This paper proposes a modeling of the energy consumption of a class of small convertible VTOL-UAVs (Vertical Take-Off and Landing Unmanned Aerial Vehicles). We consider systems composed of a set of coplanar propellers for propulsion and wing(s) to improve energy efficiency. Aerodynamics of propellers is modeled from standard momentum and blade element theories. In order to obtain simple closed-form expressions, modeling simplifications are made and a six-parameter-analytical model is proposed. The model parameters are identified from the experimental data reported in the literature. As for the wing(s), a NACA profile is selected and an approximate model of lift and drag coefficients over the entire flight domain is defined. Based on these models, the calculation of energy consumption reduces to solving a minimization problem in two variables. As an application, we compare the energy consumption of different convertible structures in the horizontal-flight range of [0; 20]m/s. This comparison provides useful guidelines for both conception and control of convertible UAVs.