I have a classic wired doorbell at home: the 230V powered transformer produces 12V on its secondary winding. The circuit on that secondary winding powers an electromechanical doorbell via a pushbutton. The bell rings (“ding-dong”) when the button is pushed (closing the circuit).
Since losses occur in all transformers, I wanted to know how much my doorbell transformer consumes in standby mode (doorbell not ringing). The primary winding is always energized, as the pushbutton (normal-open switch) is on the circuit of the secondary winding.
I made the measurements on the primary winding: 3,043 Watt. That’s more than I expected, so I double-checked, and noticed I had forgotten this:
There’s a small incandescent light-bulb in the doorbell button. That consumes power too!
Second set of measurements after removing the light-bulb: 1,475 Watt.
So with light-bulb, my doorbell consumes 3 Watt 24/7, and 1,5 Watt without light-bulb.
1,5 Watt is very similar to the standby consumption of linear power supplies. As energy experts here in Europe advice to replace linear power supplies in favor of switched-mode power supplies, I wonder why they never mention doorbells … Replacing your doorbell would not be as easy as replacing a (USB) charger though (it would best be done by an electrician), so that might explain it, but on the other hand, there are enough energy experts proposing impractical solutions.
3 Watt is 26,28 kWh for a whole year. In my case, that’s a cost of €5,89 (that’s total cost: electricity plus taxes). I could reduce this by half, just by removing the incandescent light-bulb.
Should I do this? Well, the decision has already been taken for me: I dropped the light-bulb while it was still hot, and the impact broke the filament …
For comparison: 3 Watt is at least three times higher than the individual standby consumption of our appliances like TV, fridge, freezer, …
Yet another comparison: asking an LLM to write an email requires less (< 0,3 Wh) than my doorbell over a period of an hour (3 Wh).
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Didier Stevens 
lol
Comment by Anonymous — Wednesday 6 November 2024 @ 12:32
A doorbell uses a simple electromagnetic coil to pull in the plunger. That electromagnetic doesn’t know the difference between AC and DC. As a test, I tried ringing my 24VAC doorbell with a DC power supply and got a good ding at 5V DC. Then why not replace the lossy transformer with an efficient USB power supply that produces 5V DC?
Another thought: how did you measure 1.5 watts of wasted energy? Did you take into account the fact that an unloaded transformer will consume current but that current will be out-of-phase with the voltage? This is called low power-factor. Because current and voltage are out of phase, you can’t simply multiply one number for measured current with one number for measured voltage. A good power meter will do this correctly, but even a very expensive ampmeter will not do this.
Comment by Anonymous — Monday 9 June 2025 @ 14:33
I won’t change to a more efficient power supply, because that would not only require making the change, but also documenting it and having my electrical installation re-certified. And that complete process is way too expensive compared with the cost of the light.
I made the measurements with a good bench power meter. It can also measure harmonics. Don’t know the model from the top of my head, by I reference it in other blog posts.
Comment by Didier Stevens — Monday 9 June 2025 @ 21:21
Correction: I remember now, I did the measurements with a professional energy multimeter: a Metrahit Energy.
It takes the power factor into account.
Comment by Didier Stevens — Monday 9 June 2025 @ 21:30