Do you get confused by kW and kWh all the time?

 Kilowatt vs Kilowatt-hours

kW vs. kWh: What’s the difference?

‘Two dollars’, my wife replied right away. We have a 20 watts (or may be 40 watts, I don’t exactly know to be honest) light bulb outside, above the front door of our two-bedroom apartment. I like to keep that light on in the night, but my wife doesn’t and her reason is the power bill – which had skyrocketed over the winter.

front-door-light

One night she noticed that I had forgotten to turn off that light. I refused to get out of the bed; so she got up and turned off the light and let out a big sigh of relief as she walked back to the bedroom. I asked her, ‘How much do you think we save by keeping that light switched off at night?’. Without any hesitation, she replied, ‘Two dollars’. ‘In a day?’, I asked. She didn’t respond.

It is important to understand the concept of Kilowatt (KW) and Kilowatt-hour (KWH) to know how the power companies charge us for using electricity. Furthermore, if you are looking into solar panel system, understanding your energy usage is key to determine your solar need. And if you can’t differentiate KW and KWH, it’s easy to get mixed up.

So, let’s get to it…

Definition of Kilowatt (kW) and Kilowatt-hour (kWh)

Kilowatt, kW in short, is the measurement of power. One kilowatt is 1000 watts. All things that use electricity is labeled with how much power they need to “function”. In the light bulb example above, it requires 20 watts of power to function, or in this case light up. An electric water kettle is rated much higher for power, something like 2000 watts, or 2 kilowatts.

water-kettle-power-rating

Kilowatt-hour, kWh in short, is the measurement of energy consumed; one kilowatt-hour is one kilowatt in one hour.

A 20-watt light bulb that stays on from 8pm-8am draws power at a continuous rate of 20 watts for twelve hours. What is consumed over twelve hours is expressed in kWh.

If you take a closer look at your electricity bill, you’ll see that you are being charged for kWh, not kW.

kilowatts-vs-kilowatt-hour

Kilowatt, or watt, can be found on the label of an electric appliance. Kilowatt-hour is what you find on your power bill.

So, how is kWh calculated from kW?

There is only one way to calculate kWh from kW. 

Using the same light bulb example: A 20-watt light bulb turned on for 12 hours each day will consume total of 240 watts-hour or 0.24 kilowatt-hour for that day. The calculation is simply 20 watt X 12 hours = 240 watt-hours per day.

solar-power-and-energy

If the bulb is left on for twelve hours everyday, in four days it will have finally consumed about 1000 watt-hours or 1kWh; 240 watt-hour per day X 4 days = 960 watt hour.

We currently pay 26.5 cents for every kWh we use. That means, the cost of keeping the light on would be 26.5 cents every 4 days. Thus, in a month, the total cost would be close to two dollars. So, my wife was probably right when she said two dollars…. but I doubt she meant it over a month.

Unfortunately, most electrical equipments use a lot more power than a light bulb.

For example, a tiny electrical fan heater can be rated for 2000 watts. If that heater is left on continuously for two hours, it will consume 4000 watts-hour, or 4kWh of energy.

Here is a teaser; in half an hour, this 2000-watts heater will consume one kilowatt-hour. In fifteen minutes, it would consume 500 watt-hour. Do you get it? Keep on reading…

Let’s look at it in reverse….

You can not work backward to calculate the power rating (how many kilowatt anything uses to function) of an appliance from its energy consumption value in kWh.

For example: if someone says their electric heater uses 500 watt-hour, but doesn’t say in what amount of time, you can’t be sure about the power rating of that kettle. It is not automatically 500 watts.

Instead, if that someone says their electric heater uses 500 watt-hour in 15 minutes, then in one hour, it uses…….2000 watt-hour or 2kWh.

energy-time-power

Power companies determine rates for each kWh we consume. It doesn’t matter how slowly (over 4 days in case of light bulb) or quickly (in half an hour in case of heater) you use it. But I am sure you know which way they prefer.

What other ways can we consume 1 kWh?

Here are a few.

  • Toasting bread in 1000 watts bread toaster for one hour.
  • Watching 80 watts LED TV for 12.5 hours.
  • Using 50-60 watts laptop for about 16 hours.
  • 1000-1200 watts microwave for 1 hour.
  • 2000-3000 watts clothes dryer for 20-30 minutes.
  • Turning on a 4000-6000 watt air conditioner/heater for 10-15 minutes.

On average, an Aussie family consumes about 18 kWh of energy per day. At 25 cents per kWh, that would be $4.50 per day. 18 kWh may sound a lot but think about how many things you have in the house that run on electricity; the single biggest one is obviously the air conditioning system. 

kW and kWh is same as the speed of a car and the distance travelled.

car vs kettle

If kW and kWh still sounds a bit confusing, here is an analogy…

Imagine you are driving a car. You are cruising at 100 kilometer per hour.

The speed of the car determines how far you can go in a certain time. Let’s say you go two hours non-stop at 100 km/hr. Total distance traveled is 200 kms.

Back to kW and kWh… kW is like speed of the car; it the rate at which you consume power and determines how much you can expect to consume over time.

kWh is like the total distance covered; it says how much energy was consumed in total.

A 2000-watt electric kettle will consume 500 watts-hour of energy in fifteen minutes much like a car traveling at 100 km/hour will cover 25 kilometers in fifteen minutes. Makes sense?

Size of solar panel is measured in Kilowatt (KW).

Not the physical size, but the electrical size of a solar panel is rated based on its output power, which is measured in kilowatts (kW).

Most solar panels today are rated 250 watts per panel. If you bought one kilowatt, that’s four panels. 

If toasters and kettles consume power at a certain rate, solar panels generate power at a certain rate.  

Wrapping up,

The concept of kWh and kW gets a little confusing at first. But just remember that rate of power consumption or generation is kW, and the total power consumed over time is kWh.

If you have any questions, please don’t hesitate to contact me.

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