Best practice: Calculate grid power from house consumption and PV power?

Setup, Configuration and Use Beginners
1

Hi everyone,

I’m relatively new to openHAB and have a fundamental question about calculating values.

Here’s my situation: I have two Number:Power items:

  1. Power_House_Overall – Total house consumption (from smart meter)

  2. Power_PV – Current PV power (from inverter)

I want to calculate the current grid power (house consumption - PV power). A negative result would be grid feed-in.

Which approach is considered “best practice” in the community (maintainable, performant, future-proof)?

  1. Classic rule: Simple, but triggers on every update of both items. Are there performance issues with second-by-second updates?

  2. Profile (Transform/JS): Is this even possible with two input items? I only know profiles for 1:1 transformations.

  3. Group aggregation (e.g., SUM): Can I calculate a difference (value1 - value2) via a group? This doesn’t seem intended.

  4. Completely different: Is there a binding or add-on that provides virtual meters/power values (like evcc does)?

What is the community “best practice” for such standard calculations? I’m leaning toward a rule but want to avoid making a “beginner mistake.”

Thanks for your tips!

2

?

…joined 2018 …

3

Should work just fine. When using file-based rules, for example JSRule, consider putting them into a dedicated context (see JavaScript Scripting - Automation | openHAB), this way second-by-second updates perform the calculation in a dedicated thread and not the per-file-thread.

Not really. Technically it should be possible to write a transformation that does this, but it is better suited to be a rule.

This also isn’t support AFAIK. I think this is the case because whilst a SUM is quite straightforward, for a subtraction you would need to define from what to substract what.

Another idea would be to use MULTIPLY one of the values by -1 using Math - Transformation Services | openHAB and then use a SUM group aggregation function.

4

I do it like this now

Another idea would be to use MULTIPLY one of the values by -1 using Math - Transformation Services | openHAB and then use a SUM group aggregation function.

I ammjust looking for the best practice one more question, is jsscripting or math transformation approach more resource-efficient (CPU/memory) in the long run?

5

I don’t know. I’ve never profiled such things and I also don’t care whether one is a few milliseconds faster than the other.

All I can say is that I am running a number of JS Scripting rules that calculate various values related to solar power, energy management etc. on change (and my inverter values are updated every 5 seconds). None of the rules’ threads is in the top 40 threads in openHAB when ranking by CPU time in the openHAB console …

ttop --stats=tid,name,state,waited_time,waited_count,blocked_time,blocked_count,user_time,cpu_time,user_time_perc,cpu_time_perc,lock_owner_id,lock_owner_name --order=cpu_time --millis=300