The Levelized Cost of Energy (LCOE) is widely used to define the cost of electricity generation (in $/ MWh) over the life of the solar power plant (typically 25 years).
The LCOE is calculated by dividing the present value of the capital costs + all operating costs over the lifetime of the solar plant by the present value of the electricity generated. See LCOE formula definition
The internal rate of return (IRR) and ROI are calculated from the initial capital cost and the cash flow during the period for which a return is expected.