kW and kWh: What is the difference?

Don't confuse kW and kWh. It matters!

One of the most common sources of confusion I come across when talking to non-electrical people about solar power is the difference between a kW and kWh.

Please excuse this post if you think I am explaining the bleeding obvious! But I think it is really important to write a post that describes these key units of electricity in a way that requires zero prior knowledge of anything electricity related.

So here is my humble attempt to answer the question:

What is the difference between a kW and 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 obviously a measure of time.

So kW means kilowatt which is 1000 Watts. It is a measure of power. Notice that, if you like to keep anal 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. e.g. a 1kW system can produce 1kW of power on really sunny days.

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?

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 like me up the wall. Especially when I read it in national newspapers and books!

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 which case they should have said

“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 8kW solar system. Which will cost about $18,000 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 $3,500 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: Energy is measured in Joules. Energy is the capacity of something to do work.

Power: Power is the rate at which energy is used. Power is measured in Watts.

1 Watt is a rate of energy usage of 1 Joule every second, or 1 Joule per second (J/s).