~/bookshelf/schaums-outline-of-basic-circuit-analysis-second-edition
If the calculated P is positive with either formula, the component actually absorbs power. But if P is negative, the component produces power--it is a source of electric energy.
For mesh analysis, the transformation of all current sources to voltage sources is usually preferable because there is no formula for the voltages across current sources. If, however, a current source is positioned at the exterior of a circuit such that only one mesh current flows through it, that current source can remain because the mesh current through it is known—it is the source current or the negative of it, depending on direction. KVL is not applied to this mesh.
The number of mesh equations equals the number of meshes minus the number of current sources, if there are any.