Understanding the Difference between kWh and kW

kWh = kilowatt-hour = 1 kWh
kW = kilowatt = 1 kW = 1000 J / second
1 kWh = 1 kW being used for 1 hour
J = Joule

Power Formulas

kWh to Joule conversion

What is a Watt

A Watt is a unit of Power, often shortened to just W. W = Watts. Power is measured in Joules. 1 Joule per second = 1 Watt. The formula for finding Power is given above.

kW = kilowatt = 1000 W = 1000 J / sec
Kilo = 1000
1 W = 1 J / second

Devices that consume power almost always (I believe by law) must display on the power component of the device how much power they consume. So, if this is your Laptop battery charger pack, it will show what is going in (INPUT) and what’s coming out of the power adapter and into the laptop (OUTPUT). The device I just read sitting here has these values only in Volts (V) and Current (I). Using the formula above, we multiplying these two numbers together gives you P in Watts, well Joules. If we’re talking instantaneous moments of time, 1 W = 1 J / second. If you use a 1kW device for 1 hour, you have used 1 kWh.

Now, as you’re going about the house looking at these numbers on all your electronics, there are a few things to keep in mind.

Is it DC or AC voltage that you’re reading? The formula given above is for DC voltage.

The number you get for Power, if it’s the output, you’re not taking into account the Voltage drop across the transformer itself (the black charger pack). So, the Voltage being ‘used’ will be a little bit higher then the Voltage reading on the charger pack. I would expect somewhere between 0.5 V – 2 V drop for the black charger pack, but I’m sure somebody out there would prove me wrong for some device.

The Difference Between kW and kWh

Firstly, kW is a unit of Power. kWh is a unit of energy, and thus cannot be compared to each other directly.

“kW (kilowatts) and kWh (kilowatt hours) are not compatible units, so cannot be compared.” ~ Wolfram Alpha

The power rating, kW, of a battery describes how fast it can release or absorb energy. It has nothing to do with time. It’s an instantaneous snapshot of the power.

1 kW is equal to around 1.34 horsepower for you motor enthusiasts out there.

1 kW over 1 hr = 1kWh. If you use 1 kW for 1 hour, you’ll use what’s known as 1 kWh.

So you’ve found out that your device uses 2 kW. If you use it for 8 hours, it will have used 16 kWh of electricity.

10 kW for 10 hours = 100 kWh.

So going the other way now, if you have a battery promising 200 kWh, and the light you plan to plug into it takes 2 kW, you will be able to power that light for 100 hours.

Common Household Appliance Power Usage

Common Household Appliances Power Usage | source: do-it-yourself-help.com

Get the idea?

Hope this helps!

refrences:

• DIY Electric Car forum
• Auto Blog | Difference between kWh and kW
• Do It Yourself Help | Calculating Circuit Load