As many of us take the plunge into the joys of owning an electric vehicle we’re exposed to some terminology and measurement units that we may not be very familiar with. What is a kilowatt hour? How does it differ from a kilowatt? Why do I care?
First, let me say that some of this may be specific to the US in terms of the units we see on bills, on our dash, and on our bills. Most drivers in the US have their units set to miles vs km and so far we’re all still paying in US dollars. I’m also over simplifying all the science in this to get to the basics so please cut me some slack if you already know all this!
We all should probably already be familiar with kilowatts (kW) and kilowatt hours (kWh) because our electricity bills are reported in kWh. Your EV is using units that have been on your electric bills for years so this terminology and unit of measurement is not new to us but may be something we haven’t paid much attention to in the past.
Depending on the EV display you may see Watt hours (Wh) or kilowatt hours (kWh) in some places and Watts (W) and kilowatts (kW) in others. The kilo or k is a standard prefix meaning a thousand. So 1 kWh is 1,000 Wh. If you own your EV long enough you may just get to the next level, megawatt hour (MWh) which would be a million Wh!
Now for the fundamental definitions:
kW is a measurement of power and kWh is a measurement of energy.
Energy is about how much fuel is contained in or used by something over a period of time. kWh, calories, Joules and other things like that are all units of energy and you can convert between different units. A slice of pizza has 285 calories which is 0.33 Wh of energy. Energy can be converted and change form. We can convert that slice of pizza to heat by burning it. The fuel is the pizza, but don’t try converting it in your EV!
Power is the rate at which energy is generated or used. kW is a unit of power. When you accelerate you’re using power and when you decelerate with regenerative braking you’re generating power. The Model S dedicates half the speedometer display to the unit of power on the right side. There you can see how many kW you are using (indicator is orange) or generating (indicator is green) at any instant in time. This is good to see, but you can’t easily convert this into cost — for that you need to measure it over time to convert it into a unit of energy.
Power is similar to your speed. 50Mph is your speed, but you have to maintain that for an hour to go 50 miles. Similarly, 40kW is how much power you’re using but you have to do that for an hour to use 40kWh. If you spend half that hour at 40kW and the other half at 20kW you’ll end up using 30kWh. Power usage is constantly changing and will also include things like your heater or A/C use.
Energy is power over a unit of time.
As another example, a 100W (100W of power) bulb used over 1 hour is 100Wh of energy used. If you use that 100W bulb for 8 hours every day, it will consume 0.8 kWh per day. After 30 days, it will have consumed 0.8kWh x 30 = 24 kWh. After 365 days it will have consumed 292 kWh. Your EV is the similar but it can both use and generate power over periods of time. The difference or net power used (used – generated) is what you see reported on your EV displays.
When charging your EV you’re loading energy back into your battery so you’re storing kWh for later use. EVs report charging in different ways. The natural way to report charging is kW and kWh added. So a charge rate of 6 kW is storing 6kWh up for every hour of charge. If you’re adding at 6kWh/hour and charge for 2 hours you’ll have an extra 12kWh added at the end of your charge.
While we like to think in terms of miles, not all miles are the same and there are hills and weather and other factors that vary for each mile. A kWh stored is always the same — it comes down to how you use it. This is why the purists show and measure kW and kWh for charge rate.
Purists show and measure kWh for charge rate.
The Model S offers the kW and kWh display option, but many owners choose to display charge rate in terms of miles of range added per hour, i.e. 16 mi/hr. There’s an assumption being made here about how many miles you can drive on a Wh and that assumption needs to account for charging efficiency. Tesla uses different math for this conversion depending on the situation.
Tesla varies charging efficiency assumptions in different situations.
In Tesla’s online calculator they assume 300Wh/mile average use and a 90% charging efficiency. My own measurements show the average Wh/mile usage to be bit higher (306 lifetime average) and the charging efficiency to be a bit less (81% last month).
Now you may be wondering how all this relates to volts and amps. This gets us back to the basics. Given volts and amps you can calculate Watts by multiplying the two together. W = V x A. So if you’re at a public charger and its charging at 199V and 30A like the picture above, you’re charging at 199V x 30A = 5,970W or about 6kW. That doesnt mean you’ll have 6kWh added after an hour of charging as charging efficiency needs to be factored in.
The Model S is reporting this as a rate of 16 mi/hr. Lets check that math: 5,970W/300Wh/mile standard assumption = theoretical 20 miles/hr charge rate. But that doesn’t account for charging efficiency. The Model S is reporting 16 mi/hr so its assuming an 80% charging efficiency (16/20) under these conditions.
On your electric bill, costs are expressed in price per kWh. So $/kWh. Your electric company may break it down by distribution vs generation, time of use, etc. and then have a different cost per kWh on each and the pricing seems complex but you can simplify this.
To figure out your total cost per kWh just take your total amount for the bill and divide by your total usage for the same period. That does include the various service fees, taxes, etc. but in the end you’re paying the electric company that total amount for those kWh you used however they’re adding it up. This average rate varies greatly throughout the country and world.
When you have those averages you can look at your kWh used on your EV and calculate the costs for your trips etc. But beware of those charging efficiency losses.
EVs display all the usual numbers you’re used to like miles travelled, speed you’re traveling, etc. But they also report how much power and energy they’re using. Power is like your speed and energy is how fast you’re using or regenerating power. Added to the complexity are the various differences in charging stations in volts and amps provided and then the conversion factors and efficiency losses in converting back into power and energy.
While there are a number of terms and a few simple formulas to get used to, the average EV owner often ends up better informed and more aware of the actual costs of driving and energy usage than many drivers of gas powered cars. Thanks to the extra information, EV drivers often drive more efficiently than drivers of ICE cars — even in powerful cars like the Model S.
EV drivers often drive more efficiently than drivers of ICE cars
Brian H said:
A Watt is capitalized because it’s named after a person. The same should be applied to Volts, Amperes, Joules, Coulombs, Ohms, and so on.