Using Portuguese grid data, the two organisations found wind and hydro each contributed around 45% of the 632.7GWh of domestic consumption over the five days between 7 May and 11 May.
The rest was supplied by solar and biomass. A further 153.7GWh of production was exported, which included a small element of non-renewable production.
"This shows that 100% renewable electricity production is possible and that we are on the right track," said Apren president Antonio Sa da Costa.
Portuguese "renewable energy has passed a stern test in supplying 100% of the country's consumption needs and exporting a significant percentage given the tiny interconnections between the Iberian peninsula and France," according to Zero president Francisco Ferreira.
The potential for 100% renewable production could be extended from spring into summer if the Portuguese government prioritises boosting solar capacity, both executives believe.
"With a doable ten- to fifteen-fold expansion in solar capacity from the current 450MW, wind, hydro and solar will each have an approximately 30% share by 2030," Antonio SA da Costa said.
To achieve 100% renewable electricity 365 days a year Portugal will also need to boost energy efficiency and solve the problem of storage, but Apren calculates the target can be met "without major disruption" by 2040. This date assumes a major switch to electric transportation and a corresponding surge in electricity demand.
The target could be achieved faster if, by 2030, there were to be 30% interconnection capacity between the Iberian peninsula and France.
These interconnections would allow Europe to take advantage of storage and the difference in sunlight, wind resource and rainfall across the continent, as well as the cheaper prices of Spanish and Portuguese wind and solar power.
The European Commission has made boosting these interconnections a priority, but Sa da Costa is sceptical. "Seeing is believing," he said
"If Iberia and the UK were also connected, we could provide power equivalent to the proposed Hinkley Point nuclear plant both faster and safer, but I don't see political will in France, the UK or Spain," said Sa da Costa. "Hinkley Point will never be built, and we have the capacity to replace it. Why not do it?"