Capex and opex
Beginner
Capex is money spent once to buy something. Opex is money spent over and over to keep it running, like the power bill.
Novice
Capital expenditure is the up-front cost of equipment and buildings, spread over their useful life (depreciation). Operating expenditure is the ongoing cost: electricity, maintenance, staff, rent.
Expert
For accelerators, capex per hour falls with a longer depreciation life, but only if the hardware stays useful; opex scales with measured power, PUE and price. Idle hardware still accrues capex, so low duty cycle raises cost per unit of work.
Explained in Comparing chips (Architectures).
See also: TCO (total cost of ownership).