Capacitance is typified by a parallel plate arrangement and is defined in terms of charge storage:

where

• Q = magnitude of charge stored on each plate.
• V = voltage applied to the plates.

Since charge cannot be added or taken away from the conductor between series capacitors, the net charge there remains zero. As can be seen from the diagram, that constrains the charge on the two capacitors to be the same in a DC situation. This charge Q is the charge you get by calculating the equivalent capacitance of the series combination and multiplying it by the applied voltage V.

You store less charge on series capacitors than you would on either one of them alone with the same voltage!