Beginner help with calculations - Use and Amps

Hi am a new EmonPi user, my system has been running for a couple of weeks.

I have 2 CT Sensors, one is measuring the total usage for my house and the other is measuring the Heat Pump use.

I want to calculate 2 other values that I can graph but I cant work out how to do it.

  1. I want to try and see what the maximum current I use is, I know this isn’t really available but If I assume all my usage is resistive I can get an idea by just dividing the use by the voltage (I do have an AC transformer connected to my Emonpi)
  2. I would like another use which is the total house use minus the heat pump use (difference between the 2 CT sensors)

Any help or links you can give on calculations would be great I am struggling with knowing where to look and where I should do the calculations

You do the maths on the Inputs page.


On the line for the Use c.t., click on the spanner and you’ll get a list of the processing steps. How it works is the power value comes in from emonHub at the top of the list, and each process step does something to it (maybe only sends it to a Feed) and then passes the maybe changed, maybe not (check the text for each step) down the page to the next step.

Exactly what you need to do next is guesswork, because I don’t know what’s there. Basically, you need the “Use” power and divide that by the voltage. If all you’re doing now is “Log to feed” then as the next step you can add
/ feed and in the feed box pick up Vrms.
If you don’t have the power accessible, you’ll need to zero the value handed down with reset to original and then divide by the voltage.
Then of course you need another log to feed (a different feed) so that you can use the value for the current.


Is much the same, this time subtract the second power.

It’s advisable to use Feeds rather than Inputs due to timing considerations arising from how emonCMS processes the numbers.

Hi Robert and group, thanks for this explanation, I am still stuck as I can’t find anything that describes these processes so I can choose one to do the calculations. Am I misunderstanding something? basically what process can divide the ‘use’ power from emonpi by the ‘vrms’ from emonpi. I can see both of these in the inputs section but just can’t work out how to process them.

Is there some documentation that explains these processes I can’t find it.

Ages ago, I suggested that “proper” documentation ought to be written to explain this. The result was a short paragraph that you see when you select the process.

Go to the Inputs page and the line for your “Use” c.t. Click on the spanner icon.

Here’s the process list page for my “Power1”

Select “/ feed” and “Vrms” from the drop-down lists. Click “Add” to add the process.
Now you want to save the current in a Feed. I chose “Log to feed”, picked “CREATE NEW”, called it “Current1” (type that yourself), chose PHPFina and 10 s for the interval. Click “Add” again.

and so on if you want to add any more. Don’t forget to click “Changed, press to save” before you exit the page.
(It might show the value as (NaN) - it will pick up the value eventually.) Now if you go to the Feeds page, you should see a new feed - mine is called “Current1”.

Have you found “Resources” and the emonCMS guide? In there, find Emoncms Core Concepts - Guide | OpenEnergyMonitor
A little way down there you’ll find

  • Log Locally: An example of configuring emonPi inputs, adding input processing and logging data to feeds

which says more or less what I’ve done specifically for you above.