As with other responses, you won't notice any drain on battery. I've had Apple Watch since launch day, and a pebble steel watch before that and neither has made any difference to my battery. Also bare in mind I've used several phones over the course of this period; a 5s, a 6, a 6 plus, a 6s and an SE. All perfectly fine with the Watch.
My colleague has just got an Apple Watch and is under the placebo impression his battery has taken a nosedive...even though he's getting about 20 hours out of his 18 month old iPhone 6!
Trust me, you'll be fine