Make hydrogen engines use slightly less hydrogen so they can slowly fill
There's currently an issue right now with hydrogen engines. If you have twice as many O2/H2 generators than hydrogen engines with batteries on the grid. The hydrogen engine(s) won't fill until the batteries are charged.
So I say that the engine should use slightly less what's produced by 2 generators(around 2.5% per generator). Making it easier to set it up as a backup generator on bases or ships.
Or you could just make a stockpile mode for hydrogen engines when they are off idk : /