Informational: EV units explained

[tsedge]

Hi, this came up in another discussion and may help people who are bit confused about all this talk about killer-wots and owwww-ers. The experienced will already know it.

Energy is measured in Joules (J). Power is measured in Joules-per-second (J/s), which is also known as Watts (W), a thousands watts is 1 kilowatt (kW).

If you run a 1kW appliance for an hour you'll be using 1 kilo-watt-hour (kWh) of energy, or 1000 J/s * 3600s = 3,600,000J.

Hence, with electric cars, the power of a charger is rated in kW and the capacity of the car is rated in kWh. A 50kW charger fully charging an empty 50kWh battery will take 1 hour (assuming no losses / slow downs). 50 kWh / 50kW = 1hr.

Range is simply capacity (energy stored, e.g. kWh) multiplied by distance-efficiency (distance per unit energy, e.g. miles-per-kWh or m/kWh), for example in the UK: 50kWh * 3.5 m/kWh = 175 miles.

However, efficiency can be measured in another way: kilo-watt-hours-per-100-km: kWh/100km. How many kWh does it take to drive 100km? Less is better. This is energy-efficiency-per-unit-distance.

So, on the continent, you calculate range by dividing battery energy capacity (kWh) by energy-efficiency and then multiplying by 100: (kWh / (kWh / 100km)) * 100, the kWh units cancel leaving the estimated range in km. For example, 50kWh / (15kWh / 100km) = 3.3 * 100 = 330km.

There are then more complicated usages that can get very confusing. Another way of rating charger speed is "how many miles of range will I get per hour of charging?", which is expressed as miles-per-hour, or mph (but not to be confused with speed, which is also measured in mph).

You can work out the miles-per-hour (charge rate) by taking the power of the charger and multiplying by the average distance-efficiency of your vehicle: kW * (m/kWh), which leaves you m/h or mph charge rate. For example, 50kW * 3.5 m / kWh = 175mph charge rate.

Hopefully this clears up any confusion.

 

[Gomev]

Perhaps Fast and Rapid chargers ought to be mentioned too as I see so many posts saying Fast when they mean Rapid.
Fast - like wallbox at home or destination chargers, the slow ones. (but not quite as slow as the 3 pin grannies....)
Rapid - like on motorways, the Instavolts and so on.......

It helps to mention the right type when reporting any problems or queries or even just a jolly good story.

 

[okohll]

The old water analogy is often used - e.g. skip down to 'Power and Energy' here:

Understanding the basics of electricity by thinking of it as water

A quick, visual overview of electricity... We cover the basics like voltage, current, resistance, AC, DC, power and energy, all using water as an analogy.

www.freeingenergy.com

where the battery is the equivalent of the bucket

 

[Grriff]

Perhaps Fast and Rapid chargers ought to be mentioned too as I see so many posts saying Fast when they mean Rapid.
Fast - like wallbox at home or destination chargers, the slow ones. (but not quite as slow as the 3 pin grannies....)
Rapid - like on motorways, the Instavolts and so on.......

It helps to mention the right type when reporting any problems or queries or even just a jolly good story.

I think charging is a little more complicated. If you are charging using AC, the charging speed depends on the electronics in your car which have to convert the electricity to DC. This is only 7kW for some MG4s, whatever the quoted speed of the charge point. Most AC chargers are slow, up to 11kW or fast, up to 50kW.
DC chargers are fast or ultrafast, up to 350kW. As with AC, maximum charging speed depends on the car but should be around 150kW for an MG4.

 

[Gomev]

I think charging is a little more complicated. If you are charging using AC, the charging speed depends on the electronics in your car which have to convert the electricity to DC. This is only 7kW for some MG4s, whatever the quoted speed of the charge point. Most AC chargers are slow, up to 11kW or fast, up to 50kW.
DC chargers are fast or ultrafast, up to 350kW. As with AC, maximum charging speed depends on the car but should be around 150kW for an MG4.

Which is precisely why it helps to mention the correct type of charging by definition (not speed), fast or rapid (or even ultra rapid which is still rapid) when talking about it, reporting problems, asking for advice or just telling a nice story.
Fast charging is defined as up to 22kW not 50kW and is AC as you rightly say. There is/was also 43kW AC charging (3 phase) but this is defined as rapid and I as far as I know cars capable of this were the early Zoes and maybe the Tesla Model S. I'm sure someone will know better.

 

[MoDolph]

Hi, this came up in another discussion and may help people who are bit confused about all this talk about killer-wots and owwww-ers. The experienced will already know it.....

Thanks for putting this together. It's something that really irritates me.

7kwh or 3.6 kw/h chargers and 64Kw batteries ?

Maybe somebody could refer the Audi advertising team to this thread:

If I was being pedantic I might point out that SI unit rules state that there should be a space between the numerical value and the unit symbol