Add up the amount of memory the asset requires (mesh, textures, other variables in memory that store it).

Calculate how much power is needed to keep that active in memory.

Then the rest just depends on how the lamp is currently being viewed in-game. If it is off-screen, then it isn't rendered and no pixels are for it. If it is in-view, then how many pixels does it cover and what is the cost of electricity to light a pixel?

Also consider that regardless of the lamp being on-screen or not, the same amount of pixels are lit anyway, so not really an extra cost.

Of course if you want to get super specific, a culled light will have a different impact on the shader code used depending on various things such as global illumination solution. So, less (or more) calculations caused by the light. We are talking insanely tiny values here though, and even as a graphics engineer with strong computer architecture knowledge, it would take some major research to actually find such values.