Can’t go wrong with a pi. I’m using a pi4 for mine with a sonoff usb ZigBee dongle. Can’t give you accurate power consumption but it’ll be fairly minimal I would think.

Beware, HomeAssistant absolutely eats SD cards.

A pi booting from an SD card is a good way to test it out, but HA does so much logging and other writing to storage that the write limit on the card gets used up very quickly. Took my setup only like a month or two to die when I first started with HA.

If you want to run HAOS on a pi long-term, you will need to set up a different boot drive that can handle more lifetime writes.