Power Monitoring with Tasmota

6 min readApr 18, 2023

At home, I run a server rack and a hefty desktop computer, not to mention a kegerator. This likely represents a significant amount of my power usage. The problem is, that’s a guess; I don’t actually know how much power they’re drawing. To remedy this, I spent some time working with IoT plugs, prometheus, and grafana to set up real-time power monitoring.

What we’ll need

Sonoff S31 (or any esp8266 based plug with power monitoring)
USB to TTL adapter (for flashing the S31)
jumper wires (you can’t have too many of these)
soldering iron and solder sucker

That list puts the cost of this project around $35–60 depending upon whether you already have access to a soldering kit. I also used a stand with alligator clips like this to make holding wires together a bit easier.

Opening the S31

We need to access the serial contacts on the S31. To do that, we need to take it apart. For this step, I found this tutorial extremely useful. On my device, the screws were made of very soft metal and stripped quite easily. You may want to have a screw extractor drill bit on hand. I’ve paraphrased the disassembly steps below:

Remove the darker plastic panel with the power button. It should come off easily if you pry at it carefully.
Slide out the plastic rails on the top and bottom corners, on the side with the male plug.
Unscrew the three screws that were concealed by the plastic rails.
The two parts of the case should now separate easily.

Attach the serial connection

Note: Be absolutely certain that the S31 is not plugged in or powered in any way. The ground pin is connected to the live AC wire and it will electrocute you or fry your computer if it’s plugged into the wall.

Next, solder the jumper wires to the S31. The contacts we care about are labelled vcc (3v3 power), rx (receive), tx (transmit), and gnd (ground).

The S31 has two sets of rx-tx contacts. The pair closer to gnd isn’t connected to anything. Solder the jumper wires to the rx-tx pair closer to vcc. If you’re having trouble heating the ground contact, try carefully increasing the heat on your soldering iron or using a tip with more surface area.

Let everything cool off, then attach the jumper wires to the usb-to-ttl adapter. Using the adapter linked in the first section, connect the labelled contacts as follows (formatted as S31 to adapter):

vcc to 3v3
rx to tx
tx to rx
gnd to gnd

Double check that you’re attached to 3v3 power, not 5v. Powering the S31 with five volts will turn it on, but may damage the onboard wifi module.

Flashing custom firmware

We’ll be flashing the plug with tasmota, which is open source and feature rich. Before we can do that though, there are two stumbling blocks: the default release doesn’t include a prometheus exporter and the S31 doesn’t have enough memory to directly flash the binary we’re going to compile. So, we have to load the firmware using the following steps:

Clone the tasmota repo and compile the firmware with a custom configuration
Flash the S31 with the tasmota-lite release
Perform an OTA firmware update to the binary we compiled

Note: The tasmota project has been in active development since the writing of this article, so you may be able to skip the tasmota-lite install and instead flash the custom compiled binary directly

Compiling tasmota

Below, I’ve linked to the docker-tasmota repository because it reduces the required software for compiling the firmware. If you’d prefer to compile using native tools, you can find the repository here. Additionally, if you run into any problems during this step or just want more information, the tasmota project site is enormously helpful.

After we’ve cloned the docker-tasmota repo, we need to edit the file named user_config_override.h in the project root directory. In this file, we're going to define our wifi network information and pass some configuration options. The contents of the file should include at least the following lines:

#ifndef _USER_CONFIG_OVERRIDE_H_
#define _USER_CONFIG_OVERRIDE_H_

// set up your wifi name and password
#undef  STA_SSID1
#define STA_SSID1 "your wifi ssid"
#undef  STA_PASS
#define STA_PASS "your wifi password"

// disable some default features we don't need
#undef USE_DOMOTICZ
#undef USE_HOME_ASSISTANT

// enable prometheus
#define USE_PROMETHEUS

By default, the docker-tasmota project compiles using the development branch of tasmota. If you’d prefer to use the stable branch, change the value of USE_STABLE to 1 in the compile.sh script.

Compilation can be kicked off with the following command, which will output the binary to Tasmota/build_output/firmware/tasmota-sensors.bin

./compile.sh tasmota-sensors

Flashing minimal firmware

First, hold down the small button that can be found next to the GPIO contacts you soldered earlier and plug the usb-to-ttl adapter into your computer. This will put the S31 into programming mode and allow us to load new firmware.

There are several ways to flash this board, but I found the simplest to be using tasmotizer. Check the Release radio button and choose tasmota-lite.bin from the dropdown. Ensure that Erase before flashing is true and we're ready to flash!

An even smaller binary is available as tasmota-minimal on the downloads page

Flashing custom firmware

For this step, we’re essentially repeating the previous using the binary we compiled earlier. The only difference is, in the tasmotizer UI, select the BIN file radio button rather than Release and open the tasmota-sensors.bin file.

Once flashing is complete, power cycle the device and ensure that the S31 is connected to the wifi by selecting the Get IP button in tasmotizer. The last thing we need to do is finish off the configuration details and test the web UI. The web UI will be available at the IP address shown earlier. From the main menu, select Configuration, Configure Module, and choose Sonoff S31 from the dropdown. Once complete, we're all set to desolder the connections and plug the S31 into the wall!

Verify power monitoring

With the plug fully set up, we can verify that power monitoring is functioning properly by plugging it into the wall and connecting a lamp to it. Pulling up the web UI again, we can verify that sensor measurements are available at the top of the main page. And we can check that prometheus metrics are running correctly using the /metrics endpoint.