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As many others im getting a Zero W for PiHole and i would like to power it using the usb-port of my wifi router. However, im concerned about undervoltage. I read the Zero W wants 1,2A, but usb 2.0 only has about 500mA or so. Without a screen i wouldnt recognize the Pi getting undervoltaged.
Does it really consume that much? The Zero wont get any active coolers, displays or leds.
There is a lot ill-informed opinion about Pi. See Raspberry Pi Power Limitations. This has links to Typical Power Requirements which shows Typical bare-board active current consumption 150mA.
If you are using WiFi it will certainly be more, but a stand alone (i.e. no peripherals) Pi Zero W should run on 500mA.
Whether your Fritz!Box can supply this is unknown. Strictly USB should only supply 150mA without negotiation (which the Pi can't do) but many (most?) USB devices ignore this.
You asked if it is "safe" - certainly there is no danger, it will either work or not.
I read the Zero W wants 1,2A... please provide a link to the page where you read that