EM Reprint: 3:15pm

1 12 2009

(Originally published July 21, 2008)

I have been clinging to part of the July 6 edition of the Dallas Morning News as it was chock full of Power Plus-related material, most notably a story about wind power in Texas. For the purposes of this installment, I am going to zero in on a sidebar item from the wind power story about the “grid” and peak demand.

First and foremost, peak demand in Texas comes right around 3:15pm. If it is possible to use the least amount of grid-tied electricity, that’s the magic moment. Unfortunately, it’s not “peak” demand for nothing. As the temperature rises – we’re getting into the triple-digit times now – demand for air conditioning and the like rises accordingly. Informed of the “magic moment”, I’m being more cognizant of what electrical items are running when, knowing that our air conditioner trumps all else and is quite capable of driving up a $550 electric bill for a modest 1-story house.

The second part to the sidebar item concerns the electric grid itself. I had always thought that “the grid is the grid is the grid”, and whether you stick a 9-volt battery on there or a nuclear generator, it’s all power that ultimately will be sold to you by way of your electric meter. Not exactly.

The electric companies bring power stations online from cheapest to most expensive, in accordance with demand. If a 9-volt battery supplied enough power for their service area, the electric company would theoretically use it, until 10 volts of electricity were needed, which would require the next available power source. Wind is the cheapest (commercial) power source in Texas, and natural gas is the most expensive. In the wee hours of the morning, some amount of our electricity comes from wind. At 3:15pm, rest assured that natural gas is online and ringing up the expenses. According to the newspaper item, the last thing to be added to the grid drives the market price for everything connected to the grid. If your 9-volt battery costs $0.00001 Kw/h to supply power to the grid, and along comes natural gas to the tune of [way more], that 9-volt battery power now sells for whatever natural gas is charging.

That is, lest anyone get silly with 9-volt batteries, if the power company has arrangements in place to buy power from you. If you’re sticking power on the grid for free (to them), that will be charged back as usage to you. If your state doesn’t have net-metering in effect, be wary of claims that your super-duper solar array on your roof is going to “pay for itself” because you’ll charge back the excess power to the electric company. Only if they have that agreement in place. You may still pay much lower electric bills, so there’s that.

Since I’ve gotta be me, I ran some tests today on our electricity usage during a 100F day between two separate time periods. The first was the “magic hour” of 3pm-4pm Central Time. The second was between 9pm and 10pm.

I used a stopwatch to measure how long the AC ran for each time period.

3-4pm: 45 minutes

9-10pm: 30 minutes

According to earlier research, 1 hour of constant AC use translates to roughly 5760 watts (5.8 Kw). I believe it is fair to multiply that number by .75 (for the 3-4pm record) which gives me 4320 (watts) or 4.3 Kw.

4.3 * 24 = 103.2 Kw/day (assuming the AC stopped for 15 minutes every hour)

Normally, a full hour of AC use would play out as 5.8 * 24 = 139.2 Kw. Stopping the AC for those 15 minutes during peak demand appears to be reasonably significant (139.2 – 103.2 = 36 Kw/day savings). This assumes that my math is correct. This also does not take into account our electricity demands that are independent of the AC unit.

Our non-AC usage during peak demand is roughly .89 Kw/h. Much lower than 5.8 Kw/h. A full day of our peak demand hour usage (when the AC is OFF) would be 21.36 Kw. Again, far lower than 139 or 103 Kw. Somewhere in all of this is the blend that determines our actual monthly usage.

The 9-10pm time slot played out as a 50-50 split between our normal usage versus the highway robbery of our AC unit. So I feel justified in multiplying 5760 by .50 to get 2880 or 2.9 Kw. 2.9 * 24 = 69.6 Kw/day. 139.2 – 69.6 = 69.6. I guess it stands to reason that running the AC for half of the day would result in a (gasp!) 50% reduction in power consumption (by the AC). As the night cools off (such as it does here in North Texas), the AC is expected to turn off for longer periods of time. Plus we’ll have fewer electrical devices running overnight (mostly lamps).

We are augmenting the AC with ceiling and desktop fans. Their combined power draw is peanuts compared to the cash drain of our evil AC unit. Yes, we hope to upgrade the AC unit someday. In the interim, we have to use other methods to become less reliant on the AC unit. But we simply cannot function without the AC unit. Even with fan augmentation, you could grow tomatoes in our house at 3pm. I tried doing without the AC for a whole day (between 8-4pm) and had to give in around 2pm.

This exercise isn’t about doing without electric items completely. It’s about examining current usage and determining if it is optimal. This is a topic for another installment (in the near or distant future), but generally speaking we in the US tend to draw liberally from the grid on the operating premise that, well, we can. But we also pay through the nose for the privilege, especially now (in Texas). And we pay in other ways, or we will.

Before things really heat up this summer, take a moment to examine your electrical usage especially during peak demand. Did the dishwasher or laundry room have to be used at that moment? What is the least amount of electricity you need to draw to live comfortably before crossing the line into sheer gluttony? This may seem all gloom-and-doom and like I’m insisting that we all live in the woods and eat bugs. No.

Energy, as a general area of interest, is a commodity that we have a hand in regulating. People complain often about high energy prices and spiraling demand, but we’re all part of those “market forces”.

Also, it’s easier to adjust your lifestyle upward or downward in advance of what I’ll call crisis conditions. But first, gather the data.




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