Physics 101 – what is the difference between power and energy?

what is the difference between kW and kWh?

by Solar Choice Staff on January 19, 2010

in What are the right solar PV cells for you?

For the non-scientifically inclined, all this talk about power and energy can get confusing. It gets worse when people start saying kilowatt and kilowatt hour, and then using the letters k, W and h everywhere. What does it all mean? 

When buying solar cells, there are two basic things you need to know. What is power, and what is energy. This article will (hopefully) clear this up for you by speaking in laymans terms.

What’s the difference between power and energy?

People like to use the terms power and energy interchangeably. In truth though, the two words have very precise meanings that are slightly different.

Energy

As most of you would know, energy is the stuff that allows you to do, well, stuff. When you move your arm you are using energy. When you drive your car, you are using energy. The computer you are reading this on uses energy. So the concept of energy is one that is very familiar to us.

A Joule (J) is the term we use to quantify energy, just as a metre is the term we use to quantify distance. And a joule is something that most people, too, are familiar with. We describe food it terms of how many joules of energy it contains when trying to monitor our diet. In the same way we also talk about how much energy we have ‘burnt’ when excersising in terms of joules.

In the same way we can describe petrol in terms of how many joules of energy it contains, and also electricity.

So energy talks about how much there is. It describes an amount.

Power

Power, on the other hand, is a measure of how much we CAN produce. It is not a measure of how much energy there actually is, but a way of describing how much could be produced.

This is usually talked about in reference to engines. So when we talk about power we’re measuring how much an engine can produce. For instance, when talking about the power of a car engine, you’ll hear people say it’s a 3 litre V6 or a 5 litre V8. What this refers to is the size of the engine, and as it so happens, the size of an engine is a good indicator of how much power it can produce. When people start to get more technical and precise, they will say my engine has 200 kilowatts (or kW for short) of power.

A watt (W) is the term we use to quantify power, just as kilometres per hour is the term we use to quantify speed. A kilowatt just means a thousand watts (a kilo in front of anything just means there’s a thousand of it).

Now why do you run an engine? You guessed it, to produce energy. So if what comes out of an engine in energy, it makes sense to measure the size of an engine in terms of how much energy it can produce. And that is what power is, it is a measure of how much energy we CAN produce.

Hence the two terms are linked. And a watt is defined as:

1 Watt = 1 Joule per second

which is the same as saying;

Power = how many joules of energy can be produced in 1 second

So, all up, power is the way we describe the size of something that can produce energy.

Electrical energy

Electrical energy is exactly the same stuff, it is still energy, only it is in the form of electricity. What may confuse you is that electrical energy is talked of in terms of kilowatt hours (or kWh for short).

What is a kilowatt hour? As a unit of measurment, it is actually the same thing as a joule, it is just a way of measuring energy. The reason why it is used is because when we say kWh it is immediately clear people are talking about electrical energy.

In terms of solar cells

When it comes to solar cells we use these two terms to describe the exact things they are meant to.

The power of a solar system tells you how much energy it CAN produce, and is measured by the unit of power, kilowatts (kW). This measure is telling you how big the system is, or in other words, what it’s capacity to produce power is. 

The amount of energy a solar system will produce is measured in kilowatt hours (kWh) – which remember, is just another way of talking about energy which is otherwise measured in joules.

Now how much energy your solar system will produce is dependent upon the amount of sunlight it gets, and this is the topic of another article – How much energy will my solar cells produce?

Kobad Bhavnagri
Solar Energy Consultant
Solar Choice Pty Ltd

© 2010 Solar Choice Pty Ltd

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