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The difference between kW and kWh Explained ?

A lot of people, included energy professionals, don't fully understand the difference between kW and kWh.

If you are one of them, 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.

So, what is the difference between a kW and a kWh?

Well, the difference is really very simple, although it only seems simple after you understand it.

kWh is a measure of Energy !

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 kilowatt hour (kWh) is a unit of Energy... The Calorie is a unit of energy... And the joule (J) is also a unit of energy... And these aren't the only units of energy - there's the British thermal unit (BTU), the watt hour (Wh), 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 metres, feet, km, miles and so on.  The flight distance between Sydney and Melbourne is fixed, but you can express that distance as or 713 km, 443 miles, or 713'000 meters etc. Similarly, you can express a measure of energy in joules, or Calories, or kWh, or BTU etc.

Energy can change form! We could eat Chocolate to provide us with energy. Or we could burn the Chocolate and turn it into heat energy. Given the right equipment we could turn the heat energy from the burning Chocolate 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 Chocolate to run a light bulb for several seconds.

Probably the best option would be to eat the Chocolate, but hopefully you get the general idea - the Chocolate 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 Chocolate contain a certain amount of energy - 800 Calories or 0.0008 kWh per Chocolate. But Chocolate 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, that we buy or generate kWh of energy, and we use it to fuel the equipment in our buildings.

The relationship between Energy consumption (kWh) and time.

Obviously a typical building uses more energy over long periods of time than it does over short periods of time, it's not rocket science:

  • On any day a building might have used 100 kWh.
  • Over the week it might have used 600 kWh.
  • For 12 months it might have used 10'000 kWh.

Given the three figures above, we can easily see that the building used more energy over a longer period, 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 ...etc.
  • 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.

Remember: 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, diesel or fuel 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, usually stated at kW per day.

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 what not 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).

So, a kWh is a measure of how much energy is used or generated in relation to a period of time.

......... // ...........

Given a value for any two of the following: average Power (kW), total Energy (kWh), Hours (h), you should be able to use any of the formulas above to calculate the value of the third.

You should also be able to convert between other units of Energy, Power, and Time, given that:

  • 15 minutes is 0.25 hours, 30 minutes is 0.5 hours, a day is 24 hours, a week is 24 x 7 hours etc;
  • a kW is 1000 W and a W is 0.001 kW;
  • a kWh is 1000 Wh and a Wh is 0.001 kWh;
  • a MW (megawatt) is 1000 kW and a kW is 0.001 MW;
  • a MWh (megawatt hour) is 1000 kWh and a kWh is 0.001 MWh;
  • and any other conversions between units that measure the same thing (like different units of energy, or different units of power, or different units of time) are widely available online.

The relationship between energy and power

The relationship between energy and power is a lot like the relationship between distance and speed:

  • Energy is like distance. The amount of energy that you used over a specific period of time is like the distance that you travelled over a specific period of time.
    (e.g. when driving to work you travelled 4 kilometres between 08:04 and 08:57 - horrible horrible traffic!)
  • Power is like speed. Your instantaneous power is like your speed at a specific instant in time.
    (e.g. right now). Your average Power over a specific period of time, is like your average Speed over a specific period of time.
    (e.g. when driving to work you travelled at an average speed of 4 kph - you might as well have walked...)

Both distance and speed are useful measures, as both are closely related. Sometimes it makes sense to talk in terms of distance, and sometimes it makes sense to talk in terms of speed. It's the same for energy and power - you need both, but usually one makes more sense than the other.

Newbies to energy often try to use energy (kWh) for everything (sometimes calling it kW by accident), but more experienced people tend to use power (kW) a lot more.

The equation connecting energy and power:

Following is the fundamental equation that connects energy and power.

energy = power x time.

We can express this equation in terms of kW, kWh, and hours (h):

  • kWh/p = kW x h / p
  • Where;
    • kWh is the energy.
    • kW is the power.
    • h is the time in hours.
    • p is the Peak Energy x by a set time.

You might also remember from school that equations can be rearranged:

  • power = energy x time.

Calculating cost:

At the simplest level cost is usually expressed in terms of $/kWh or c/kWh or £/kWh or €/kWh or p/kWh (or whatever-unit-of-currency-you-have per kWh). It makes sense that cost should be calculated per kWh (not per kW), because cost is a cumulative item - the more energy you use, the more it costs.

Let's work in $/kWh... To work out the total cost over a specific period, calculate the total number of kWh over that period, and multiply that by the $/kWh. That will give you the total cost in $.

...It's that Simple.  e.g. You use 10kWh of energy in 24 Hours, and it costs you $0.30cents per kWh , so 10kWh x $0.30cents =$3.00.

However, cost calculations are usually more complicated:

  • Prices change over time. One month you're paying one price, next month you're paying more. Unfortunately in Australia the power cost is greatly inflated due to the deceptive & hidden fees the so called "Electricity Retailers" enforce on consumers, manipulating energy prices to rise over time.
  • Electricity costs can depend on the time of the day, the day of the week, and the time of the year. Energy suppliers/utilities come up with complicated tariffs to define these rules. Just like prices, tariff structures can also change over time.
  • Electricity costs can also depend on the maximum demand, or peak load, across a period. For example, given half-hourly data for a month, the peak load or maximum demand could be defined as the half-hour period that had the highest average kW. The higher the peak load, the higher the peak-load charge (or maximum-demand charge).
  • Electricity tariffs often charged at different rates depending on how many kWh you use. For example, the first 100 kWh might cost $0.30kWh, the next might cost $-.28kWh.
  • There are often standing charges - regular fixed fees that aren't related to how much energy you use.

So, All in all, if you're looking to reduce energy consumption it's usually much easier to spend most of your time working in units of kW and kWh. The occasional cost figure can be useful for showing to non-technical folks (everyone understands dollars and cents), but cost figures aren't very good for accurate, in-depth analysis of energy-usage patterns (e.g. to find opportunities to save energy and to track progress at doing so).

:: Want it Less complicated, then Purchase a Battery Bank & forget it all. ::


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