Solar energy has become an important renewable energy source for reducing the use of fossil fuels and to mitigate global warming, for which solar collectors constitute a technology that is to be promoted. The use of nanofluids can increase the efficiency of solar into thermal energy conversion in solar collectors. Experimental values for the specific heat, thermal conductivity and viscosity of alumina/water nanofluids are needed to evaluate the influence of the solid content (from 0.25 to 5 v%) and the flow rate on the Reynolds, Nusselt and the heat transfer coefficient. In the laminar flow regime, thermal conductivity enhancement over specific heat decrement is key parameter, and a 2.34% increase in the heat transfer coefficient is theoretically obtained for 1 v% alumina nanofluid. To corroborate the results, experimental tests were run in a flat plate solar collector. A reduction in efficiency from 47% to 41.5% and a decrease in the heat removal factor were obtained using the nanofluid due to the formation of a nanoparticle deposition layer adding an addition thermal resistance to heat transfer. Nanofluids are recommended only if the nanoparticle concentration is high enough to enhance thermal conductivity, but no so high so as to avoid wall deposition.