How do I calculate what it costs to charge my i3?

[RobJones]

I've had my i3 for about a week and am loving it. We named it "Watney" after Matt Damon's character in The Martian because BMW really scienced the shit out of this car!

I've been trying to figure out how much it costs me to charge the car, assuming a fully depleted battery. But I'm not an electrical engineer and have been struggling with the math. Here are the rates from my electric company - Flint Energies (http://www.flintenergies.com):

kWh Charges:

October through May:
All kWh per month: 10.0 Cents per kWh

June through September:
First 4,000 kWh per month: 10.0 Cents per kWh
All kWh over 4,000 kWh per month: 12.0 Cents per kWh
(including Energy Conservation Adder)

*A kilowatt-hour (kWh) is a measurement of electricity. One kWh runs a 100-watt light bulb for 10 hours.

So how can I use this data to figure out what it costs to charge the car's battery?

 

[websterize]

The relative (usable) battery capacity is 19 kW, so about $2 USD per "fill-up."

Watch the neighbors’ eyes widen when you tell them that.

 

[RobJones]

Ah, I see now. Thanks! So, in the worst-case scenario of recharging a completely drained battery every day, it'll cost me roughly $60/month in electricity. Of course, I won't be draining the battery every day so it'll be a lot less than that.

So for roughly the equivalent of filling the tank in my X5 4.6is once, I can "refill" the i3 for a month. Pretty sweet!

 

[jadnashuanh]

Because things aren't 100% efficient, if you needed to fill the batteries with 19Kw, you might need to supply 21Kw or so, give or take a bit. The conversion from acv to dcv is not perfect, and then you may need to run the fan or the cooling circuits during the process. But, as a rough estimate, at $0.10/kw, a couple of dollars is about right. My electrical costs are nearly double that, but it's still a bargain.

 

[Jeffj]

Easy way to estimate is as follows:

Reset your trip computer, drive for a week. See what it says for your "efficiency" - its displayed in mi/kWh. Know that its lower in the winter than the summer, since it takes energy to heat the batteries and car. Depending on where you live, it could be a 30% difference summer to winter.

Estimate your miles/month and divide by the mi/kWh shown. Multiply by 1.25 to account for inefficiencies, pre-conditioning, etc.

So, if you drive for a week and show 3.9 mi/kWh, and you estimate that you drive ~900 miles/month, you'd get
900/3.9 = ~230 kWh * 1.25 = ~288 kWh. @ $0.10/kWh, that's just under $30.