What are Volts, Watts, kW and kWh? – Beginners Guide To Conversions

Beginners guide to solar termsOne of the confusing things about choosing a solar or battery system can often be the terminology and electrical terms used.

Whilst most of these are simple, for those of us who aren’t electricians or in the electrical industry, it is hard to remember what we were taught at school, assuming we were even listening.

When deciding whether or not to buy a solar system, there are some terms you really should understand (like kilowatts, for example) and there are others you might be interested in that may only help a little to know.

We are going to cover off some of the basic info first, (you can skip over the first bit if you’re not really interested) and then move onto the info that is relevant for solar.

The basics of Power – Volts, Amps and Watts

To understand electricity and power, it may be easier to think of electricity like water running through a hose. In this pretend comparison:

What is a Volt?

Volts would be pressure of the hose (eg. if you were to squeeze a bag full of water, the water will shoot out with force). Volts are the standard measure of electromotive (ie: electrical movement) force.

In Australia, our power supply is set at 240 Volts (it actually varies up and down a little from this, but that’s another story).

What is an Amp?

Amps (amperes) would be the volume/amount of water flowing through the pipe. It’s the amount of electricity you have in your wire. 

To assist with this, a high drawing appliance such as a heater or air conditioner would draw (use) more Amps than a low drawing appliance (such as a computer or TV screen), even though the voltage is the same.

What is a Watt?

Watts are a combination of Amps and Volts. It’s calculated by Amps x Volts. 

A Watt is the standard measurement of power that we generally use in day to day terms. 

How to convert Watts to Amps – The Power Formula

Here’s how to calculate Watts from Amps and Volts:

Power (Watts) = Volts x Amps

Therefore, an appliance that uses 2 Amps on a 240V power supply will be using 480Watts.  

Watt is a kiloWatt? (pardon the pun) – Convert kW to W

A kiloWatt (kW) is simply that – 1,000 Watts. We use kiloWatts for the same reason we use kilometres – a Watt is generally too small to be usable just as measuring the distance from Sydney to Melbourne in metres would be impractical.

What is the difference between Power and Energy? (Read this part..)

So, this is the bit that is worth knowing.

Power is the amount of electricity (in this case) that is used at a particular point in time.

Energy is the amount of power that is used over a period of time.

To use the water hose analogy above, Power is the amount of water coming out of the hose (whether it is flowing quickly or slowly). Energy is the amount of water to have flowed out of the hose in a period, say one hour.

Power is measured in Watts or kiloWatts (the speed at which the water leaves the hose).

Energy is measured in Watt/hours or kiloWatt hours (the amount of water in the bucket).

What is a kiloWatt hour (kWh)? – Convert kW to kWh

A kilowatt-hour (kWh) is 1,000 Watts of Power used over the course of an hour.

A kiloWatt hour could, therefore, be used by:

  1. An appliance using 1kW for 1 hour; or
  2. An appliance using 10kW for 6 minutes; or
  3. An appliance using 100W (0.1kW) for 10 hours.

Why is this important? 

This is important to understand because when we buy electricity from our supplier, we are (at least for residential customers) buying Energy, not Power.

In other words, on our bills, we are charged for kiloWatt hours – not for kiloWatts.

Therefore, when we install a solar system, we are looking to reduce the Energy Usage, not necessarily the Power Usage.

How are the terms kW and kWh used when buying solar?

When we talk about the size of a solar system we generally talk in kiloWatts (eg a 5kW or a 10kW system).

When we talk about how much a solar system will save you in energy usage and dollars, we talk in kiloWatt hours.

Simple!

Why are solar systems sold by kiloWatts, not kiloWatt hours?

When a solar system is described as a 10kW solar system, it doesn’t actually refer to the amount of power that it will produce.

Instead, this is referring to the Peak Output of the panels in a test (or laboratory) environment.

The reason for this is twofold:

  1. Every rooftop is different, and every location around the world (ie Sydney versus Oslo) gets a different level of sunlight.
  2. Solar panels are manufactured to different quality levels, using different technologies and their performance may vary wildly from one panel to another over time.

When a solar panel is manufactured, they are tested as they leave the production line. This test is done at 25 degrees Celsius, with 1,000Watts of Solar Irradiance per square metre, and an air mass of 1.5 (known as AM1.5 but we won’t even try and explain that one here).

Importantly, the kW rating of a solar system only refers to the potential output of the panel, the actual output of the system will usually be lower once the power has been converted to AC power, and run through cables etc.

It is also worthwhile noting that it will rarely, if ever, be 25 degrees on your rooftop with 1,000 W/m2 and an air mass of 1.5.

And, as mentioned above, the quality of the panel will mean the output after a few years could well be dramatically different.

However, what the use of this Standard Test Condition does is allow us to know that, at least when new, 1kW of panels should output 1kW in the same conditions at which they were tested. Whether or not this is actually 1kW is not really important, rather that similar output panels should all output approximately the same, and that everyone uses the same measurement.

Why is the Yield or Benefit of a Solar System measured in kW hours, not kiloWatts?

As we discussed above, 10kW of solar panels can have different levels of output based on where they are located.

A 10kW solar system in the Northern Territory – operating at full capacity on a sunny day, will output a much higher level of power than an identical system operating in Hobart, for example.

In any event, when we buy electricity, we buy it in kiloWatt hours, so it makes sense to measure the benefit of solar power also in kiloWatt hours.

In Australia, for residential customers, we generally estimate the yield (or energy production) of a solar system using guidelines published by the Clean Energy Council.

In Sydney, for every 1kW of panels installed facing north without shade, the CEC estimates it will produce 3.9kWh of energy per day on average across the year. In Melbourne, the same panels would be expected to produce 3.6kWh per day and in Brisbane 4.2kWh.

These numbers, of course, vary based on the quality of the system, shade and other site-specific issues. In reality, we find most of our system generally outperform these numbers in the right conditions.

So, What Does All This Mean?

What all this really means is that a solar system is designed to reduce the amount of money you pay for your energy, not necessarily run any given appliance at a particular point in time.

In other words, it is about having the most amount of water in the bucket at the end of the day rather than how fast the water pours out in the middle of the day.

Therefore, good quality panels, advanced technology inverters, good design and installation together are usually much more important than just the number of kiloWatts the panels may have produced at the factory.

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