We're talking about Watts at full throttle, or available Watts. Same thing.

As a general rule of thumb, 100 Watts per pound will give you roughly the same performance as a glow engine. Doesn't matter how big the plane is. 3 pound plane? 300 Watts. 15 pound plane? 1500 Watts. All you need is a good estimate of how heavy the plane will be when it's ready to fly. Most airplane kits come with a weight range specified on the box. Go with the highest value on the box, maybe even add 10%.

Watts, Amps, and Volts are always measured at the battery, at full throttle. That's where the rule of thumb is based.

The reason we don't really care about thrust is that it's only half the picture. You can have a 2lb airplane with 8lbs of thrust, but there's the possibility that it won't fly. Yes, it'll hover and go straight up, but it won't FLY. Why? There's also a SPEED component to the thrust from a propeller. If the speed component is less than the stall speed of the airplane, the airplane's wing won't make lift, and the plane won't fly.