Concept
Operating expenditure (opex) is the recurring cost of running a plant — feedstock, energy, labor, maintenance, and overheads spent year after year — as distinct from the one-time capex of building it. It’s the annual cost flow that, added to annualized capital, makes up the total cost of production.
A flow set against a stock. Opex recurs every year (or per unit of output); capex is a one-time stock. The two combine only after capital is put on an annual footing with the capital recovery factor, so the annual capital charge and annual opex can be added.
Its two halves — split by how they respond to output (the fixed-vs-variable razor):
How it’s estimated at the maturity anchor. Variable opex = consumption per unit (from the mass and energy balance) × input prices. Fixed opex is factored, like capital: maintenance as a percentage of ISBL per year (a few percent), labor from an operator count × wage, overheads as a factor on labor and maintenance.
Where it sits. Annual opex + the annualized capital charge (CRF × total capex), spread over the plant’s output (set by the capacity factor), gives the levelized cost. Opex here is usually the gross operating cost; netting byproduct credits is a separate step.
Green ammonia opex is dominated by one variable line. The plant’s electricity cost — power for electrolysis, air separation, and compression — runs ~$400/t NH₃ and scales with output. Fixed opex — maintenance (a few percent of ISBL/yr) plus labor and overheads — is far smaller, ~$50/t at design output. Gross opex ≈ $450/t, almost all variable electricity.
Edge case: gray ammonia rearranges the same structure around a different input — natural gas as both feedstock and fuel becomes the dominant variable line — so its opex tracks the gas price where the green plant’s tracks power, fixed opex playing the same minor role in both.