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Another Duke Energy Bill Problem - Part 2

By Sid of Stone Marmot

March 23, 2016 [Corrected Mar. 25, 2016]

Table 1 shows my surplus power production for the months of March and April since I've had my photovoltaic solar electric system installed. The data in the March and April columns is from the “Total KWH Received” line of the appropriate bill, which is the difference between the “Current Received” and “Previous Received” of the same bill in kwh. The last two columns are the number of days in that month's billing period for that year. Since it is too early to receive an April 2016 bill, n/a (not applicable) is in those spaces.

Table 1 - Surplus Power Generation For March And April

Year March April Mar. + part Apr. Days for Mar. bill Days for Apr. bill
2010 240 265 306 30 32
2011 272 260 335 31 29
2012 215 312 298 29 34
2013 200 244 281 28 30
2014 207 252 286 28 32
2015 166 268 275 25 32
2016 0 n/a 314 32 n/a

The number of days in each period makes a big difference in the total power for that period. That is one reason why March 2015 is so much lower than the rest (it was also a lot rainier and cloudier than normal that period last year). Since my most recent bill, March 2016, was for 32 days, and it was six days since Duke claims they read the meter and I read it upon receiving my bill, for 38 days total, I standardized the data in the “Mar. + part Apr.” column for 38 days. I did this by subtracting the number of days in the March billing period from 38, dividing this result by the number days in the April billing period, and multiplying this result by the kwh in the April column. This result was then added to the kwh in the March column, giving the value in the “Mar. + part Apr.” column.

For example, there were 28 days in the March billing period for 2013. 38 minus 28 is 10 days. 10 days divided by the 30 days in the April 2013 billing period gives 0.3333. 0.3333 times 244 from the April 2013 column is about 81, which is then added to the 200 kwh in March 2013 to give 281 kwh in the “Mar. + part Apr.” column for 2013.

You will notice that the data for the most recent bill, March 2016, is zero, which is way out of line with all the other data. But the 314 kwh in “Mar. + part Apr.” column for 2016, which is the difference between what I read on the meter immediately after receiving the bill six days after Duke claims to have read the meter and the “Current Received” on the bill, is within the same range as the other values in the “Mar. + part Apr.” column for all the other years. This indicates the meter is probably working correctly and the “Current Received” on the March 2016 bill was not entered correctly.

Incidentally, all the power flowing into and out of my house flows through my SMA Sunny Island 5048U inverter. This 5048U inverter continually measures the power flowing into and out of the house and stores it once a minute onto an MMC/SD memory card. This data is stored for the past 100 days before the oldest data is written over with the most recent data. So it is possible to extract all the same data that Duke's power meter measures from this 5048U inverter data for the March billing period.

[Correction (Mar. 25, 2016): All the surplus power generated does flow through the 5048U. But only the power used by circuits in the critical loads panel in excess of the power generated by the solar panels flows through the 5048U. Since I have a small house, all the 120 VAC circuits in my house except the one going to my clothes washer are in the critical loads panel. The clothes washer and all my 240 VAC loads go to my main circuit breaker panel and then the power meter, bypassing the 5048U.

I only have four 240 VAC loads: Water heater, oven/stove, clothes dryer, and heat pump. The water heater is only on when I'm taking a shower. The clothes dryer is typically used for one load every three weeks. The oven hasn't been used in over 19 years. The stove is only used for an average of four meals a week, with most of those meals taking less than ten minutes to cook. Since my house is well sealed and insulated, with no phantom loads and terrazzo floors, the temperature stays fairly constant and I almost never use the heat pump any more (once in the past 2 years). The clothes washer is typically only used for two loads every three weeks.]

For the record, Figures 1 and 2 show my power meter readings as of 4:39 pm today. This is to show that my meter is working correctly. See my article Stone Marmot Goes Solar: Part 6, Modern Power Meters Don't Run Backwards for info on how to interpret these readings.

Figure 1- March 23, 2016, meter reading of net power flowing out of house (same as current received on power bill).

Figure 2- March 23, 2016, meter reading of net power flowing into house (same as present actual on power bill).

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