Difference between Power and Energy

You thought power and energy were the same? Well, they are not. Most people think they are the same thing, but there is a difference that is important for you to know.

Let’s start with Power. What is power?
Power is measured in watts. Kilowatts (kW), which you’ve probably heard of, is simply a thousand watts. So, what is power? Let’s think of an appliance. Power is the rate at which the appliance uses electricity. In other words, how much energy it uses every second.

Here is an example: Think about the humble old-school incandescent light bulb. Everyone knows that a 20-watt light bulb uses less energy than a hundred-watt light bulb. We also know that a 20 watt light bulb is dimmer, about five times dimmer, than a hundred-watt light bulb. Why is that? Because we are pushing electricity into the 20-watt light bulb slowly at 20 watts and we are pushing the electricity into a hundred-watt light bulb five times faster. We are pushing a hundred watts into that light bulb; so the light bulb is five times brighter.

Now, let’s take it to Solar. As well as measuring the usage of an appliance, we also use watts or kilowatts when we are talking about solar to measure how much electricity something produces, the rate at which it produces that electricity. Consider a six-kilowatt solar system, a typically sized solar system in Australia. Six kilowatts means that’s how much electricity that solar system will pump out on a perfectly sunny day in the middle of the day when the sun’s the strongest. We size them using power measured in kilowatts and we size them based on the amount of electricity they will produce at midday on a perfectly sunny day.

Now, solar systems are made of lots of solar panels, so how do we measure how much power does a solar panel produces? Well, it is actually really simple: A solar panel, like a solar system, is sized by its power; but because they’re smaller we size them in watts, not kilowatts. So a typical solar panel will output at midday on a perfect solar day between about 250 and 300 watts – a bit bigger these days. A six-kilowatt solar system will use either twenty-four 250 watt panels – if it’s a bit of an old system – or more likely around twenty 300 watt panels because 20 times 300 is 6,000 – meaning six kilowatts.

So in few words, power is measured in watts or kilowatts. Solar power systems are sized in kilowatts.

Now that we have clear what power is, let’s move on to explain what is energy.

Energy is how much electricity you have used over a period of time. It is measured in kilowatt-hours (kWh), which admittedly is very easy to confuse with kilowatts. But that little “h” on the end is really, really important.

So, imagine you are using a kilowatt of power. Imagine you use that kilowatt of power for one hour. Guess what? You have used one kilowatt-hour of energy. Simple, right?

Let’s make it more tangible – picture the 100-watt lightbulb. In order for it to work, you have to make 100 watts to run through the lightbulb, right? So far it is really easy. Now if you wonder “how much energy am I using?” The answer is really simple, it depends on how much time you keep the lightbulb On. For example, if you turn it on for one hour, you would be using 100 Wh (watts per hour). If you leave it on for five hours, you would be using 500 Wh. If you leave it on for ten hours, then you would have used a kilowatt-hour (kWh) of energy. And so on.

So basically, the difference between Power and Energy is the way they are sized. Kilowatt (kW) measures power – the rate at which you use or produce electricity. Kilowatt-hours (kWh) measure energy – how much electricity you’ve used or generated over time.