Call Back from the Power Company

Based on the note to the Corporate Commission, I got a call from APS yesterday.  A really nice lady that totally understood what I was talking about discussed what I had found.  She described the measurement as taking the kW reading every minute, averaging it over sixty minutes and then holding the value.  It would keep the highest number between meter readings.  That corresponds exactly to the definition that is both on the web and in their tariff.  When I told her that was NOT what I was seeing; I saw it keeping the highest reading as I previously described; she said that she would have to check more deeply and would get back to me.

She agreed that my measurements (as I described them) made sense, but that should not be what the meter recorded.  We discussed the recirculating fans in my house and how they only worked for a few minutes, far less than an hour and how an average should always be less than the peak.  I also told her that one of their engineers told me that it held the highest current usage and that a manager in metering told me it held 15 minute interval averages.  Seems people there don't have a clear understanding of how Demand is supposed to be measured much less how it actually is.

The unfortunate part is that I didn't get her name.  I was working on my pool at the time and took the call in the back yard where I didn't have anything to write on.....darn.

Just to keep it in the record, this is the detailed explanation of the meter's operation I received via email from a person working in the metering department at APS:

The explanation of “Demand” on the APS web site, although accurate is a simplified definition.  The residential meter is set up to record the average demand over a 60 minute period. Your example below of a 1000W light bulb is a good one to start with.  If the 1000 W bulb was on for 15 minutes during the 60 minute interval, it would record only ¼ of the 1000W because the meter needs the entire 60 minutes to “build up to” the 1000W load.  This example is assuming that there are no other loads on before or after the 15 minutes the light bulb was operating.

In most homes, there is some load on all the time (refrigerators, clocks, remote control devices etc) so the actual situation would look more like this:

4pm – 4:15 pm – 1000 watt light

4pm – 4:10 pm – 5000 watt A/C

4pm – 5pm – 500 watts misc ceiling fans, etc

4:20 pm – 4:30 pm - 5000 watt A/C

4:30pm – 5pm – 1000 watt television

4:40 pm – 4:50 pm - 5000 watt A/C

From 4pm to 4:10pm the meter sees (1000 x 1/6) + (5000 x 1/6) + (500 x 1/6) = 1083 watts (1.08kW)

From 4:10 pm – 4:15 pm the meter sees (1000 x ¼) + (500 x ¼) = 375 watts (0.37 kW)

From 4:15 – 4:20 the meter sees (500 x 1/3) = 167 watts (0.16 kW)

From 4:20 – 4:30 the meter sees (500 x ½) +( 5000 x 1/6) = 1083 watts (1.08 kW)

From 4:30 – 4:40 the meter sees (500 x 2/3) + (1000 x 1/6) = 500 watts (0.50 kW)

From 4:40 – 4:50 the meter sees (500 x 5/6) + (1000 x 1/3) + (5000 x 1/6) = 1583 watts (1.58 kW)

From 4:50 to 5pm the meter sees (500) + (1000 x ½)  = 1000 watts (1.00 kW)

The Demand for this 60 minute period is 1583 watts.  The meter will record 1.5kW.  If this reading is the highest for the month, the meter places this reading into memory and it stays there until a higher reading is recorded, or the monthly demand reset happens.  The monthly demand reset returns the reading to zero and we begin the cycle again.

However, if you remember your definition of average from high school, the explanation above doesn't cut it. The explanation is describing a measurement of the maximum current draw for a short period.  Since the meter would hold the highest measurement, for the entire month this could cost you a bunch.  People should remember that an average is a sum of several items divided by the number of items.

If one were to analyze this minute by minute there is a very different number:

1000W 5000 W 500 W 1000 W
         Bulb        A/C          Misc          TV          Usage     Demand
0 1000 5000 500 108.3 6500
1 1000 5000 500 108.3      6500
2 1000 5000 500 108.3      6500
3 1000 5000 500 108.3 6500
4 1000 5000 500 108.3 6500
5 1000 5000         500 108.3 6500
6 1000 5000 500 108.3 6500
7 1000 5000 500 108.3 6500
8 1000 5000 500 108.3 6500
9 1000 5000 500 108.3 6500
10 1000                       500 25 1500
11 1000                       500 25 1500
12 1000                       500 25 1500
13 1000                       500 25 1500
14 1000                       500 25         1500
15                                  500 8.3 500
16                                  500                       8.3 500
17                                  500 8.3 500
18                                  500 8.3 500
19                                  500 8.3 500
20                   5000 500 91.7 5500
21                   5000        500 91.7 5500
22 5000        500 91.7 5500
23 5000 500 91.7 5500
24 5000 500 91.7 5500
25 5000 500 91.7 5500
26 5000 500 91.7 5500
27 5000 500 91.7 5500
28 5000 500 91.7      5500
29 5000 500 91.7 5500
30 500 1000         25         1500
31 500 1000         25         1500
32 500 1000 25 1500
33 500 1000 25         1500
34 500 1000 25         1500
35 500 1000         25         1500
36 500 1000 25         1500
37 500 1000         25         1500
38 500 1000         25 1500
39 500 1000         25 1500
40 5000 500 1000 108.3 6500
41 5000 500 1000 108.3      6500
42 5000 500 1000 108.3 6500
43 5000 500 1000 108.3      6500
44 5000 500            1000 108.3      6500
45 5000 500 1000       108.3      6500
46 5000 500 1000 108.3 6500
47 5000 500 1000 108.3 6500
48 5000 500 1000 108.3 6500
49 5000 500 1000 108.3 6500
50 500 1000 25 1500
51 500 1000         25 1500
52 500            1000 25 1500
53 500 1000 25 1500
54 500 1000         25 1500
55 500 1000 25 1500
56 500 1000 25 1500
57 500 1000 25 1500
58 500 1000 25 1500
59 500 1000         25 1500

KW used in hour 3750
Max Demand 6500
Avg Demand 3750

Granted, my number is higher than the guy's from the power company, but it is still far less than the 6500 peak demand used for a few minutes in the example above.  Somebody somewhere is doing something wrong.  It may be me, but I don't think so.

I want people that read this to keep in mind that I have consistently been treated politely both by APS and the Corporation Committee.  Each person I talked to was considerate and actually tried to understand my concerns.  Usually when dealing with someone on a item like this people seem to get testy and often they put you on hold until your patience expires and you slam the phone down in disgust.  This kind of thing has not happened.