Depending on the concentration, it would hurt as it's a bit of an acid, plus ATP outside of the cell is one of the mechanisms that drives inflammation, but it won't give you extra energy or anything.

ATP is used to transfer energy more than store it, more like a wire than a battery. The average adult has about 250g of ATP in their body (for my fellow Americans: about one rather chunky hamster) but it's recycled about 200 times a day, so would require 50kg (6 watermelons or two average labradoodles) if it was used and discarded.

ATP has been around since the beginning of life or near enough, and evolution is a deranged, cat-piss-soaked hoarder that makes use of whatever is already lying around, so ATP also does several things beyond energy transfer. This also means where ATP is allowed and in what quantity is fairly controlled. To that end, there's a class of enzyme called ectonucleotidases that's found on the outside of cells. One of the things it does is keep the level of circulating ATP and things like it low, so whatever was injected would get chopped up pretty quick.