Electricity math/cost question

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Applewins

Active member
Joined
Dec 16, 2012
Messages
44
Location
Grand Rapids, MI.
The MiEV has a 16kW battery, correct? If my typical recharge is 75% of total capacity I'd be using 12kW of electricity. If my rate is .07¢ PkW hour (my rate for charging off peak hours), would my electrical cost only be 84¢?

Second question: How much does a preheat cycle cost if I'm at this rate? If the pre-heat time is 1/2 hour and if it's a 5000kW heater, would it be 2.5 x .07 = 17.5¢?

Having fun with the MiEV!
 
Applewins said:
The MiEV has a 16kW battery, correct? If my typical recharge is 75% of total capacity I'd be using 12kW of electricity. If my rate is .07¢ PkW hour (my rate for charging off peak hours), would my electrical cost only be 84¢?
You have that right, but it's a 12kWh (energy) and not 12kW (power) battery. On that 12kWh and 84¢ you probably drove close to 50 miles vs. a couple of gallons and $8 in an ICE for the same distance. :cool:

Applewins said:
Second question: How much does a preheat cycle cost if I'm at this rate? If the pre-heat time is 1/2 hour and if it's a 5000kW heater, would it be 2.5 x .07 = 17.5¢?
Your math would be correct IF the heater was running for the full 1/2-hour; however, once warmed up, the heater cycles resulting in a lower average energy consumption and therefore lower cost. My guess would be 10¢.
 
IF you preheat during the off-peak period with 7 cent electricity, 0.5 hr x 5kw x $0.07 = $0.175

BUT, it won't preheat unless plugged in, and the charger will kick on immediately, consuming 1.65 kwh x peak rate cost (assuming you leave home during a peak period, detail follows).

Unless it's verry ccold, I'm pretty sure that my preheat cycle is not 5kw continuous- it cycles the heating element on and off.
Even if it was continuous, the 2.5 kWh that a 30 minute preheat consumes would only be partially replenished by your EVSE. 0.5 hr at 3.3 kw= 1.65 kwh, leaving 0.85 kWh to be supplied by your pack (which you already paid for and won't be replenished until the next plug-in, but it's still a cost of approx 2 miles of range and 6 cents).

SO, if you preheat and lose a bar by the end of your driveway, yep- it consumed 5 kW continuous!

*this assumes a charge/discharge efficiency factor of 100%, which is about 20% optimistic. :geek:
 
JoeS said:
Applewins said:
The MiEV has a 16kW battery, correct? If my typical recharge is 75% of total capacity I'd be using 12kW of electricity. If my rate is .07¢ PkW hour (my rate for charging off peak hours), would my electrical cost only be 84¢?
You have that right, but it's a 12kWh (energy) and not 12kW (power) battery. On that 12kWh and 84¢ you probably drove close to 50 miles vs. a couple of gallons and $8 in an ICE for the same distance. :cool:

Applewins said:
Second question: How much does a preheat cycle cost if I'm at this rate? If the pre-heat time is 1/2 hour and if it's a 5000kW heater, would it be 2.5 x .07 = 17.5¢?
Your math would be correct IF the heater was running for the full 1/2-hour; however, once warmed up, the heater cycles resulting in a lower average energy consumption and therefore lower cost. My guess would be 10¢.
Also, you can never preheat at 5Kw - The heater may sometimes draw that from the battery in normal operation (if everything is set to max) but during preheating, the heater runs off the 3300 watt charger and not the battery, (so that you're not depleting your battery while preheating) so the preheat will never draw more power than the EVSE is putting out

The maximum power draw from your outlet would be about 3300 watts, *if* you are using a 14 or greater amp L2 EVSE. If you're preheating using the OEM EVSE, you're never using more than 960 watts at any time since that's all the EVSE is capable of, so half hour of that would be roughly 3 cents

Don
 
Don said:
Also, you can never preheat at 5Kw - The heater may sometimes draw that from the battery in normal operation (if everything is set to max) but during preheating, the heater runs off the 3300 watt charger and not the battery, (so that you're not depleting your battery while preheating) so the preheat will never draw more power than the EVSE is putting out

Actually, I don't think that is accurate. I've noticed preheating on the stock level 1 will use some battery power. This is evidenced by the RR decreasing after using the preheater, and after turning off preheat, the charger will stay on to top the battery back up. You could be right about level 2, though. I've never fully changed on level2.
 
PV1 said:
Don said:
Also, you can never preheat at 5Kw - The heater may sometimes draw that from the battery in normal operation (if everything is set to max) but during preheating, the heater runs off the 3300 watt charger and not the battery, (so that you're not depleting your battery while preheating) so the preheat will never draw more power than the EVSE is putting out

Actually, I don't think that is accurate. I've noticed preheating on the stock level 1 will use some battery power. This is evidenced by the RR decreasing after using the preheater, and after turning off preheat, the charger will stay on to top the battery back up. You could be right about level 2, though. I've never fully changed on level2.

I have level two charging and I do notice the charger staying on for a short time after the preheat. Now, I'm not sure if it's actually charging or doing some other function like battery balancing. I have not seen that pre-heating the car effects the battery on the estimated mileage. Also, on preheat, for me, there are no heat adjustments except for the seat being on or not. Heat is on hot with no adjustment which is fine with me because when it's 15 degree's out it's nice to step into a very warm car.

Great discussion!
 
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