This paper establishes coupling loss factor of linear vibration energy harvesting systems in a framework of statistical energy analysis under parameter variations and random excitations. The new contributions of this paper are to define the numerical ranges of the dimensionless force factor for the weak, moderate and strong coupling and to study the connections of dimensionless force factor, coupling loss factor, coupling quotient, critical coupling strength, electro-mechanical coupling factor, damping loss factor and modal densities in linear vibration energy harvesting systems. The motivation of this paper is to enable statistical energy analysis of linear vibration energy harvesting systems for reliable performance predictions and design optimisation under parameter variations of materials and manufacturing processes and random ambient environmental excitations.