Nokia 7 Plus, starting 7am, now 4pm, tells me wireguard ate 27% (1026mAh, using 4G most of the time: 9am - 4pm).

Though I'm not sure how much data I actually used. My phone does persistent keepalive so that I can use https://messages.android.com/ . Usage should go down quite a bit if you don't need remote connection to the phone.

Not directly comparable of course because it's WiFi only, but my iPad reports 3% over the last 10 days (much of which would have been Wireguard being open and exporting logs when I was debugging whilst setting it up) and 1% over the last 24 hours. I was also concerned about this but it's been fine.