Pop Quiz – what’s the difference between a kilowatt (kW) and a kilowatt hour (kWh)? I’m not sure many people know. The term kW is a measure of power, like the horsepower of a gas engine, while kWh is a measure of energy, like the energy potential in a gallon of gas. I read a professionally produced, glossy magazine about electric cars. It compared the latest electric vehicles (EVs) on the market and did a beautiful job. Yet, several times, it mixed up the terms kW and kWh. The still unfamiliar kW and kWh will replace familiar terms like miles per gallon, horsepower, and fuel tank capacity. We will have to get used to them.
What two questions would a prospective EV buyer ask?
- How far can I go?
- How fast can I charge?
How Far Can I Go?
Range depends mainly on the capacity of the battery in kWhs. EV weight, motor power, efficiency, and many other factors also matter. On average, a kWh runs a standard EV vehicle about 3 miles. So if the battery has the capacity of 100kWh, then the range is about 300 miles.
How Fast Can I Charge?
It’s simple math. To calculate how fast, take the battery capacity in kWhs divided by the power supplied to the battery in kW. So for our example above, with a 100kWh battery and power supplied by a 350kW commercial fast charging station, it takes 100 divided by 350 or about 17 minutes. Not bad. This is an oversimplification since fast charging isn’t linear. DC fast charging is not performed at a constant rate from 0 to 100% because EV battery management systems don’t allow it. It is not good for the battery’s health. Here is more precise information on fast charging.
If I use a home charging station that supplies 2kW, it takes 50 hours.
To fully get at the answer, it’s important to examine the entire charger system, which consists of three components:
- Charging station
- Onboard charger (OBC)
- Battery and Its Management System
Charging Station
The charging station connects the EV to the source of power. There are three types:
- Very slow, Level 1 (L1), powered by ordinary 120-volt circuits. Power is limited to about 2kW.
- Slow, Level 2 (L2), powered by heavy-duty 240-volt circuits like those used for electric clothes dryers. Power is limited to about 10kW or less.
- Fast or Level 3 (l3) are special commercial stations powered by direct current (DC). Modern fast chargers can supply up to 350kW of power per station. That limit will likely grow. However, the power available from a fast charging station can vary widely. That power also depends on how many EVs are charging simultaneously.
The more power in kW delivered to the EV, the faster the charge.
Onboard Chargers (OBC)
The next limitation on the power in kW to the EV battery is the OBC.
The OBC is the brain of the EV. It converts the AC to DC, performs safety checks, communicates with the charging station and the battery management system, and delivers the power to the battery. OBCs ratings can range from 3kW to as high as 22kW. Nearly every EV sold in the US has an OBC rating limited to 11.5kW.
The OBC bypasses AC to DC conversion when EV owners charge from a fast charger station. Thus the above limits don’t apply. This bypass is why fast chargers are fast. It’s the equivalent of intravenous medicine versus swallowing a pill.
Battery and Its Management System
Even though a fast charger can supply 350kW, most EV battery management systems limit the amount and rate of power they can accept. Some small EVs limit the power supplied to as low as 50kW regardless of the charger station’s power capacity.
So how fast to charge? It depends on the power in kW from the charging station, the OBC limitation, the battery and its management system.
Why This Matters to Utilities
As the number of commercial charging stations increase, utilities must keep up with the demand. For example, a charging center might have ten fast charging stations, each with a rating of 350kW. That’s a total demand of 3500 kW.
Assuming each new EV draws, on average, about 10kW for extended periods for L2 residential charging, utilities may have serious issues with local low-voltage networks and their distribution transformers. Five to eight EVs simultaneously drawing 50 to 80kW could seriously overload utility equipment.
The key for electric utilities is to model the electric network completely, from generation to consumption, including all distributed energy systems. Unfortunately, utilities have historically minimized detailed modeling of low voltage systems where the EVs connect. GIS can help. ArcGIS Utility Network is not simply a mapping system. It is also an advanced network modeling system.
In addition, utilities need a solid understanding of the transition from gas cars to EVs. ArcGIS can overlay the network model with a demographic model to help planners decide where and when the number of EVs will grow. That will help them plan for the power (kW) and the energy (kWh). Then, they can meet the challenge with the proper tools in place.
ArcGIS promotes collaboration around these concerns.
SIDEBAR
Why is the k in kW lower case while the W is upper case? Blame it on the metric system (the International System of Units (SI)). Prefixes in SI stand for a number. The k stands for kilo or a thousand. The SI rule states that the letter is lowercase for values less than a million. Why is Watt capitalized when meter is not? The rule is that if the term is based on a person, the abbreviation is upper case. Since the terms hour, gram, and meter are not based on people, the abbreviation is a lowercase letter. Thus kWh is the proper abbreviation for kilowatt hour.