Why Wh/mi?

[PD46]

To me it seems counter intuitive to express efficiency in terms of a number that you want to minimize. The ICE equivalent is miles per gallon, so why not adopt the inverse of Wh/mi?

As the capacity (energy) of our batteries is expressed in KwH, why not state efficiency figures in terms of something to be maximized like miles/KwH?

Cost figures are also then computed nearly identically with ICE vehicles (Currency/Gallon) / (Miles/Gallon) = Cost Per Mile = (Currency/KwH) / (Miles/KwH)

 

[cwerdna]

Many other US market BEVs (all 3 models I’ve had) go by miles per kWh.

 

[LoudMusic]

Kilometers / kWh are common in Europe as well. I often do that math on the fly in my head.

About 50 miles per hour uses about 250Wh/mile which is 4miles/kWh.

About 75 miles per hour uses about 330Wh/mile which is 3miles/kWh.

It would be nice as an option to view consumption that way on the car's display.

I think for marketing purposes they might have wanted to stay away from such tiny numbers. Considering that basically no one understands what a kWh is, saying the car gets 4 miles to the kWh is a tough comparison to a car that gets 35 miles to a gallon. So just make the number a completely different measurement and you don't have that problem.

 

[jjrandorin]

I think for marketing purposes they might have wanted to stay away from such tiny numbers. Considering that basically no one understands what a kWh is, saying the car gets 4 miles to the kWh is a tough comparison to a car that gets 35 miles to a gallon. So just make the number a completely different measurement and you don't have that problem.

This has always been my impression as well. Explaining to someone why 4 miles / kWh is better than 30MPH would be an extreme frustration, and most regular people wont even do it, they will just see "tiny number... bad".

So.... "large number.... good", since it will at least make people ask "WTF is that large number?"... where if the number was 3.5 Miles / kWh they are not even gonna ask, because... "its single digits, it MUST suck, I was stupid for even thinking about buying an EV"...

 

[beatle]

The "resolution" of wh/mi is higher than mi/kwh, at least unless the manufacturer uses several digits after the decimal. I know Ford only used one digit after the decimal. That's probably "good enough" but not for data nerds.

 

[Rocky_H]

The ICE equivalent is miles per gallon,

Not always. You are generalizing that to all gas cars, but that is kind of a United States-centric view. In Europe, they commonly use a consumption figure of liters per 100 kilometers. So that is a consumption figure just like the wh / mile.

 

[LoudMusic]

Not always. You are generalizing that to all gas cars, but that is kind of a United States-centric view. In Europe, they commonly use a consumption figure of liters per 100 kilometers. So that is a consumption figure just like the wh / mile.

Which I find hilarious because we're essentially swapping consumption figures with them.

 

[Rocky_H]

And more practically, I think it's for resolution and usability. People's brains just kind of shut off if they encounter decimal points and strings of numbers after that, so they subconsciously truncate them off, and they disappear.

So people are going to say their car gets about 4 miles per kWh or 3 miles per kWh, but if you pressed them, they probably couldn't remember much more detail than that. But with the consumption figure, they will usually be able to remember numbers like 250, 280, 310, or 350. Those have more resolution and are more informative, but people may still remember them and use them.

 

[zroger73]

Adding to the complexity is where the power is being measured.

For example, my vehicle shows 271 Wh/mi over its lifetime, but the app shows 2,600 kWh/8,384 mi which is 322 Wh/mi. The 271 Wh/mi doesn't account for charging losses or power used to heat or cool the cabin, for example. This makes the Wh/mi figure displayed by the vehicle look better than it really is.

When you calculate fuel economy for a gasoline-powered vehicle, you don't subtract the gas used to run the A/C.

 

[maximizese]

I would think mathematically inclined people see it both ways and aren't hindered by looking at figures through the inverse. My Fiat 500e gets about 4.5mi/kWh; our classic S85 uses about 305Wh/mi. In writing this, I suppose the inverse of "gets" is "uses" when talking about consumption (which again, I suppose the inverse of consumption is production or generation).

Here's another weird one I use for my non-EV friends when talking about efficiency comparisons:

One hour usage of my house's central A/C unit is the equivalent energy consumption to go about 24.3 miles in my Fiat, or about 17.7 miles in the Tesla. That's an even weirder way to rate EV efficiency but it's fun for me.

 

[dwbv]

Here's another weird one I use for my non-EV friends when talking about efficiency comparisons:

One hour usage of my house's central A/C unit is the equivalent energy consumption to go about 24.3 miles in my Fiat, or about 17.7 miles in the Tesla. That's an even weirder way to rate EV efficiency but it's fun for me.

Then you follow up and say "sometimes I turn off my A/C at home for an hour so I can go joyriding for 24.3 miles and break even".

Units of measure still net the same output at the end of the day. It's what makes most sense for your target audience. If we lose weight after months of exercising, it's still the same amount of weight loss whether we measure in ounces, kilograms, or pounds.

 

[beatle]

I always seem to lose weight in ounces but gain it in pounds.

 

[bfayer62]

In the US the EPA requires manufactures to report in Wh/Mi in the certification documents. The EPA coverts this to KW/100 Miles (rounded) for their public documents.

Tesla could display it in some other form, but the choose to use the same format the EPA requires.

Example below.

 

[Midpack]

In my first month, so preliminary, I'm at 3.48 miles/kWh. That's a little lower than I expected, but I've had sentry mode on whenever the car is parked away from home, and I know that uses some battery.

 

[zroger73]

My '23 MY has 12,174 miles. The app reports 3,762 kWh. That's 3.24 mi/kWh.

• Sentry Mode rarely used - the car spends most of its time asleep at home and work
• The car has yet to sit in temperatures below about 50°F
• Chill Mode used almost exclusively
• Mix of city and highway driving - speed rarely exceed 60-65 MPH on the highway

3.48 mi/kWh seems "high" to me. How did you arrive at that number?

 

[maximizese]

In my first month, so preliminary, I'm at 3.48 miles/kWh. That's a little lower than I expected, but I've had sentry mode on whenever the car is parked away from home, and I know that uses some battery.

Are you making a lot of short trips (under 10 miles)? Like an ICE car, there's a good amount of energy used to warm up the coolant, battery, DU, never mind the cabin if you like to stay comfy. You'll notice higher energy consumption on the "Trip" display for first few miles when the car is not warmed up.

 

[Midpack]

My '23 MY has 12,174 miles. The app reports 3,762 kWh. That's 3.24 mi/kWh.

• Sentry Mode rarely used - the car spends most of its time asleep at home and work
• The car has yet to sit in temperatures below about 50°F
• Chill Mode used almost exclusively
• Mix of city and highway driving - speed rarely exceed 60-65 MPH on the highway

3.48 mi/kWh seems "high" to me. How did you arrive at that number?

Same way you did... miles / kWh. But I have less miles than you, that's why I said "preliminary."

I just did a search and found the MYSR can get up to 4.0, the MYLR up to 3.8 - both IF you do everything possible to maximize (I haven't, e.g. I've had sentry mode on parked away from home at all times).

 

[zroger73]

Same way you did... miles / kWh. But I have less miles than you, that's why I said "preliminary."

I just did a search and found the MYSR can get up to 4.0, the MYLR up to 3.8 - both IF you do everything possible to maximize (I haven't, e.g. I've had sentry mode on parked away from home at all times).

Oh, okay - the reason I asked is because if one takes the kWh figure reported in the vehicle (which is lower than the one reported in the app which is lower still than the actual amount of power being metered and purchased to charge the vehicle), the vehicle will appear to be more efficient than it really is.

For example, my vehicle reports 3,269 kWh used over 12,174 miles - that's 3.72 mi/kWh.

The app reports 3,762 kWh used over 12,174 miles - that's 3.24 mi/kWh.

The Wall Connector reports even higher usage. For example, the app shows 20 kWh on December 14, but the Wall Connector measured 21.2 kWh. On October 26, the apps shows 23 kWh, but the Wall Connector measured 25 kWh. This about 7% higher than what the app reports, which would be about 4,042 kWh - that's 3.01 mi/kWh. See how these efficiency numbers keep dwindling as the accuracy of the information improves?

Lastly, even the figure reported by the Wall Connector doesn't account for the losses between the metering point and the Wall Connector itself. For me, that loss should be minimal since there is only a few feet of wire between my meter and the Wall Connector, but can be very high. There's a NEMA 14-50 receptacle at a local park that I charge at that is a few hundred feet from the meter. At 40 amps, the voltage drops from 240 to 220 - that's another 8% loss (800 watts being lost as heat in the wiring between the metering point and my vehicle). Even though the electricity company is billing for that 800 watts, it isn't going into my vehicle. So, the real efficiency is closer to 2.76 mi/kWh when you account for all the losses.

To recap:

3.72 mi/kWh based on the vehicle
3.24 mi/kWh based on the app
3.01 mi/kWh based on the Wall Connector
2.76 mi/kWh based on the actual amount of power metered by the electricity provider

 

[dsm363]

kWh/100 miles would be better. As would gallons/100 miles for gas cars. Comparing 7mpg to 8mpg doesn't seem like a big jump but it's a bigger percentage than 35mpg to 38mpg.

 

[fhteagle]

My OpenEVSE claims ~2200kWh into the Model Y at home in 11 months, Tesla says ~1900kWh at home in the same time period, or a ~17% discrepancy. Some of that is charging inefficiency, OpenEVSE metering might be a bit off, etc.

Add Supercharging and away from home L2, and for car in motion I'm at ~280Wh/mi , overall ~321Wh/mi including energy used up in charging inefficiency and preconditioning. Cold mountain climate here, so preconditioning is significant.