kwh monitoring in CMS

Hello all,

Happily running with my IoTaWatt, but noted that the CMS needs to accumulate power data from IoTaWatt on 10 second intervals and calculate energy. IoTaWatt is already doing this, but the inputs available from IoTaWatt in CMS don’t expose the energy data, just the power data.

It would seem more efficient and much more accurate to input the energy directly from IoTaWatt.

There are other measurements I would like to see in CMS, like power factor.

Am I just missing something in the input configuration?

1 Like

This is already possible with the current alpha version of the firmware: 02_03_04 new ALPHA


Hi installer,

I decided to answer this in the Emoncms forum because it straddles the technologies, but as a new IoTaWatt use, I encourage you to continue your questions and dialogue here in the IoTaWatt forum.

1 Like

Thanks, appreciate it.

My box is 02_03_13 Alpha but it does not show option Wh and kWh neither.
Is there anything needs to be done other than updating the code?

Any comment will be appreciated.


wh and kWh are only available when calculating output values to send to EmonCMS or influxDB. That’s because in those cases data is being sent for specific intervals of time, and energy is an integration over time.

When defining a regular output for the status display, there is no time interval to calculate Wh. Any attempt to produce Wh would require imposing an arbitrary interval. For example it could be:

  • Wh in last 60 minutes
  • Wh in current hour
  • Wh since device last started
  • Wh since device installed

None of which are particularly meaningful. If you want to use the defined outputs in the local graph utility, the graph provides the time reference, so if you select a Watts output under the Energy tab, you can get the kWh there with respect to the time interval defined in that graph.

Thank you for answering.

I tried to find “Energy tag” but was not able to find in graph utility.
Could you please explain in more detail where “Energy tag” is?

In the upper left margin of the graph utility, there are a series of tabs: Voltage, Power, Energy, and (sometimes) Outputs:

If you click on Energy, all of the inputs and outputs with Watts units will be listed:

When you plot these, you are plotting total kWh for that input since installation, during the period selected in the graph. Here’s my total kWh for the past week:

This isn’t really very useful yet, but it does have some good information that I’ve highlighted at the bottom:

  • The Max figure is your up-to-date running kWh total.
  • The Diff figure is the total kWh used during the week.

Now if you are interested in something like daily kWh usage, you can use the graph functions to produce that from what you have. Using the various options at the bottom:

  • Select period Type “Daily” in the top line.
  • In each detail line, select Type “Bars” and check the “Delta” box. This tells graph to plot the difference in kWh in each day instead of the cumulative total at the end of the day.
  • Press “Reload” in the top line:

    Now you have a histogram of daily kWh over the last week. If you are interested in the numbers you can display the CSV:

    There is a caveat here. Days begin and end at 24:00 UTC (GMT) rather than your local time. That’s something that needs to be addressed in a future release.

Thank you very much for that detailed information.
I found that option in local box thanks!

When I follow the directions above , I get something very different. I seem to get a bar per cycle, not per day.

Here is what I see.

Looks like your screenshot didn’t upload. If you try again I’ll look at it. Be sure to show the entire screen so I can see the options you selected.

From influxdb you can get energy with cumulative_sum and group by time what you are feeding to influx. Example influx feed with 5s update, grafana line would be:

SELECT cumulative_sum(mean(“value”)) AS “Wh” FROM “xxxWh” WHERE $timeFilter GROUP BY time(5s) fill(linear)

Unfortunately this will take a lot of cpu when having long timerange (array gets really big). Fortunately there is better way:

SELECT cumulative_sum(sum(“value”)) AS “Wh” FROM “xxxWh” WHERE $timeFilter GROUP BY time($__interval) fill(linear)

This lets influxdb sum values in interval and then cumulatively add those interval-values to get energy graph for timerange.

Does that work better than:
SELECT integral("value")/3600 FROM "heat_pump" WHERE $timeFilter GROUP BY time($_interval) fill(null)
“heat_pump” measurements in watts so there is no need to save Wh in influx.


Does that work better than:
SELECT integral("value")/3600 FROM "heat_pump" WHERE $timeFilter GROUP BY time($_interval) fill(null)

Well, that query returns: error:"unsupported call: integral"
So, sum works much better :wink: Upgrading influxdb probably would help, but currently I get it from repository… Anyway, using sum is simpler and probably gets more accurate result. Downside is using a bit more storage space.
For longterm kWh energy calculations, making hourly aggregate (group by) with continous query would make getting those values really fast.