USB power

My iPad's battery had drained so I bought a spare USB sync/power cable from the Techknowspace on Front Street near Spadina. Used the cable to connect iPad to a Mac Pro. It took approximately three hours to power up the battery to 30 percent -- which was enough for reading on the way home from work. But I had thought the USB power from a Mac Pro would charge faster than that. When the screen was on, the iPad displayed "not charging", so I kept the screen off for the charge.