I'm looking to add an occupancy sensor switch in a staircase that has a pair of 3way switches. 

The power source comes in at the light,  then goes to the switches,  see Diagram:

The main issue I've been running into is that all occupancy switches I can find needs either a Neutral or a Grounding conductor,  both of which are not in my switch boxes (and not in the budget to be added). 

Reading up on the post below I noticed UMS-OPS6M-DV that says it only needs a 25w load to run. 

http://diy.stackexchange.com/questions/39853/is-there-a-motion-sensor-light-switch-that-does-not-need-ground

 What does this mean exactly? My LED lights run a Load of 18w total...

1) Will 18w work,  if not I can increase the bulbs
2) Does this minimum load mean there is always (let's say)  25w being always used to just power the switch?  Or why does this minimum exist?