Properly computing the net kWh

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eventhorizons
Posts: 5
Joined: Mon Jul 23, 2018 1:08 pm

Properly computing the net kWh

Post by eventhorizons »

I installed a V.3 meter at my house and am extracting the NET kWh to graph as my solar system pushes power out and as my house consumes power.

The meter has a "Total kWh" register and a "Reverse kWh" register.

Based on the definitions at "https://help.ekmmetering.com/support/so ... data-terms", the Total kWh register is the SUM of the Forward and Reverse kWh, e.g. Total kWh = abs(Forward kWH) + abs(Reverse kWh), whereas the Reverse kWh is only the power in the reverse direction.

To get the Net kWh, I am using the following formula from the above link: Net kWh = Total kWH - (2 x Reverse kWh)

Is this the correct way to calculate the net kWh?

I'm seeing data that looks about right but I have no idea if it is actually correct.

I ask this because I just installed another V.3 meter on a travel trailer hookup that should never see reverse power. I'm using the same data gathering script and I am observing the reverse readings slowly increasing over time. This particularly stands out when power consumption is very small.

The meter is sampled every five minutes.

For example, at 09:50, the Total kwh was 63.6 and the reverse was 10.4
Then at 09:55, the Total kwh was 63.7 and the reverse was 10.5
The next read, at 10:00 was 63.8 and 10.5

This translated to reporting 100wH in the reverse direction during the first period and 0wH total in both periods during a period of time that the long term reported load was about 200 watts. (I will try to check the load with a clamp-on but it can be tricky if loads are not consistent.)

I noted that by 18:00 today the Total kWh had reached 68.7 and the Reverse was at 11.0.

I guess another question would be "Is it normal/expected for there to be this amount of drift in the reverse reading?" Basically 500wH over a period of 12 hours or about 40 watts.
Jameson
Posts: 823
Joined: Fri Nov 04, 2011 7:42 pm
Location: Santa Cruz, CA
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Re: Properly computing the net kWh

Post by Jameson »

To get the Net kWh, I am using the following formula from the above link: Net kWh = Total kWH - (2 x Reverse kWh)

Is this the correct way to calculate the net kWh?
Yes, this is exactly correct!
I am observing the reverse readings slowly increasing over time.
If you are NOT sending electricity back out through the meters (towards the grid) via solar panels, etc., then you should NOT be seeing any INCREMENTING kWh on display #02 (Reverse kWh) of the meter itself. Here are some things to check (this assumes a “single phase, “120/240” system):

(hint: 80% of the time it is issue #2 below)

1. Make sure the arrows on the CTs are facing towards the load.
2. Make sure you have CT1 on line 1, CT2 on line 2, etc.
3. Make sure the voltage reference from Line 1 goes to terminal 7 on the meter, make sure the voltage reference from Line 2 goes to terminal 8 on the meter, etc. Connect your neutral securely to terminal 10.
4. Make sure your CT wires are connecting into the meter in the right order: For OmniMeters it should be black-white, black-white from left to right, going into terminals 1 through 4 on the meter itself.
5. Make sure that the CT on Line 1 (phase 1) is connecting in to terminals 1 and 2 on the meter, and not 3 and 4. Make sure the CT on Line 2 (phase 2) is connected to terminals 3 and 4 specifically.

Here is a video showing proper wiring of a 120/240 volt 3-wire system:
http://www.youtube.com/watch?v=_TFxszMp ... re=related

Here is a page dedicated to 120/240 volt 3-wire system wiring: https://help.ekmmetering.com/support/so ... v-metering

If you are still having trouble, send us clear photos of the wiring and/or the data from either your EKM Dash or EKM Push system (if you have your meter connected)

Note that once you have it working correctly the existing Reverse kWh will not go away — just not increment any further.
Is it normal/expected for there to be this amount of drift in the reverse reading?
You should not see Reverse kWh incrementing if you are not sending energy back to the grid. This indicates that something is not properly installed.

Let us know if you get it sorted out!
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371
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