
. On
weekends
, when people are at home, and mostequipment in the building is switched off, the average power might be lower, maybe 5 or 10 kW.The instantaneous power of a typical building varies
all the time
. If you try to monitor instantaneous power you get lost in thenoise. And ﬁgures of energy consumption are meaningless unless you know the length of the periods that they were measuredover. But average-power ﬁgures smooth out the constant ﬂuctuations of instantaneous power, and make it possible to comparethe efﬁciency of different periods, like for like, without worrying about how long those periods were. For example:The building used 41 kW (on average) across the whole of last week.The building used 19 kW (on average) across all the Saturdays and Sundays since March 2017. (We don't need to care howmany Saturdays and Sundays there were since March 2017.)The building used 38 kW (on average) between 09:00 and 17:00 on Tuesday, December 25th, 2018. That's double theaverage kW of a typical weekend, and that's bad because December 25th was a public holiday and the building was closed.You can easily use these average-kW ﬁgures to compare the energy consumption of different periods and even differentbuildings (we use the term energy consumption loosely because really we're talking about average power, not energy). It's a bitlike comparing the fuel consumption of cars:On long journeys my car does an average of 45 miles per gallon. My brother's car does an average of 41.Around town my car does an average of 32 miles per gallon. My brother's car does an average of 29.These average-mpg ﬁgures would typically be calculated across multiple different journeys, each covering different distances...But you can compare the ﬁgures
like for like
, without worrying about the details of the speciﬁc journeys that they were calculatedfrom. Average power works in the same way, but with energy instead of distance.
*
In this instance energy usage refers to the rate at which the energy is used (i.e. power). To remove ambiguity we might call it average power , or load , or demand . Energy usage and energy consumption are somewhat loose terms that can be used torefer to the rate of energy usage (e.g. 10 kW) or the total amount of energy used over a speciﬁed period (e.g. 240 kWh on February 21st, 2018).
The beauty of average-kW ﬁgures is that you can compare them fairly in an instant. The length of the time period doesn't reallymatter. So you can look at the average-kW ﬁgures from 15-minute interval data and compare them directly with the average-kWﬁgures from 60-minute data or from half-hourly data. Or you can instantly compare the average kW from last month with theaverage kW from yesterday and the average kW from the whole of last year. If these were kWh ﬁgures, the fact that they comefrom periods of different length would mean that you'd need to normalize them before you could compare them fairly. kW ﬁgurescome ready normalized.
Different names for power - load , demand etc.
People often refer to power as the load or the demand . So you might hear average power referred to as average load , or average demand .
Average power enables you think of complicated things, like buildings, as if they were simplethings, like light bulbs... Average power, typically measured in kW, is a great way to look at the energy usage
*
of abuilding. In many ways average-kW ﬁgures are easier to work with than kWh ﬁgures.

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