The conclusion is a blow for proponents of sun and wind energy because as more of it comes on tap, better storage will be needed to keep the power produced when it is sunny and windy so it can be used at other times.
The 30 million euro 'Nice Grid' pilot is one of the biggest in a European Union-backed 'Grid4EU' scheme in which France's EDF, Italy's Enel, Spain's Iberdrola Czech Republic's CEZ, Sweden's Vattenfall and Germany's RWE are testing the power grids of tomorrow.
In the Mediterranean village of Carros on the outskirts of Nice, EDF's power grid unit ERDF has connected compact batteries to solar panels on rooftops and utility-size batteries to its local power distribution network.
The pilot has shown that although the technology works perfectly it is still too expensive for wider rollout.
Philippe Monloubou, chief executive of French grid operator ERDF utility told Reuters that "The economic model of the batteries is not mature yet."
A quarter of Europe's power already comes from renewables and is forecast to rise to 50 percent by 2030. But the intermittent nature of solar and wind power requires flexible grids and the provision of cheaper power storage.
French company Saft, which sold the batteries for the Nice pilot, has already installed 80 MW of battery storage around the world, mainly in remote areas in Canada, South America and Africa, or on islands where they compete with expensive diesel generators as a back-up source of power. In Europe, the battery storage comes up against cheaper back-up power from gas-fired power plants and large, efficient grids.
In Carros, which has total solar capacity of 2.5 megawatt/hour, ERDF has connected 20 lithium-ion batteries to rooftop solar panels. Residents themselves have no control over the 4 KWh batteries - which are similar to Tesla's 7 KWh Powerwall batteries - which are run by ERDF.
ERDF has also hooked up two 100 KWh batteries to soak up solar power of several dozen residences, two 600 KWh batteries linked to the low-voltage grid and one linked to the high-voltage grid, for a total cost of under two million euros.