I saw this topic come up a few times (and have wondered about it myself), so when I happened across the following, I thought it was interesting enough to post here:
In DCRainmaker's Enduro review, he noted the battery drain from several activities:
From a run activity (at 3:25), using wrist OHR, GPS + Glonass, the watch drained 1.2% battery per hour.
Then, he did a ride with a chest strap (4:05), resulting in 1.07% per hour drain.
Assuming everything else is the same/comparable, I thought it really interesting confirmation that the full-powered wrist OHR used during activities is slightly more power hungry than running the watch with the ANT+ antenna, receiving and parsing data from a paired accessory. Btw, this matched my experience as well with watch stamina across several weeks. I typically saw a day or two more in smartwatch mode when running with a chest strap than with OHR.