The interest in high temperature electrolysis technology is sustained by promises of low cost, low carbon mass hydrogen production. To support the deployment of pre-industrial systems, CEA is devoting significant efforts to scale-up its proprietary stack base design. Partially supported by E.U. project MultiPLHY, this experimental report thus presents galvanostatic and thermoneutral operation of stacks in the 10-to-20 kWdc range. The laboratory first attempt at manufacturing a larger stack yielded an assembly of three 25-cell substacks [1]. They comprised commercial cathode-supported cells with a 200 cm² active area. After about 700 h of preliminary measurements, the stack was operated for 3.2 kh at -0.65 A.cm-2, 60% SC and 12.6 kWdc [2], during which a linear evolution of the stack temperature was recorded at a rate of +15 K.kh-1. The overall sequence generated 1.1 ton of H2, a first in the laboratory history. The end of the test is discussed. Based on internal advances in design and manufacturing control, a 20 kWdc stack was subsequently manufactured in one go. Moving away from the substack approach allowed reducing the production time by approximately 80%. The stack was then operated in thermoneutral conditions for 5.9 kh at -0.94 A.cm-2 and 18.6 kWdc, a 50% power boost compared to the first stack. The recorded temperature evolution rate, akin to apparent degradation, had been impressively lower than 1.7 K.kh-1 over 4.8 kh. In recent years, the standardization of stack tests targeting durations beyond 5 kh [3,4] was enabled by CEA’s ability to produce dedicated, purpose-built benches with near-perfect availabilities. Correspondingly, the data of the present work was obtained on a new generation of in-house equipment designed for long-term durability assessments of power-stacks. Bench availability results are discussed.