This is, in my mind, one of the benefits of laptops over micro computers: integrated UPS. Even an old, degraded battery will probably get you a couple of hours with the screen off.

IME, power consumption is going to be worse overall, for any laptop likely to be in the recycle bin, it's probably double the consumption of an ARM SBC. The integrated UPS and usually decent power conditioning of the power supply saves you more money with a laptop. Plus, keyboard and screen for emergencies - I just generally expect that, over there life of a micro I'm going to have to drag out and plug in a spare keyboard, mouse, and monitor because something in a device, or an upgrade, or BIOS flash, is preventing a boot.

There are a lot of good reasons to use laptops instead of SBCs, if you don't mind the extra power draw and (as she says) don't have size requirements.