kW and kWh: What Does it Mean?

This article was originally published on Solar Quotes, the original post can be found here.
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So, what is a kW & a kWh?

And what is the difference between a kW and kWh?

An older style meter showing kWh

Let’s start with what each letter stands for:

  • k stands for kilo – which means “one thousand”.
  • W stands for Watt – which is a measure of power.
  • h stands for hour – which is a measure of time.

So kW means kilowatt which is 1000 Watts, a measure of power.

Notice that, if you like to keep pedantic electrical engineers like me happy, the correct way to write it is always with a small k and a capital W.

The size of a solar system is defined by its peak power, often denoted as kWp (the p standing for ‘peak’), e.g. a 1 kWp system can produce 1 kW of power per hour when operating in line with the testing parameters.

kWh stands for kilowatt-hour; a kWh is a measure of energy (not power).

If your solar panels (for example) continuously output 1kW of power for a whole 60 minutes, you will have produced 1 kWh of energy.

The amount of electricity you use (or generate) is defined in kWhs. e.g. “My solar system produced 4 kWh of electricity today!”

So at the highest level: kW measures power, and kWh measures energy.

Why is the difference between Energy and Power important?

  • Power is the rate at which work is performed or energy is converted
  • Energy is the ability to do work on objects

It is very common for people to mistakenly interchange the terms energy and power as if there is no difference. Most people do it all the time without noticing. It drives electrical geeks up the wall.

For example, if someone is talking about their electricity usage and says, “I used 8kW yesterday”, they probably mean that they used 8 units of electrical energy yesterday. In this case they should really say, “I used 8kWh yesterday”

Yeah, yeah I know what you are thinking: Who cares?

Well it is actually quite important if you are buying a solar system. If someone says they need a solar power system to produce 8kW, they might end up being quoted an 8kWp solar system. Which will cost about $24,990 + installation at today’s prices and produce about 32kWh per day.

If, what they actually meant was that they need one to cover an energy usage of 8kWh per day, then they really need a 2kW solar system which costs about $8,375.00 + installation at the time of writing!

So please don’t confuse kW and kWh. If you do you may end up with a solar system that is completely the wrong size!

Top tip for filtering out the worst solar salesmen: Ask them to explain the difference between a kW and kWh. If they get this wrong how on earth are they gonna understand your requirements? A lot of cold calling door knockers will fail this test in my experience.

The technical bit for those that are interested:

  • Energy – measured in Joules (J); energy is the capacity of something to do work.
  • Power – the rate at which energy is used; power is measured in Watts (W).
  • 1 Watt – the rate of energy usage, being 1 Joule every second (J/s).