I have a dataset that contains GHI, DHI, and DNI columns. The problem is there are null values in DHI column and I want to find the most accurate way to replace those null values. Is using this equation DHI= GHI-DNI. cos (θ) to fill missing values accurate or I should use a function from pvlib?
If you have partial null values in the DHI column, then the most accurate approach would be to examine the time steps where you do have all three values to see how consistent they are, and combine this with your knowledge about the instruments and maintenance to adjust your filling strategy.
But as a first step, just try what you proposed and keep in mind Adam's comment about the sun position.