We introduce a limit for the strength of coupling light into the modes of solar cells. This limit depends on both a cell’s thickness and its modal properties. For a cell with refractive index n and thickness d, we obtain a maximal coupling rate of $2c\sqrt{n^2-1}/d$, where c is speed of light. Our method can be used in the design of solar cells and in calculating their efficiency limits; besides, it can be applied to a broad variety of resonant phenomena and devices.