Can the unit be plugged in to an external power source? 

No. The SPOT device operates with Energizer 1.5V non-rechargeable size AA lithium batteries. These are required due to this technology's unique ability to maintain a high cell volt throughout the life of the battery. Additional benefits are a larger operating temperature range, up to 8 times longer life than alkaline, an outstanding 15-year shelf life and a 30% weight reduction.

If you are interested in SPOT units that are capable of being powered by an external source, you may consider the SPOT GEN3 or the SPOT X as both have this capability.