That belt looks pretty tired. Not at all important to use OE here; any good-quality V-belt will do. Conti, Gates, Dayco, etc. The alt belt on these is short and wears more quickly than the others.
Getting it tight and keeping it that way takes some tricks... I like to loosen the large (13mm head) bolt that goes thru the end of the lower adjuster strap into the oil pump, loosen the adjuster nut, tension the belt, tighten the adjuster nut while leaving the bolt at the other end loose, let off my prybar to let the belt pull the slack out of the joint where the bolt is, tighten the bolt, then loosen the adjuster nut and tighten belt again. Keeps the adjustment from shifting; without doing this I usually find it needs re-tensioned pretty soon afterwards. Also be careful not to overtighten that long bolt through the oil pump and squish the gasket, and don't pry against the aluminum pump body, use the square chunk of iron engine block right next to it. R&R of the oil pump on this motor is not fun. (edit: see
here)
Interesting but I guess kinda not surprising to know how much of the juice goes to accessories vs driving -- I assume this was calculated over time for in-town driving? At low speeds, with not much wind/road resistance and plenty of regen braking, the numbers make some sense, even as odd as they are....