A normalized model for piezoelectric and electromagnetic vibration energy harvesters is described, allowing a unique expression of the harvested power to be written. The normalization used points out three characteristic parameters of any generator: its coupling coefficient, its losses coefficient and its mechanical quality factor. The evolution of the normalized harvested power is theoretically studied as a function of these parameters. Experimental results and a bibliographic study present typical orders of magnitude of coupling and losses coefficients values for piezoelectric and electromagnetic generators, and put forward unusual dualities: while piezoelectric generators have low coupling and low losses coefficients, electromagnetic generators exhibit high resistive losses compensated by high coupling coefficients. For both generators almost the same normalized power can be harvested with these dual parameters. Finally, the scaling effect is investigated: it is notably shown that the power density of an electromagnetic generator does not decrease proportionately with its volume as it is commonly accepted.