Absolute Solar Power Basics – The Difference Between Power And Energy

When it comes to solar power, the absolute fundamental thing you need to know is the difference between power (kW) and energy (kWh) says SolarQuotes founder Finn Peacock in his latest video.

Video transcript:

Sounds ridiculous, right? You thought power and energy were the same. Most people do. But here’s why it’s really important.

What Is Power?

Let’s start with 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- how much energy it uses every second.

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’re pushing electricity into the 20-watt light bulb slowly at 20 watts and we’re pushing the electricity into the a hundred-watt light bulb five times faster. We’re pushing a hundred watts into that light bulb; so the light bulb is five times brighter.

The Water And Hosepipe Analogy

You probably still haven’t got a tangible feel for what electrical power is; so my favorite analogy is: electricity in a wire, water through a hose pipe.

Imagine you’re in your back garden, you’re holding your hose pipe, you turn on the tap a little bit. The water’s just dribbling out the end of the hose pipe. That’s the equivalent of the wire sending electricity to the 20-watt light bulb. Electricity is flowing really slowly through the wire, just like the water’s flowing really slowly through the hose pipe.

Now turn the tap onto full whack – the water is surging through the hose pipe. Hopefully if you’ve got good water pressure, you can push that water to the other side of the garden. That’s like the hundred-watt light bulb. The water’s flowing five times faster; the electricity’s flowing five times faster into the light bulb. You’ve got a much brighter light bulb that’s using a lot more electricity and it’s costing you a lot more to run.

Solar Power And Kilowatts

Now as well as measuring the usage of an appliance, we also use watts or kilowatts when we’re 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.

Look at the picture behind me, that’s a graph from midnight to midnight the following day of how much power that solar system is producing on a perfectly sunny day. It only produces six kilowatts at midday at solar noon – and that is how we size solar systems. 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, that solar system is obviously made up of lots of solar panels.

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 – is six kilowatts.

So to summarize, power is measured in watts or kilowatts. Solar power systems are sized in kilowatts.

What Is Energy?

So I’ve described what power is. Now, what is energy? Energy is how much electricity you’ve used over a period of time. It’s 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’re using a kilowatt of power. Imagine you use that kilowatt of power for one hour. Guess what? You’ve used one kilowatt-hour of energy.

Let’s make it more tangible – let’s go back to the water analogy. So we’ve got water in a hose pipe. The faster that water’s flowing through, the higher the power. I’m sure you get that by now. Now imagine you’re filling a bucket with that hose pipe. The amount of water collected in the bucket – that’s equivalent to the amount of electrical energy stored in a battery.

So, you’ve got a hose pipe going into a bucket.; you’ve got a wire going into a battery. How do we size the bucket? We size the bucket by how many litres of water it will store.

Well, we size our battery by how many kilowatt-hours of energy it will store. The longer you keep the hose pipe going into the bucket, the more water you get in the bucket. The longer you put electricity into the battery, the more energy is getting stored by that battery. The faster you squirt the water into the bucket, the quicker it fills up. The higher power you put into the battery, the faster you’ll fill up the battery.

So kilowatts measures power – the rate at which you use or produce electricity. Kilowatt-hours measure energy – how much electricity you’ve used or generated over time.

The difference between power and energy is one of the many topics covered in Finn’s book, The Good Solar Guide, which is available to read free online in its entirety.

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About Michael Bloch

Michael caught the solar power bug after purchasing components to cobble together a small off-grid PV system in 2008. He's been reporting on Australian and international solar energy news ever since.

Comments

  1. Tony Gunn says

    I have a slightly different analogy for comparing power to energy that involves power factor, another difficult but important concept. Beer in a glass and different size glasses. The froth is the power factor – wastes space in the glass and does not contribute to getting drunk. Number of glasses consumed equates to drunkeness, Small glasses, get drunk slower than consuming large glasses. You’ve probably heard it before.

    • Not a great analogy as you can still consume the same amount of beer and at the same rate from either small or large glasses.

    • …….. But we should never forget that the rate/state of drunkeness depends upon who’s doing the drinking, and under what circumstances. It’s not possible ~ and would be pointless anyway ~ to demand that one single ‘measurement’ like ‘drinks-per-hour-per-glass-size’ apply accross the board..
      Even if a ‘standard’-size bucket were the measure.

  2. Energy is the amount of Work done or transferred.

    Power is the rate of doing Work.

    The hose analogy is a pretty good one for explaining electricity because it can also be used to explain:
    – charge (the water)
    – current (water flow)
    – voltage (a pressure differential, such as is caused when you have a water reservoir up higher than the hose outlet)
    – resistance (hose cross sectional area and inner roughness causing drag through the hose)
    – work done / energy (amount of water delivered through hose)
    – power (rate water is flowing / amount of water per unit time)

  3. ????? There’s a hole in your bucket, dear Henry, dear Henry.
    Semantics matter:- “We’re pushing a hundred watts into that light bulb; so the light bulb is five times brighter.”.
    The reality is that the light-bulb is PULLING 100 Watts, because it is engineered to USE 100 Watts. Try ‘pushing’ 100 watts into a 20-Watt lightbulb.

    ‘ KWh’ is a purely for-convenience ARBITRARY term that has no actual value beyond an acounting one. The point was made elsewhere that (nominally) a 200-ah battery can run a 5-Watt radio for days or a welder for perhaps a minute. Changing those (work-done) values to kWh measurements have no meaning other than as an accounting exercise. Y’could, in fact use any word you liked, an it’d make NO difference to the power-in/power-out equations.

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