Hi,

While it true that no work is done to lift the object, work is done to
overcome the inefficiency of the lifting method. If this is done by gas
pressure (air ?) and the shape and the gap are constant then there will be
some law that ties the pump power to the mass being lifted. This problem
is very similar to the Pitot problem ;-). There is also a theoretical
minimum power required (assuming a gas flow of a certain density, at a
certain pressure) to exert a force on a surface. This results directly
from the Bernoulli equation. The power that will be really needed will
likely be significantly greater than that. I guestimate that a pump will
not be better than 40% efficient (electrical -> air power), and the
lifting contraption is not likely to reach 90% unless it is a very
precisely machined piston.

Otoh, there exist 'standard' power/mass ratio numbers for vehicles (such
as helicopters, airplanes, cars, ships), that give the power/mass
(probably averaged at the present stage of technology) required for each
type of vehicle. The numbers are usually expressed in HP/kg and make a lot
of assumptions. I think that you can extract such numbers from transport
engineering books (textbooks or studies). I doubt whether the numbers will
help you a lot. For air lifting, I think that the number for helicopters
will be most interesting. I do not remember exactly but it was something
like 0.8HP/kg I think. That would be 40HP to lift 100lb. I think that
these numbers are a bit high (i.e. you'll need less HP for 100lb, as the
number for choppers includes a hefty safety factor for vertical lifting
acceleration). I am looking at light helicopters in Kitplanes and I see
one with 2700lb gross weight and a 315HP engine. In kg that would be about
0.4HP/kg at max gross weight and max engine power, i.e. the real number
will be lower. The number gets worse (larger) the smaller the system is
(smaller systems have larger losses). wrt 120W and 100 lb, I think that
you'll need more like 20-30HP (15-22 kW) to lift 100lb in free air. So it
all depends on the efficiency of the lifting method used. I seem to
remember that model helis weigh in about 4kg wet and the engine makes
about 1.5-5HP on full throttle. But these do not have safety factors
and they do totally crazy things in the air ;-)

hope this helps,

        Peter