An electrical device which comprises a power generating element containing a unit cell layer containing: a positive electrode in which a positive electrode active material layer containing a positive electrode active material is formed on the surface of a positive electrode current collector, wherein the positive electrode active material layer contains a solid solution positive electrode active material and the content of the solid solution positive electrode active material in the positive electrode active material layer is 80 to 98% by weight, and the solid solution positive electrode active material has as a basic structure a composition represented by the following formula: Li 1.5 [Ni a Mn b Co c [Li] d]O z, wherein z represents the number of oxygen atoms satisfying a valence, a + b + c + d = 1.5, 0.1 ≤ d ≤ 0.4, and 1.1 ≤ [a + b + c] ≤ 1.4; a negative electrode in which a negative electrode active material layer containing a silicon-containing negative electrode active material is formed on the surface of a negative electrode current collector, wherein the negative electrode active material layer contains negative electrode active material represented by the following formula: α Si material + β carbon material wherein the Si material is formed of one or more kinds selected from the group consisting of SiO x which is a mixture of amorphous SiO 2 particles and Si particles (x represents the number of oxygen atoms satisfying a valence of Si) and a Si-containing alloy, α and β each represent % by weight of each component in the negative electrode active material layer, and 80 ≤ α + β ≤ 98, 3 ≤ α ≤ 40, and 40 ≤ β ≤ 95; and a separator, wherein in the one or more unit cell layers constituting the power generating element, the electrical device satisfies formula: 0.91 ≤ C/A < 0.99 when the area of the negative electrode active material layer is A [m 2] and the area of the positive electrode active material layer is C [m 2], and the electrical device satisfies I C /I A ≥ 0.40 when the irreversible capacity per unit area of the negative electrode active material layer is I A and the irreversible capacity per unit area of the positive electrode active material layer is I C. a positive electrode in which a positive electrode active material layer containing a positive electrode active material is formed on the surface of a positive electrode current collector, wherein the positive electrode active material layer contains a solid solution positive electrode active material and the content of the solid solution positive electrode active material in the positive electrode active material layer is 80 to 98% by weight, and the solid solution positive electrode active material has as a basic structure a composition represented by the following formula: Li 1.5 [Ni a Mn b Co c [Li] d]O z, wherein z represents the number of oxygen atoms satisfying a valence, a + b + c + d = 1.5, 0.1 ≤ d ≤ 0.4, and 1.1 ≤ [a + b + c] ≤ 1.4; a negative electrode in which a negative electrode active material layer containing a silicon-containing negative electrode active material is formed on the surface of a negative electrode current collector, wherein the negative electrode active material layer contains negative electrode active material represented by the following formula: α Si material + β carbon material wherein the Si material is formed of one or more kinds selected from the group consisting of SiO x which is a mixture of amorphous SiO 2 particles and Si particles (x represents the number of oxygen atoms satisfying a valence of Si) and a Si-containing alloy, α and β each represent % by weight of each component in the negative electrode active material layer, and 80 ≤ α + β ≤ 98, 3 ≤ α ≤ 40, and 40 ≤ β ≤ 95; and a separator, wherein in the one or more unit cell layers constituting the power generating element, the electrical device satisfies formula: 0.91 ≤ C/A < 0.99 when the area of the negative electrode active material layer is A [m 2] and the area of the positive electrode active material layer is C [m 2], and the electrical device satisfies I C /I A ≥ 0.40 when the irreversible capacity per unit area of the negative electrode active material layer is I A and the irreversible capacity per unit area of the positive electrode active material layer is I C.