Although achieving efficient solar-thermal conversion with nanofluids (NFs) plays an important role in developing photothermal technologies, full-spectrum energy usage is still challenging currently. In this work, dimensionless methods of designing NFs are proposed in complex permittivity (epsilon) space, to make the most of solar energy. Firstly, an effective regime Omega' for each size parameter x is determined, in which an ideal photothermal efficiency can be guaranteed if only the NPs' permittivity falls into this area. Besides, on the other hand, by introducing a dimensionless parameter Q(i), our methods also allow to actively chose properties of NFs with a desirable solar-to-thermal efficiency, including volume concentration of NPs (f(v)) and thickness of NFs (L). Based on the proposed principles, results show that SiO2@Ni and SiO2@TiN NFs perform better with efficiency 15-30% improved than other widely used materials, such as SiO2@Ag(Au, Al) counterparts. Further, genetic algorithms are employed to verify the above results. Consistently, it suggests that near-perfect full-spectrum photothermal efficiency of SiO2@Ni or SiO2@TiN NFs can be obtained, reaching to 99% under certain conditions. In addition, regarding these two superior NFs, appreciable performance can also be protected with considerably small values of f(v) and L, which helps to reduce costs and is attractive in practice. The proposed principles offer a powerful tool to actively design optimized NP-related NFs for achieving perfect photothermal conversion, which will also potentially stimulate the development of other advanced photothermal technologies.