I was testing this on creative because I thought the fill rate is the same even though I suspected it wasn't, so I tested it on my multiplayer survival playthrough and it's indeed slower
The fill rate is about 500L/s which is about 15,000,000L per 30,000 seconds or 1 H2 generator will fill up 1 large hydrogen tank per 8 hours 20 minutes, thanks to Andersen in the comment for reminding me c=
A short video showing how much time it takes for a single H2 gen to fill up a single large hydrogen tank on Space Engineers and a suggestion on how many H2 gens you need for every single large hydrogen tank
Correction: With 1:4 large hydrogen tank:h2 generator ratio, it will take about 12.5 minutes and not 8, and with 1:3 it will be 16.6 minutes which is pretty close to my 16 minutes estimate c=
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