Has one SATA power connector. Uses the +5V and +12V. And probably < 10 watts when writing Standby power probably <2 watts (for manuf specs on wattage go to WD site, normally not a concern when used inside a computer. If installing in a external enclouser, then use a power adaptor.
AFAIK, WD uses 12V 3A switchmode AC adapters for their MyBook externals.
According to the following document, the WD15EARS has a peak current demand of 1.55A on the 12V supply. The average total power requirement for both the +5V and +12V supplies during Read/write is 4.5W.