Calibrating ESPHome Power Meters

8bit

I recently tried to calibrate my LocalBytes ESPHome Power Meter (PM) plugs following the calibration instructions on GitHub, but would appreciate some clarification on how the calibration system works.

I opened an extension lead plug, connected my multimeter in series over live, plugged the extension lead into a wall socket, and then plugged the PM into one of the extension lead sockets and a toaster into the PM to act as the consistent load.

My multimeter read 6.5A with the toaster active, so I input this into the calibrate_current field in Home Assistant as per the Github instructions, submitted, and gave the PM a few seconds to calibrate with the toaster still active. The PM now closely agreed with my multimeter (about 0.01 - 0.05A out).
Current calibration diagram

I then connected the PM straight into the wall and connected my multimeter into the socket next to it to calibrate voltage, and once again got the PM closely agreeing with my multimeter.

However, the power/wattage seems to disagree with the voltage/current which the PM is reporting.
For example, a calibrated PM is now reporting 0.289A, 241.3V, 43W. However, 0.289×241.3=69.7W.

I assume power needs to be manually calibrated and is not automatically derived from the voltage and current. Out of curiosity, how is power measured by the PM?

To calibrate the PM power, could I just derive the power from the now calibrated voltage and current, and submit this into the calibrate_power field in Home Assistant?

Also, I have noticed that my other Local Bytes PM plugs which have not been calibrated by myself and therefore still contain Local Bytes factory calibration also have discrepancies between their voltage×current and power. For example, a factory-calibrated PM is now reporting 0.35A, 251.7V, 69W. However, 0.35×251.7=88W.

Hope someone with more knowledge on ESPHome power meters can explain this, thanks :)

archie

So, i think it goes without saying, but be extremely careful using a multimeter like that with mains electricity. For anyone reading this and hoping to calibrate their own plugs, I would like to suggest using something like this from Amazon.

To address your question directly, I am not entirely sure how the chip calculates the power it reports, but the calibrate_power field will not affect the reported volts or amps.

It is also worth noting that VoltAmps (voltage x current) and power W are not necessarily the same for DC applications. I am not, and will not pretend to be an electrical engineer, but you may find some useful information here: https://www.electronicdesign.com/markets/energy/article/21801657/whats-the-difference-between-watts-and-volt-amperes.

There is also a thread on the HomeAssistant forums that you may find helpful: https://community.home-assistant.io/t/try-to-understand-power-sensor-component-with-esphome/337510/2

AAllport

Hi @8bit,
Thanks for bearing with us whilst I've been away.

archie I am not entirely sure how the chip calculates the power it reports, but the calibrate_power field will not affect the reported volts or amps.

This is entirely correct.

The IC we're using is the hlw8012, which converts the voltage/current/power it can see into a PWM-ish signal for the ESP8266 to interpret.
As such, the calibration procedure ensures the correct scaling values for those signals.

The chip itself has onboard power monitoring and voltage/current. Again, this is a PWM-style output.
Hence, there is a discrepancy between your calculated and reported power values.

I believe this very discrepancy allows the Tasmota variant to estimate the power factor of the load.