You assume that it is all linear which it may not be. Osmosis at power P may produce X litres of water per time unit while osmosis at power 4P may produce only

You assume that it is all linear which it may not be. Osmosis at power P may produce X litres of water per time unit while osmosis at power 4P may produce only 2X litres per time unit. And thus be viable only when the electricity price is N times lower.