This is the best portable charger you can buy. Seriously. It charges off of micro-USB (which is the most common adapter size- currently used by like 90% of all non-Apple chargers in the world) but the BIGGEST and most important thing that makes this a great product is that you do NOT have to use your own watch charger cable coiled up. Other products sometimes are simply shells but you have to cough up another $20 or $30 bucks for the actual wiring inside. This watch charger will charge your Apple Watch at about 1.2% per minute: you can expect a full charge in about an hour or so.
As always, remember that your Apple Watch charges faster if you put it in airplane mode, turn down the brightness, and even enable grayscale mode!