Unless your DC is pretty close to the circuit box, you are already seeing
some voltage drop on 18a. Adding another 10' is not a great idea. I would
recommend using a 10 gauge extension cord, or (even better) wiring the DC
for 240v.

A voltage drop calculator is at:
http://www.electrician.com/vd_calculator.html
You don't want to go over 3%. Length is house wiring, extension cord, and
DC cord.

And finally, I sure hope you are not using more than one tool on a 20a
circuit, and that there is nothing else on the DC circuit, like lights.