Best practice for handling energy consumption in a single cummulative value?

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Dear community,

I struggle with some specific topic and do not know if there is a “best practice solution” for this.

Let’s explain:

  • I have a heatpump that delivers various values via the Luxtronik/Novelan heatpump binding. Reading the delivered values and persisting them works great.
  • The used energy of the heatpump (KWH) is only delivered via a sinlge cummulative value that steadily grows. This value is never reset or broken down to hours or days or months, …

What I want o achieve:

  • Somehow get values from this cummulative energy value broken down to hours or days or months
  • Persist these broken down values (does this make sense? Or calculate them somehow in realtime when needed?)
  • Chart the values (preferrably Bar Charts, via Grafana). Can this be achieved with the big cummmulative value?

Does somebody else have this constellation with single values that need to be broken down and visualized?

Thank you and best regards,
Armin

What your item gives you is the energy E used by the device in kWh. What you would like to have is the power P currently drawn by your device. Since energy is the time integral of power, you need to calculate the time derivative of that value: P = dE/dt.

However, because the value for E that you get from your device is updated in intervals (probably seconds to minutes?), you can not calculate an exact value for P, but an estimate for the average value between an update and the update before that: P = (E(now) - E(previous update)) / (t(now) - t(previous update)).
It should be possible to perform this calculation in a rule on every update of E and set the value of P accordingly.

Assuming the item that receives the energy usage value from your device is called EnergyUsageItem and it is persisted in a database (e.g. InfluxDB, important!), and the item for the current power draw is called PowerItem, you could try the following:

rule "calculatePower"
when
    Item EnergyUsageItem received update
then
    val deltaE = EnergyUsage.state - EnergyUsage.previousState
    val deltat = now - EnergyUsage.lastUpdate
    PowerItem.postUpdate(deltaE/deltat)
end

You can then also persist PowerItem values and do any analysis you like using InfluxDB and Grafana.

Please be warned that I did not test the rule and I’m not sure if the calculation of deltat will work like this because I don’t have experience with timestamp values in openhab.

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