Kurt Kinetic makes a gadget that converts speed of the roller into wattage, as the power curve from trainer to trainer of their brand is supposed to be consistent.

14 MPH = about 140W. 21 MPH is in the neighborhood of 290W. On flat ground sans headwind I can go faster than 14 MPH from 140W, I think. I don't know for sure because I don't own a power meter, just the gadget for the trainer.

From the manufactor's site: Kinetic Road Machine:
P = (5.244820) * S + (0.019168) * S3
S=speed, S3=speed cubed.