KW and kWh explained

[Fluffykins]

Saw this today. Quite cute.

 

[Deleted member 6627]

Never has made sense to me. Speed is related to time but not in the case of kWh.... other than the fact that it is.... but it's not. It's like somebody got it wrong many years ago and refused to accept they got it wrong..... so much so that it stuck.

 

[Alb]

Not a good analogy to me.
Speed equates to Amperes.

 

[Gomev]

I think it can be confusing because both abbreviations use kW.
breaking it down and removing the 'k' (x1000) can make it simpler for the less well acquainted with the terms.
Watt is just power, similar to the brake horses in an ICE, Watt hours is how fast that power is consumed. E.G a 60 Wh battery will be totally emptied by using 60 (W)atts of power for an (h)our.

Some expert will now tell me I'm completely, or bitly wrong

 

[tsedge]

Saw this today. Quite cute.

View attachment 12410​

Love it. And if you don't get it, drink another glass and repeat...! ?

 

[tsedge]

I think it can be confusing because both abbreviations use kW.
breaking it down and removing the 'k' (x1000) can make it simpler for the less well acquainted with the terms.
Watt is just power, similar to the brake horses in an ICE, Watt hours is how fast that power is consumed. E.G| a 60 Wh battery will be totally emptied by using 60 (W)atts of power for an (h)our.

Some expert will now tell me I'm completely, or bitly wrong

You are quite right.

Energy is measured in Joules (J). Power in Joules per second (J/s). Watt is the Si unit for power and equals Joules per second. So to get back from power in Watts to total energy, you have to multiply by time, hence Watt hours (Wh) and kWh.

 

[Vipar]

ALL bloody Chinese to me?. Just need to work out mpg.

118 mile cost me £1.86 to charge back to 80%
What mpg have I got?

 

[tsedge]

ALL bloody Chinese to me?. Just need to work out mpg.

118 mile cost me £1.86 to charge back to 80%
What mpg have I got?

It cost you 1.5p per mile. Typical ICE car cost would be 15-20p per mile.

£1.86 buys you a litre of diesel or 0.22 gallons. So you got roughly 532mpg.

 

[Vipar]

Surely that must be wrong or it would be in the Guinness book of records. I must have got my numbers wrong ?.
I’m going to do a trip of about 120 miles tomorrow so I will write the numbers down and repost the question.

 

[Coulomb]

Watt hours is how fast that power is consumed. E.G| a 60 Wh battery will be totally emptied by using 60 (W)atts of power for an (h)our.

Some expert will now tell me I'm completely, or bitly wrong

You did indeed get it partly wrong.

Watt•hours (and more conveniently for cars, kWh) are the unit for energy, nothing to do with how fast that the energy is used or replaced.

Yes, a 60 Wh battery will be emptied by a 60 W load for an hour. So it's the power (in this case 60 W) that sets the speed of discharge.

Edit: the spot ("·") represents multiplication. That's less common than / for division (e.g. mi/h, miles per hour), but there are plenty of others. For example torque, whose metric units are Nm (Newton·metres). You can double the torque by doubling the force (more newtons), or by doubling the distance at which the force acts (more metres). Same with a battery. A battery with twice the capacity can deliver twice the power for the same time (more kW), or the same power for twice the time (more hours). Hence the unit for battery capacity is the kWh. As mentioned above, the "k" (kilo, lower case k) is just a 1000x multiplier.

So kW/h (kilo-watts per hour) is wrong, unless you have say a charger that very slowly changes its charge rate over some hours.

 

[JimD]

Think of it like a tea urn. Kwh is the amount of water in the urn, kw is the rate at which the water flows out of the tap.

 

[Deleted member 7316]

W is instantaneous power and has no time component. Wh is that power delivered for a period of time, the units say it all.

A 1 kW bar fire feels hot as soon as you walk up to it, once you are warm or even too hot then you have experienced the effect of kWh

 

[Fluffykins]

Not a good analogy to me.
Speed equates to Amperes.

Yup.

And Amperes at constant Volts gives Watts of course (Volts X Amps = Watts)

So yer 350kW charge point can fill a suitably capable battery more than twice as quick as a 150kW charge point.

350kW going into a big, big battery for an hour would pump in 350kWh (simplistically, of course)

Still simplistically, using a 150kW charger it would take 2 hours 20 minutes to push in 350kWh

 

[Vipar]

I drove 150 miles and to bring battery back to start 80% cost £2.01
What was my mpg?

 

[tsedge]

To convert to mpg, we have to assume a price for petrol or diesel for the comparison.

Let’s assume £1.80/litre. That means you paid the equivalent of 2.01/1.80 = 1.12 litres of fuel. That is 1.12*0.22 = 0.25 gallons (a quarter of a gallon).

For that “fuel” you went 150 miles so you achieved 150/0.25 = 600mpg.

 

[tsedge]

The normal economy figure to quote for an EV, however, would be miles per kWh, which you can get from your trip meter display. People are typically getting 3.5-4 miles per kWh with their MG4s.

 

[Fluffykins]

Then to chuck a curved ball across, there's mpge

https://www.kbb.com/car-advice/what-is-mpge/

- And remember that's US gallons. 1 imperial gallon = a tad over 1.2 US gallons so the mpge calculated against the formula on that website above would be worse by a factor of 1.2 if you are using Imperial gallon quantities. (Same miles but bigger gallons = fewer mpg)

 

[Vipar]

I got 3.5 mpk