We use a lattice-based self-consistent field (SCF) theory to model one-phase microemulsion systems composed of solvents with limited miscibility and a non-ionic emulsifier. All relevant degrees of freedom are accounted for in a mean-field description; all molecules can distribute freely over the two bulk phases, accumulate at the interface and take all possible conformations, but cooperative fluctuations of the interface are ignored. The only constraint imposed on the system is a fixed geometry of the droplets. The constraint equilibrium is based on the thermodynamics of small systems. We consider systems with equal compositions of oil, water and surfactants in lamellar, cylindrical and spherical topology. We take the Gibbs energy of these three systems to evaluate the mechanical properties of the monolayers. We show that a Helfrich-type description of the microemulsion is possible in this SCF framework. However, the predicted mechanical properties of the system are not classical. Usually it is assumed that the mean bending modulus kc and the spontaneous curvature J0 are surfactant-dependent constants. We find that kc and J0 also depend strongly on the surfactant concentration. However, neither the product kcJ0 nor the saddle-splay modulus text-decoration:overlinek depend on the composition as long as the interfaces do not interact. These results can be rationalised, as both kcJ0 and text-decoration:overlinek can be found from the lateral pressure profile of the flat, relaxed interface. Another important observation is that the Gibbs energy of the microemulsion is not exactly a quadratic function of the imposed curvature, causing kc to depend weakly on the topology of the interface.