Prius 101 for the Physics Syncophant

Actually, that's one of three definitions for mass. See this excellent Wikipedia entry for the other two. I would argue that "Passive Gravitational Mass" is more relevant in the current discussion.

 

Good link.
However, since the OP was trying to calculate power (Kilowatts), only the "inertial mass" is relevant as the physical equations cited use it:
Power = Force (Weight: Inertial mass * gravitational constant) * Velocity (Rate of ascent)

 

Make sure you compare your hill climbs to level runs at the same speeds.

Note:
1kW = 1.341hp, 26kW = 34.9hp
The ICE has 76hp @ 5000 rpm (57 kW @ 5000 rpm) so you're looking at about an additional half the capacity of your ICE to ascend at 1 foot per second over the power required for a level run.

Edit: P.S. Dont forget to add 60kg (your body) plus the weight of any cargo into your calculations.

 

If the guy standing on the street with his kilogram is suddenly
the guy in the space capsule, weightless, he can still guesstimate
that he's holding a kilogram by trying to wave it back and forth.
He can still feel the force needed to accelerate and decelerate it.
So it's still a kilogram even though it doesn't weigh anything.
.
The real challenge is to put it down on the frictionless surface
and then walk away!
.
_H*