Grid parity occurs when an alternative form of energy generates power at a levelised cost of electricity (LCOE) that is less than or equal to the price of buying electricity from the grid.
LCOE is a measure of the lifetime generated energy and cost of electricity from a certain technology. The cost is typically given per kilowatt-hour or megawatt-hour. It includes the initial capital, discount rate, as well as the costs of continuous operation, fuel, and maintenance. LCOE is used to assess if an alternative form of energy is headed for grid parity and a viable energy source for consumers.
Since 1970, solar PV costs have dropped by a factor of 150x while oil, coal, natural gas and nuclear have increased 35x, 6x, 15x and 10x respectively. In effect unsubsidised solar PV has improved costs by thousands of times relative to most conventional forms of energy.