Compared with inorganic or perovskite photovoltaics, the key limiting factor for organic solar cells (OSCs) is large voltage loss, which is usually over 0.7 V. A significant contribution of the large voltage loss in OSCs is due to strong non-radiative recombination, the origin of which has been puzzling the community for almost one decade, limiting rational design of materials to overcome this critical issue. We systematically investigate a range of low voltage loss systems, where the voltage loss is around or below 0.6 V. We find that the quantum efficiency of electroluminescence in these systems have been significantly increased (up to 0.03%), leading to non-radiative voltage losses as small as 0.21 V. Combining spectroscopic and quantum chemistry approaches, we formulate the key rules for minimizing voltage losses: (i) low energy offset between donor and acceptor molecular states, and (ii) high photoluminescence yield of the low-gap material in the blend.