[bigstrat2003]

I have an electric (though not robotic) lawn mower, and it turns out that it's not much quieter than a gas powered one. No engine noise obviously, but the blades spinning and hitting grass still makes a lot of noise (and indeed in my case it turned out to be the vast majority of the noise). So it wouldn't be a very good idea to run your robot lawn mower at night.

[lowkj]

Maybe your specific model? I have a Makita electric mower and the difference is night and day. It's very relaxing and I mow with no hearing protection. I've tested to see how far the noise carries and none of my neighbors can even hear it. Meanwhile, I hear gas mowers from my neighbors on all sides and several lots away from me.

[arcticfox]

This is uninformed - my Navimow is close to silent. It makes a slight clicking/snipping sound as it cuts through grass.

The mechanical components are entirely different, instead of a helicopter-like blade it is a ~silent solid disc with some razor blades on the edges.