What Watts? EV Charge Rate Explained

When you drive an old-fashioned gasoline powered vehicle, you know how many gallons your tank holds and what your typical miles per gallon (MPG) is in town or on the highway, so it’s easy to plan for how much gas you’ll need based on where you want to go. It’s also easy to know that when you pay for 14 gallons at the pump, that you put 14 gallons in the tank. It’s tempting to try to translate that “MPG” model directly to EVs, but refueling your EV is bit more complicated. With an EV, you’re filling up with Watts (Watt-hrs to be precise!) instead of gas or diesel. We’ve all seen the social posts wondering why the public charging station charged for xxx watts, but the EV says it got xxx watts. Let’s take a closer look at where all those watts actually go.

First, it’s important to ground on a few facts about EV charging:

1. No EV charging is 100% efficient. Not plug-in charging, not wireless charging, not DC Fast Charging (DCFC), not AC home charging. Regardless of how or where you charge, not all the electrical energy pulled from the grid – those watts – will make it into the “fuel tank” (battery).

2. Electric vehicle batteries store and produce DC electricity, not AC electricity. DC (direct current) and AC (alternating current) are different types of voltage, or current, used for the conduction and transmission of electrical energy. This is important because EV batteries are charged with DC power. Since Thomas Edison’s time, most electricity transmission to the point of use is AC power. That means somewhere in the process of moving watts from the electric grid to the EV battery, that AC power must be converted into DC power. (More info here)

3. Every EV has an on-board charger (OBC) that receives AC power from the Level 1 or Level 2 charging cord and converts it to DC power so the battery can accept it. (For those new to EVs, the Level 1 charger is generally what comes with your EV and plugs into a normal household outlet. A Level 2 charger charges more quickly and plugs into a 240-volt outlet, like an electric clothes dryer would. (More info here)

4. OBCs are not 100% efficient at converting AC to DC, as anywhere from 5%-15% of the power from the grid ends up as heat during the AC to DC conversion process.

5. DC fast chargers do the conversion from AC grid power to DC charging power off the vehicle, in the charger itself. (That’s part of the reason that they are so expensive, but also why they can charge a battery more quickly.) They lose about 5%-10% of the power from the grid to heat during the AC to DC conversion process.

6. Every vehicle has an appetite for energy to monitor and control the charging process and to condition the battery. Not all the electric power from the charger goes into the battery storage for driving.

7. A cold battery will not perform as well as a warm battery, impacting acceleration, driving range, regenerative braking, and the ability to safely accept DC fast charging.

All of this matters because the speed of charging – regardless of whether the EV is being charged wirelessly or through a plug – depends on where the AC to DC conversion happens, how much power the car WANTS to draw, and WHERE the car wants to send that power. We’ve already explained that offloading the conversion of AC to DC to an off-board charger, rather than in the car, is one reason DCFC is faster. But beyond that, if the car is cold, a lot of energy may go toward warming the battery rather than filling it. If the battery is already warm from driving or because it’s really hot outside, some energy may go toward cooling the battery rather than filling it. (Yes, it is a little bit like Goldilocks.)

But why does the battery need to be warm? The rate at which energy can be pulled from the battery depends on its temperature, and a warm battery can provide more power than a cold one. A cold battery impacts acceleration, regenerative braking performance, and driving range. Let’s go back to the DCFC: a warm battery is particularly important if you’re going to use a DCFC to charge a car since the battery temperature should be at least 50 degrees Fahrenheit (10˚ C) to avoid permanent damage to the battery. In fact, when you connect a DCFC cable, the vehicle will first check the battery temperature and will divert charge power to battery heaters to get the battery to a suitable temperature to begin fast charging. The same will happen if your vehicle has been sitting in a cold garage. Your car might even start to heat up the battery if you use the nav system to find the nearest DC fast charger!

With all this in mind, the best way to think about charge rate is that it’s really what’s left over for driving usage AFTER your car has used what it needs to simply charge. And this varies based on type of vehicle, temperature, and other factors. This is why car companies suggest that you keep the car plugged in as much as possible so you’re not using battery power for auxiliary operations like battery conditioning or cabin pre-conditioning for comfort.

As a result, you have to remember to plug and unplug even if the battery is well charged. With wireless charging, you can park and walk away—and know that your vehicle will use grid power for its own energy appetite—so your battery is fully charged to satisfy your driving appetite!

So, as the car commercials used to say, “your mileage may vary.” EV charging rates vary, too.