The ornamental design for a powerinverter, as shown and described.
FIG. 1 is a front, left isometric view of a power inverter showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof; and,
FIG. 6 is a top view thereof.
The broken lines shown in FIGS. 1-6 are included for the purpose of illustrating environmental subject matter only and form no part of the claimed design. The bottom view is omitted and forms no part of the claimed design.