Simply put, grid parity is when an alternative energy source, such as solar, can directly compete with power from the electricity grid in terms of cost. Solar levelized cost of energy (LCOE) is used to calculate grid parity, which can be framed in multiple ways.
LCOE calculations can be compared to retail electric rates. In this sense, solar has reached grid parity, in which the cost of solar is equivalent to the retail rate. Retail rates can vary from one utility to another and depending on the customer class (residential, commercial & industrial, etc).
Grid parity can also be determined at the wholesale level. Broadly speaking, the solar LCOE is equivalent to the wholesale rate of electricity. But wholesale rates can be parsed out into various levels, such as average wholesale rate, wholesale rate of a certain conventional power plant, baseload vs. peak load wholesale rate, and many other wholesale comparisons.
Another aspect of calculating grid parity is whether to include any direct subsidies into the LCOE of solar energy. LCOE may vary when incentives are included.
Make sure to apply these considerations when reading or calculating grid parity.