They really don't have to. Watt is joule/second i.e. power is energy/time. Because the time is so short (10 -9 of second), Power is so big (2*10 15 Watts). So, storage needed is power * time = 2*10 6 Joule, which is about the amount half-depleted car battery has. I don't know how much it is in Library of Congress units, though.