Calculation for cost of charging?

[pwebb]

Hello would like to confirm if my calculations are correct. I wanted to know how much it cost to charge my FFE. I used 23 kw battery x (0.06) and got $1.42. I am lucky and have a low electric rate in Loveland Colorado at 6 cents a kw/hr. Then I wondered if I should be calculating it by 19.6 for the battery as I have read this is the true usable kw of the battery. Any thoughts?

 

[dmen]

You're in the ballpark.
Yes, about 19.5-20 kWh are available when the car battery is brand new. However not all the electricity your EVSE sends to the car ends up in the battery. Charging today's EVs is estimated to be 80% efficient. So you're probably buying more like 19.5/0.8 = 24.375 kWh to charge from empty to full, costing you $1.46. Since you'll rarely actually charge a truly empty battery, another way to look at it is that you'll buy 1.25 as much electricity as you are putting into the battery. So each kWh you add costs you 1.25*6 = 7.5 cents. Efficient city driving (200Wh/mi) will then cost you less than 1.5 cents per mile, and inefficient winter driving with a toasty cabin (500Wh/mi) will cost nearly 4 cents per mile!

 

[jmueller065]

Your best bet is to turn on the enhanced trip meter and reset it whenever you are fully charged. The enhanced trip meter will show you how much electricity was consumed, then you can use the formula to estimate how much that charge will cost.

I discuss the enhanced trip meter (along with some other dash stuff) in more detail here:
http://jamiegeek.myevblog.com/2013/09/25/butterflies-really-what-your-dashboard-can-tell-you/

 

[WetEV]

Charging today's EVs is estimated to be 80% efficient.

With a 120V L1 charging station, 80% is about correct.

With a 240V L2 charging station, more like 90%.

If you drive enough to use $25 per month, an L2 station will save you about $2.50 per month. Pays off in just 30 years! :lol:

The real reason to get an L2 is to come home after work and put 6 to 12kWh into the battery before going out later.

 

[jmueller065]

The real reason to get an L2 is to come home after work and put 6 to 12kWh into the battery before going out later.

Exactly!

 

[goofygrin]

My juice is $0.079/kwh. I've calculated that for the first couple weeks of driving I'm spending about 2-3 cents/mile. I drive short distances mostly so I drive more aggressively (so have a higher average usage than I could have).

If you think in terms of $/mile, the FFE is very, very hard to beat (my CTSV or Jeep is $0.26-$0.28/mile -- both get 13-14 around town on $3.39 or $3.69 gas [V requires 93 octane]).

In Loveland, you have hills and cold which will change this calculation heavily fwiw.

 

[dmen]

With a 120V L1 charging station, 80% is about correct.

With a 240V L2 charging station, more like 90%.

I thought L1 was a bit less efficient than L2 but this is the first I've heard it quantified. Do you have a reference? 90% doesnt seem right. When I use chargepoint L2s where I can compare what i'm buying to what my battery's storing, I see more like 80-85%.

I got 80% (actually 81% but I rounded) from fueleconomy.gov. It didnt specify L1 or L2 but I assumed L2, as most EV makers recommend L2. Anyway the 81% was said to result from the charger being 90% efficient and the battery being 90% efficient, so 90% of 90% is 81%. I wonder if your 80% with L1 is charger only, so consumer sees 90% of 80% for net 72%? That would fit with my observations: charging my car from near empty on L1 takes almost 20 hours, where it should only take 16 if L1 is 80% efficient.


Back to original poster, also remember to account for energy you might be using from plug to maintain battery temp and precondition the car with go times. Jmueller made some helpful observations a while back. If i recall, overnight battery warming only used an extra kWh or two, but heating the cabin used a good 5 kWh. I havent seen numbers for cooling battery and cooling cabin, but would guess cooling battery takes a bit more, while cooling cabin takes a lot less.

 

[WetEV]

I thought L1 was a bit less efficient than L2 but this is the first I've heard it quantified. Do you have a reference? 90% doesnt seem right. When I use chargepoint L2s where I can compare what i'm buying to what my battery's storing, I see more like 80-85%.

You are correct in that this is charger efficiency only. This source is for the 2011/2012 Leaf, with a 3.3kW charger, not the 2013/2014 Leaf with a 6.0kW charger, or the FFE with a 7.2kW charger, both of which are slightly better efficiency. I own a Leaf, but have driven a FFE a lot, and the cars are very similar.

http://www.mynissanleaf.com/viewtopic.php?f=31&t=8583

 

[pwebb]

Thanks for the helpful replies.

 

[EVA]

The other option is to install a Kill-O-Watt on your charger (I think they can handle the 110V amperage - I seem to recall somebody installing one on their charger). That will tell you exactly how much electricity you use, no guessing, no estimates.

On the other hand, if the calculation is simply for scientific curiosity, you got enough of an answer.

 

[jmueller065]

If you don't have a L2 EVSE yet you could get one like the Juicebox:
https://www.kickstarter.com/projects/emw/emw-juicebox-an-open-source-level-2-ev-charging-st

Which has a display showing how much power has been consumed (and the rate).

 

[hybridbear]

I'm very interested in calculating relatively accurate cost of charging as part of our consideration of the Focus Electric. In a post in the "Introductions" thread I mentioned that we live in an apt and that they're willing to install a 240V charging station for any resident who buys a BEV/PHEV. The initial comments to me by the Community Manager is that the property owners would pay for the cost of the charging station (assuming that if the resident moves out the charging station stays with the apartment and doesn't leave with the resident) and then they'd expect the resident to pay for electricity. They wouldn't incur the expense of installing a separate electric meter for the charging station but would rather just add circuitry into their existing 240V circuit(s).

They don't really have a specific plan of how to bill residents, it could be a fixed rate per month or a usage based bill based on data from the car. I'm curious for suggestions from Focus Electric owners about options of how to propose billing should we decide that we'd like to get a Focus Electric.

One idea I had so far: reset one trip odometer monthly. I could e-mail them a copy of the kWh used according to the car and then divide that kWh value by the estimated charging efficiency of x%. This could then be multiplied by an agreed upon price per kWh.

We haven't yet driven a Focus Electric, but from reading posts online it sounds like this info is easily accessible on the trip meter screens. I know that it is available on the trip meter screen of my parents' C-Max Energi so I assume that it is on the Focus Electric too.

 

[jmueller065]

Yes that would work, the enhanced trip meter will give you the information you're looking for.
http://jamiegeek.myevblog.com/2013/09/25/butterflies-really-what-your-dashboard-can-tell-you/

 

[hybridbear]

Yes that would work, the enhanced trip meter will give you the information you're looking for.
http://jamiegeek.myevblog.com/2013/09/25/butterflies-really-what-your-dashboard-can-tell-you/

Thanks. After making my post above I went to your blog and read a number of your articles. Very informative, thank you!

 

[magudaman]

Just FYI Everyday for the last 3 months I have been comparing my trip meter KWh to what the charge point system reads after completion. It calculates to a very nice average of 85% efficiency pretty much every time. 30% fill or 100% fill it doesn't seem care it is almost always 85%. I am curious if I will see that number drop as we get into the hot summer this year but for now it 85%