Help me with the math

Someone help me with my calculations.

I usually spend at least 3 minutes at the gas station when filling my Prius. The gas station pumps are rated at 10 gallons. per minute, so that means I must be using 30 gallons (yeah ri---ght I know, but using the OP's logic ok). Now I travel 500 miles on that gas so when I do the maths my poor Prius is only getting about 16 MPG.

 

perhaps someone with a leaf or volt will jump in here with some actual user experience.

 

Hi Judgless, the figures I've seen are less than half of what you calculated, about 25kW-hrs energy (total including losses) to fully charge the battery.

The problem with your calculations is that batteries don't generally charge at a constant rate. They charge at the higher current early in the charging cycle, but as the state of charge increases they have to slow down to avoid damaging the battery. They will also charge at a higher rate if you only connect to the charger for a short time (as in only doing a partial charge). This is because during charging the battery may heat up and then the charger reduces the current flow to prevent the temperature rising further.

Anyway, as a previous poster mentioned, I know your calculations are way out (about double) but your basic premise is correct that electric cars aren't necessarily as low cost to run as some people think. Obviously it depends a lot on what you're paying for electricity. For someone paying 20c per kW-hr an EV is probably not all that much different to a standard Prius in terms of cost per mile.

On the other hand however I think historically we'll look back at the current gas prices and think wow it was cheap back then. If gas prices rise significantly over the next few years as many are predicting then it should shift the balance more towards EV's for better cost per mile.

 

Do not presume that battery charging always uses a constant current for the full cycle. Some chemistries take a tapered current profile.

Go back to Post #6 in this thread, where I posted Nissan's recharge cost claims. Then pull out your calculator and derive kWh. Depending on which Q&A is used, the differing significant digits lead to 24 or 25 kWh, half your estimate.

Then go to the Leaf Comes Home thread, post #57, where Dave's real life results fall in between yours and Nissan's estimates: "i am averaging 3-3.45 miles per KWH from the wall and 4-4.8 Kwh on the car ...".

 

just read the blog from the guy who owns a leaf and a volt. they just don't want to get into it. 'figure a penny a mile for electric.' what does that even mean?

 

The LEAF will not draw 30 A @ 240V the entire time it is charging. Perhaps it is constant current for a large portion of the charge time, but near the end it will drop off.

A good estimate is the EPA's estimate of energy used per mile, which is 0.34 kWhr per mile.

 

The LiFePO units I played with last year had a much longer tapered tail than that. So did the more traditional chemistry units a few years earlier.

He has a Kill-A-Watt meter to measure the energy at the wall plug.

I also don't automatically assume that the stated 30 amps is all going into real power. While anything of that size should have power factor correction to minimize reactive power, someone needs to actually measure both components.

 

if you were paying .12 per kw that would be about .04 per mile? 50 miles would be $2.00 compared to a gallon of gas for the pri? seems reasonable. am i correct? of course, at .40/kw, that would be a problem!

 

That means...what? Like anywhere from about $0.04 to $0.10 per mile if the EPA numbers are correct. This is depending on how you calcuguess your rate per kWh, after you back out the portion of the non-metered monthly charges that are represented by your additional electrical usage (if you're intellectually honest enough to do so) and not including any additional costs if you want any special charging equipment.

Let's say we just compare kWh costs to gasoline costs.

My raw electrical rate per kWh is $0.11, so that's like 3.74 cents per mile in electrical costs---not factoring in anything else.

My local gas price is $2.79 (this morning) so if I take today's flight data of 57.6mpg (over my last 73 miles) I get...(crunch-crunch-crunch) 4.84 cents per mile. That's a delta of (about) $0.011---or a projected savings of about 1,100 bucks over 100,000 miles. Zat sound right?
So....over a 100,000 mile lifetime yes...you can save about enough to install an up-armored charging station, or pay for some of the difference in the MSRP price between a G3-II and a LEAF.

Hmmmm....if you live in an area where electricity is cheap, the numbers aren't as goofy as I originally thought. In fact...if you can put up with the range anxiety of the LEAF, and gasoline shoots up to $5 per gallon, and electricity stays at $0.11 per kWh, you could alllllmost save enough money make up the MSRP difference between a LEAF and a G3-II.

Lotsa if's in there I know.....but it's fun to think about.

 

ETC, your problem is you make your calculations on your costs, and then project them to others.
Costs may be much higher, or much lower. And, where you use EPA numbers for the Leaf, you use your own numbers for the Prius.
I could compare my electricity (4.8c/Kwh for EV charging) at 4miles/Kwh to get a fuel cost for my EV of just over a penny a mile vs the EPA of the Prius to get much better numbers.

The problem with comparing fuel costs of EVs vs other vehicles is it is HIGHLY variable on electricity costs and driving behavior and weather.
Now, if you compare the costs of producing that fuel, the case for EVs (and Hybrids to a lesser extent) become much more clear.

 

I only have these numbers handy because I was doing the same calculation a few days ago, after looking at my own electricity bill. And yes, they are only applicable to comparing LEAF against Prius, in my locale.

Note: I am adding 10% to the electricity usage for charging losses (Conversion losses, Power Factor, etc.)

For instance, last month I paid $0.1285 per kWhr of electricity. At the highest Tier, I pay $0.31 per kWhr.

0.34 kWhr per mile (* 1.1 for charging losses) @ $0.1285 per kWhr = $0.048059 per mile

In a Prius, I can go easily 50 mpg @ $3.30 per gallon, current prices, which is $0.066 per mile

My utility offers some kind of program for owners of electric vehicles. It basically involves installing a new meter and wiring, and it charges $0.068 per kwhr of electricity as a night-time rate. This means I can only charge at night between 10 PM - 6AM. I will also be paying the utility probably something extra per month for meter. But for the sake of argument:

0.34 kWhr per mile (*1.1) @ $0.068 per kWhr = $0.025432
This is the absolute rock bottom $ per mile I can expect from a LEAF given my location. This assumes I only charge at home at night with the extra meter. This is about 1/3 the cost of gasoline @ $3.30 per gallon with 50 mpg.

At what price per kWhr is electricity as a fuel generally more expensive than gas as a fuel, when gas is $3.30 w/ 50mpg?

($3.30 per gallon / 50 mpg) / (0.34 * 1.1 kWhr per mile) = $0.1765 per kWhr

Of course if you live let's say in the pacific northwest where there is lots of Hydro, electricity is not only cheap but clean. The analysis will be different.

 

I've read Dave's thread and just to be clear, he only has a 110V charger, so the kill-a-watt is a perfectly fine tool to measure his electricity usage.

Also note the kill-a-watt can measure volt-amps in addition to watts, and also shows the power factor, and as far as I know it is equivalent to what you propose.

He's measuring at the outlet before the charger, this is the right thing to do to measure more accurately. The kill-a-watt is very accurate, maybe more-so than the utility meter.


FYI for anyone trying to compare a Volt, EPA is 0.36 kWhr per mile.

 

Go to Nissan Leaf Forum - Online forum for the Nissan Leaf Electric Car, join the forums and ask there. Dave has already given his figures for L1 charging. L2 should give better results, but owners there might be able to give you L2 numbers.