Monday, September 12, 2011

Charge Scheduling

One nice thing about the Volt is all the different charging options.

If you pay a flat rate for electricity, you can charge your Volt at any time. Just set it to charge when you plug it in, and plug it in when you get home. Actually, you should always have your Volt plugged in whenever possible, as under certain circumstances it may need to turn itself on to cool the battery!

However, if you're really into saving money and energy efficiency, you'll be on a Time-Of-Use (TOU) electricity plan, where you get charged different rates for peak and off-peak usage, and the peak hours are different in summer and winter. You can save a considerable amount of money by moving to such a plan as long as you make an effort to shift some of your big appliance usage (the washer, the dryer, the dishwasher, etc) to those off-peak hours.

For example, here in North Carolina, you can pay 10.153 cents per kWh flat rate, or you can pay 16.65 cents per kWh peak and 5.003 cents per kWh off-peak.

If you decide to go to an TOU plan, the Volt has you covered. You can program in the peak and off-peak schedules for weekdays and weekends (generally all off-peak) and for summer and winter, and the Volt will try its hardest to only suck amps during off-peak hours. I use a 15-minute buffer; if off-peak actually starts at 9pm, the Volt thinks it starts at 9:15.

So far, so good. However, there is another rate plan that may be available that you'll want to consider -- Time-Of-Use Demand (TOU-D). In a TOU-D plan, you get charged lower peak and off-peak rates, but there is an additional charge based on your maximum peak-time demand.

I'm on TOU-D because I have a solar array (which you can monitor online) that feeds power back into the grid when possible (the Volt is just part of our household energy efficiency plan, admittedly the coolest part however). I pay 6.377 cents per kWh peak, 5.003 cents per kWh off-peak, but in addition there is a $5.02 per kW max peak-time demand charge during the summer (less in the winter). What this means is that Progress Energy looks for the 15 minute period during peak hours (but not during off-peak!) where we are drawing the most energy from the grid, and charges us based on that peak usage. So if we max out at 10 kW of demand, we get billed an extra $50.20. But remember, we're paying 10 cents per kWh less during peak time and there are 330 peak time hours per month in the summer, so it's hard not to come out ahead.

The end result is that TOU-D really encourages you to even out the demands you place on the electrical grid, by shifting big appliance use to off-peak hours or by staggering their use. Electric companies really like nice even demand, because it reduces their costs.

One other interesting thing: the Volt lets you define time periods as peak, mid-peak, or off-peak; you can tell it to use both mid- and off-peak charging, and it'll try and minimize the cost. I'm sure there are areas of the country where the power company uses mid-peak billing (all the different local energy costs are their version of "your mileage may vary"), but my power company doesn't.

However, I may still have a use for the mid-peak setting. Keep in mind that under a TOU-D plan, there isn't that much different between peak and off-peak rates, it's the peak demand that you want to avoid. My summer off-peak hours are 9pm through 10am, which is plenty of time to recharge the car on 110 volts, but the winter off-peak hours are more scattered -- 1pm  through 4pm and 9pm through 6am. I may want to set some of the winter peak hours to be mid-peak, as long as I'm reasonably sure those hours are relatively low demand, so the extra 1 kW or so that Volt will suck down won't increase the demand charge.

However, before I do that, I need to get a better feel for the daily power usage fluctuations of the house, as well as what my typical Volt driving patterns are.

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