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Energy Saving Fallacy

As mentioned in a previous article about CO2 emission monitoring I’m using Shelly Plugs to monitor the energy usage of devices.

But of course these Shelly Plugs are smart plugs and allow to turn devices on and off based on a schedule. So for the time I have them now, I used the plug to turn the station of my vacuum robot off and on, trying to optimise the energy usage.

The setup

The setup is quite simple. The vacuum robots has two schedules, one for the main room early in the morning and one for all the other rooms later during morning. Therefore turning the Plug on from 6 to 12 is completely sufficient.

So the station of the vacuum is plugged into the Shelly Plug and the Shelly Plug and the Shelly plug turns on power at 6 in the morning. This allows the vacuum to recharge its battery before it schedules starts.

After returning from the first schedule, the robot will just dock to its station, charge before going for the second run. After returning, the robot has a enough time to recharge, before the Shelly Plug turns power off at 12.

To make everyone’s life a bit nicer, the vacuum robot doesn’t run on the weekend. Therefore during the whole weekend power is also turned off.


18 hours without power and the resulting use of sleep states for the vacuum should reduce its energy usage. Further the timing of the charging especially in summer can help to use early solar power in an effort to reduce CO2 usage.

Additionally the downtime during the weekend should just improve that effect.


It kind of works, but it doesn’t. The theory is correct, turning the device off, reducing the energy usage. However, when looking at absolute numbers, it saves ~5 watt-hours per weekday and around 13 watt-hours for the weekend. That’s not much, meaning the vacuum robot is quite efficient with its standby.

But the whole setup becomes silly, when you consider that the Shelly Plug I use needs a bit less than 1 watt of power to do anything. Meaning it needs roughly 24 watt-hours per day.

To conclude: It’s not efficient to save 38 watt-hours per week by investing ~168 watt-hours. Look closely at the power usage of the devices around your home and only use smart plugs where it makes sense. Sometimes the devices standby is more efficient than the smart plug.

And of course: If you really want to save energy, there is nothing better than putting effort into it and just unplugging the cable or getting a classic switch that you use.