An unrecoverable error occurs when the Steam Power "Desired power output in MW" is less than 7.2MW but greater than 0MW. It apparently is setting a stack size of less than 0 somehow. Anything more than 7.2MW appears to work and 0MW gives no items in the inventory so it works.

Note that I'm trying to get 1 boiler and 1 steam engine (0.9MW) as a result.

That sounds strange, it's probably a rounding error in the code somewhere. I'll look in to it. Until then it seems that 7.2 MW is the minimum.

It was a rounding error, it set the amount of offshore pumps to 0.

As for trying to get 0.9 MW, it's not possible atm, since the calculation is based upon the boilers. So you'll always end up with atleast 2 steam engines. I'll see to it that I'll change that when I rewrite the calculation in the near future.

Nice. I can live with the 2 steam engine results.