I know induction AC motors can have efficiency ratings in the low to mid 90% range. DC motors using brushes might be a little lower but not sure. And no, a motor can't put out more mechanical power then it draws in electrical power, does that surprise you?
Lefty
No, it does not surprise me, I just wasn't sure if you could convert like that. Horsepower is an expression of Torque @ given RPM/5252, and I have never seen Watts expressed as ft/lb or N/m, so I thought I might be missing a conversion factor somewhere.
I've not used motors this large yet, though I own a pair as part of a mobility chair transaxle assembly. I do have a word of advice:
Purchase a controller, don't attempt to build your own as such a controller for large motors isn't easy to design. Unless you have this experience, of course, your time and money (and sanity) will be far better spent on other aspects of the robot. Just note that such controllers are far from inexpensive (but likely less expensive than something you could design yourself - unless, like I noted, you have experience in such design work).
Good luck with your robot.
Motor Controller is already ordered (sabertooth 2x25), wasn't too expensive, and while I can build a board from a schematic, I have pretty much zero experience in circuit design 
I've used 24volt 600watt motors to drive a large electric loco. Actually used 4 of them, one on each axle. The motors were originally from electric scooters and probably made in China. Very nicely put together, even if some of the engineering was a bit crappy.
As part of the project I conducted an efficiency test of electrical power in vs mechanical power out test by having one motor drive another, which acted as a generator ie an elctrical brake rather than a mechanical one. By applying resistive load to the generator I could measure it's electical output power. At the same time I measured the input current and voltage on the drive motor to measure its electrical input power. Efficiency transfer of motor input electrical power into mechanical power and then generator efficiency of mechanical power into electrical power could thus be determined by halving the motor to generator electrical transfer ratio.
I no longer have the actual data but I can confirm that the generator delivered 600 watts of electrical power. It therefore follows that the drive motor was producing in excess of 600 watts of mechanical power.
I used a 4QD controller (rated at 36 volts/300amps) to drive the motor. They are a UK based company but will export overseas at very competative prices. Their web site also contains loads of good information on typical applications 4qd.co.uk
They also have USA based distributors
jack
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Your post is very informative, thank you. I thought I remembered there being something else in the equation as far as electrical consumption vs output power, but it has been many many years since I was in school. I am using a 24v bridge/setup. Deep cycle marine batteries are crazy expensive here and I havent decided yet on 4x6 or 2x12, waiting until I get all the pieces for basic mobility before I start physically building.
