In response to global climate target, over the last decade renewable electricity generation from solar energy has grown rapidly and hence measures to deal with its non-dispatchable nature. Integrating concentrating solar plants in conventional fossil power plants is a widely researched solution to tackle solar energy intermittency. This results in hybrid power plants whose total electricity production consists of two different contributions (solar and fuel-electricity) that can be evaluated separately by implementing numerical methods based on the so-called "with and without solar energy" approach. Nonetheless, such evaluation cannot be carried out in actual power plants operation where fossil and renewable contributions are not discernible from each other within the overall production. To overcome this limitation, and consequently allow the total electricity generated to be properly partitioned among energy sources, this study proposes and validates an alternative "on-line and real-time" method to quantitatively assess solar and fuel-electricity by subdividing the overall cycle efficiency into subsequent ones related to the different energy conversion processes during real plant operation. Required input can be derived from available operating data except for factor of internal losses that, however, showing negligible dependence on solar energy and power plant load, can be reasonably assumed constant and estimated in dedicated calculations.