The power storage device includes a positive electrode, a negative electrode, an electrolyte, and an exterior body. The positive electrode includes a positive electrode current collector and a positive electrode active material layer in contact with the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode active material layer in contact with the negative electrode current collector. The positive electrode active material layer and the negative electrode active material layer overlap with each other. The positive electrode, the negative electrode, and the electrolyte are surrounded by the exterior body. When a length of the positive electrode active material layer is Py, a width of the positive electrode active material layer is Px, a length of the negative electrode active material layer is Ny, and a width of the negative electrode active material layer is Nx, Py>Px, Ny>Nx, and Ny>Py+Nx−Px are satisfied.