If you buy something like the Western Digital MyBook 2TB 2.5" USB 3.0 drive, then no you don't need power. Very few, if any, laptop drives need power connected via Thunderbolt or USB 3.0.
I've been using that above 2TB drive with no problems, and also two Thunderbolt SSDs. No power cables.
Above is a portable (2.5") drive. To my knowledge 2TB is the largest portable drive available. They are found in laptops and don't require external power. on the other hand, desktop (3.5") drives needs power. That's generally the quick way to figure whether you need external power when you are shopping.