Economics don't work out...

[Bob]

Please help me find the error in my math.

One hour of using my 120V Nissan EVSE gives me 4 miles of range
At 12A, that's 1.44kWh.
At $0.11 per kWh (national average), that's $0.158 for 4 miles, or $0.0396 per mile. (I'm actually paying $0.17/kWh)
Today's national average price for gas is $3.49 per gallon. (I'm actually paying $3.30/gal)
At that rate, I calculate 88.13 eMPG. (and 54 eMPG for me)

Yet I read of 109 to 129 eMPG.

Any thoughts?

Bob

 

[DNAinaGoodWay]

Not all of the electricity coming out of the wall makes it into the battery. You lose some in the charging process and running the cooling system. I think I've seen up to 15%. could be L1 is even less efficient.

 

[Randy]

No surprise. It all depends on what numbers are used for those calculations...I think all that really matters is that it is possible in most cases to drive cheaper than gasoline...Some people pay more and some people pay less...

The more miles you drive electric means the more you avoid paying for gasoline costs...

 

[fooljoe]

Bob, your math is sound, but you have 3 avenues for improving your eMPG:

1. You can drive more efficiently. Do you really get 4 miles/hour on L1 or is that a gross estimate? If you can bring it up to 5 miles/hour (about 4 miles/kwh at the dash - easily doable), then you'll be getting 104 eMPG.

2. You can use special EV rates / time-of-use metering to achieve a lower effective price per kwh. Investing in solar can make it even cheaper, especially when combined with time-of-use metering. Bringing the price down to, say, $.08/kwh would bring the eMPG to 143.

3. You can use L2 charging instead of L1. L2 is more efficient as constant loads like the fan are a smaller percentage of the overall draw, and the charger is "tuned" to work more efficiently at higher power. This might translate to about 5% more efficiency, which would bring the eMPG up to about 150.

Also, you should consider that it's probably reasonable to assume that gas prices will rise more quickly than electricity prices, and electricity prices can be "locked in" with solar. So even if the eMPG seems small now, it will probably grow higher in time.

 

[UkrainianKozak]

It all depends on how you drive and what you pay, you can't get the top results from hypermilers braggins who use solar to charge for free and apply it to regions with high electricity cost and low gasoline cost while doing aggressive driving and using max AC/heater at all times.

MPG ratings are actually gathered using standard EPA techniques. Those are standard.
You can't really compare MPGe and MPG using price of the gallon vs price of kw of electricity. Those are consumption rates, not price comparisons. You can compare standard/average energy usage, but those ratings are not taking in account the cost.

Here you can read bit about MPGe vs MPG stuff:
http://www.autoguide.com/auto-news/...ctric-car-fuel-economy-ratings-explained.html

You can go to EPA website to get more technicalities if you need to.

What I know is if you drive in the same way, in most cases electricity will be cheaper then gas, and electric motor is by far more efficient than ICE engine in converting energy into motion of the car.
By how much, it is really dependent on many factors.

 

[evnow]

Please help me find the error in my math.

One hour of using my 120V Nissan EVSE gives me 4 miles of range
At 12A, that's 1.44kWh.

4 miles for 1.44 kWh => 2.78 m/kWh at the wall or 3.27 m/kWh at the dash. That can certainly be improved upon. My life time avg is above 4 m/kWh.

 

[LEAFer]

Bob:

Your math is correct. And that's how anyone using "common sense" would do the calculation.

Notwithstanding all the good and valid comments already made by the others above ... I would like to add the following.

It's interesting that you should compute a unit you "created" called eMPG. Interestingly the "official" unit is MPGe.

The difference may have been intentional on your part, or accidental. In either case, here's one crucial difference. An "MPGe" is computed based on *NOT* the pricing of gasoline, but the energy equivalence of a gallon of gasoline. A U.S. gallon of gasoline is said to contain 33.41 kWh (and I have seen people use 34kWh).

For lack of time ... I'll let you re-do the math if you'd like. But that basically would explain the difference between you computation of 88 eMPG and Nissan's official (and I haven't verified it) 109 MPGe (not, as you stated "109 eMPG" ... your only "error" ...).

HTH.

 

[garsh]

I did a similar calculation for myself after buying the Leaf:

I plugged the included L1 charger into a killawatt meter, and saw that it was drawing about 1350 watts. My electric company charges me about 11¢ per kWh (distribution & transmission). According to the Leaf wikipedia page, the Leaf will gain about 5 miles for every hour that it charges via L1.

Putting that all together, it costs ($.11/kWh)*(1.35kWh/h)(1h/5miles) = $0.0297/mile -> 3¢ per mile.

My old car averaged 27mpg. Gas is about $4/gallon. That gives ($4/gallon)*(1gallon/27miles) = $0.1481 -> 15¢ per mile.

I used to spend ~$2000/year on gas.
Now I usually charge at work, at zero cost.

 

[fooljoe]

The difference may have been intentional on your part, or accidental. In either case, here's one crucial difference. An "MPGe" is computed based on *NOT* the pricing of gasoline, but the energy equivalence of a gallon of gasoline. A U.S. gallon of gasoline is said to contain 33.41 kWh (and I have seen people use 34kWh).

Whether or not it was intentional, he's right to focus on the miles per cost-of-a-gallon metric rather than the miles per energy-of-a-gallon metric. It's just a coincidence that the two metrics do give fairly similar results. $$$ is what matters to practical-minded consumers considering whether or not to buy an EV, not energy content of fuels.

 

[smkettner]

At $0.11 per kWh (national average), that's $0.158 for 4 miles, or $0.0396 per mile. (I'm actually paying $0.17/kWh)

I have virtually stopped charging except at super off peak (10 cents) or if provided free.
Out of power... I take the other car as the marginal rate is closer to 35 to 45 cents.
Makes the loss of range even more fustrating.

 

[planet4ever]

Please help me find the error in my math. One hour of using my 120V Nissan EVSE gives me 4 miles of range. At 12A, that's 1.44kWh.

4 miles for 1.44 kWh => 2.78 m/kWh at the wall or 3.27 m/kWh at the dash. That can certainly be improved upon. My life time avg is above 4 m/kWh.

Sorry, evnow, but you have an error in your assumptions. Your estimate of 3.27 m/kWh assumes 85% efficiency from wall to dash. That is about right for 240v charging, but the efficiency is approximately 75% with 120v charging. So 4 miles per 120v charging hour really corresponds to about 3.7 m/kWh at the dash.

I agree he can improve on that, but I do take issue with people who claim we can "easily" get 5 miles per 120v charging hour. That would correspond to an average dash reading of about 4.6 m/kWh. There are some who maintain that high an average, but not many, at least not many who spend a significant fraction of their time on freeways.

Ray

 

[Epi117]

My May numbers were:

1,169.6 miles
Avg 5.2 miles per Kw
Cost of Kw in Oregon 14 cent

total cost for month $31.22

would have cost me +$80 in gas in my Prius

 

[fooljoe]

I agree he can improve on that, but I do take issue with people who claim we can "easily" get 5 miles per 120v charging hour. That would correspond to an average dash reading of about 4.6 m/kWh. There are some who maintain that high an average, but not many, at least not many who spend a significant fraction of their time on freeways.

Good point Ray. I knew that L1 efficiency was less, but didn't know how much less. I agree 4.6 at the dash is not easy (although I've done 4.7 in 2 years almost exclusively on the freeway). But FWIW, since I only estimated 5% better efficiency going from L1 to L2 in my item #3, I think my final "eMPG" result is still quite attainable.

 

[SanDust]

Please help me find the error in my math.

The problem isn't the math it's that can't use your efficiency and your prices sometimes and then national averages and EPA MPG at other times. Stick with the official numbers. The EPA sticker says the Leaf uses 34 kWh/100 miles. That's from the wall BTW. At $.11/kWh that means it costs $3.74 to go 100 miles. That's the cost equivalent of getting slightly more than an EPA rated 93 MPG.

Note that the 34 kWh/100 miles is using 120v charging. If you use 240v charging you can apparently increase the efficiency or decrease the number of kWh needed per 100 miles. (SAE J1634 says "normal AC source").

Your 88 MPG is close. Could be off because you drive more aggressively or because the charging draw isn't 1.44 kW.

 

[LEAFer]

The difference may have been intentional on your part, or accidental. In either case, here's one crucial difference. An "MPGe" is computed based on *NOT* the pricing of gasoline, but the energy equivalence of a gallon of gasoline. A U.S. gallon of gasoline is said to contain 33.41 kWh (and I have seen people use 34kWh).

Whether or not it was intentional, he's right to focus on the miles per cost-of-a-gallon metric rather than the miles per energy-of-a-gallon metric. It's just a coincidence that the two metrics do give fairly similar results. $$$ is what matters to practical-minded consumers considering whether or not to buy an EV, not energy content of fuels.

I agree. (Notice what I said in my first paragraph, 2nd sentence.) However, I wanted to point out that the government's way of calculating MPGe doesn't (unfortunately?) work that way, and that comparing the "common sense" method to the government's method is ... well ... non-sensical. (His method was -- eerily -- ?? -- eMPG rather than the standard MPGe ...)

'nuff said. Or ... like I say to Joe ICE Public: "If you give me $3, I can go 100 miles. If I give you $3, what can you do with it ?" ( I should TradeMark this already )

 

[bytrain]

Hi Bob,

My question concerns how accurate (to how many significant figures) is the range you receive per hour of charging. Is it 4.00 or 4.25 rounded to 4.00? 4.00 seems really low to me because I get closer to 5 miles of range per hour. At 5, that would be ~110 eMPG.

Please help me find the error in my math.

One hour of using my 120V Nissan EVSE gives me 4 miles of range
At 12A, that's 1.44kWh.
At $0.11 per kWh (national average), that's $0.158 for 4 miles, or $0.0396 per mile. (I'm actually paying $0.17/kWh)
Today's national average price for gas is $3.49 per gallon. (I'm actually paying $3.30/gal)
At that rate, I calculate 88.13 eMPG. (and 54 eMPG for me)

Yet I read of 109 to 129 eMPG.

Any thoughts?

Bob

 

[DaveEV]

I have virtually stopped charging except at super off peak (10 cents) or if provided free.
Out of power... I take the other car as the marginal rate is closer to 35 to 45 cents.

Why would you do that? I get about 3.8 mi/kWh from the wall on average. But my worst month was 3.4 mi/kWh, so let's use that. That would come out to about 4.0 mi/kWh on the dash which is easily achieved even with freeway driving unless you speed.

At night time rate of $0.14/kWh, that's about 4.1c / mi. At 35-45 c/kWh thats about 10-13c / mi.

Right now regular gas is $4.00 / gallon. To match 4.1c / mi, I'd need a car that can get 97 mpg. Not going to happen. Now 13c/mi I'd only need a car that gets 31 mpg which isn't hard to do in just about any 4 cyl car. But you'd really need a Prius/Insight/HCH to beat 31 mpg by any significant degree.

But for occasional trips - who really cares? At most you might do an out and back trip of let's say 70 miles for 140 miles total. The miles on the way out you'd do at 4.1c / mi. Say you have to pay $0.50 / kWh or 15c/mi to charge back up for the 70 miles home - now you've spent $2.87 getting there and $10.50 getting home or $13.37 total for an average cost of 9.5 c / mi. Now you're cost / mile is basically the same as a Prius.

None of that takes into account the other benefits of driving an EV.

I'd rather drive the LEAF! (tm)

 

[smkettner]

I have virtually stopped charging except at super off peak (10 cents) or if provided free.
Out of power... I take the other car as the marginal rate is closer to 35 to 45 cents.

Why would you do that?

Chart does not even go 35 cents much less 45 cents.
Is the chart wrong? Even if LEAF goes twice as far on "gas equivalent" we are well over $10 per gallon.

Would you pay $15 per gallon to get 100 mpg in your other vehicle?

 

[DaveEV]

Is the chart wrong?

Short answer: YES

Long answer: Read my previous post for a purely economical analysis. From an energy purchased point of view - yes, as the chart states - $4.00 / gal gasoline = $0.12 / kWh electricity. But your typical EV is way more than even twice as efficient as your gas vehicle. It's more like 3-4 times as efficient. 70-80% of the energy you buy in a gallon of gas is wasted - only 20-30% goes towards moving the vehicle.

 

[lkkms2]

Don't forget if you buy gasoline we are still importing about 40% from overseas. So if you buy a gallon, or spend $1000, $2000, or $3000 dollars a year on gasoline, after refining expense are considered, you are sending about 30% of your dollars overseas.

You (and everyone else) end up sending $300, $600, or $900 a year overseas away from our economy.

Using electricity, all your fueling dollars stay here and are circulated back into the economy.

That is my cleaner economic choice .