The ornamental design for a energystorageunit, as shown and described.
FIG. 1 is a front, left-side perspective view showing our new ornamental design.
FIG. 2 is a front view thereof;
FIG. 3 is a left-side view thereof;
FIG. 4 is a right-side view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines in the drawings illustrate boundaries and/or environmental structures of the energy storage unit that form no part of the claimed design.