Grid is capacitive in the evening
The grid is generally inductive, not capacitive.  This means the current lags the voltage a bit.

The grid is specified as a voltage source.  The power producers set the voltage, then the loads draw whatever current they draw.  Ideally this current is instantaneously proportional to the voltage.  A resistor has this characteristic.  However, many loads on the grid are motors, which have a inductive component.  Some loads are capacitive, but generally the inductive loads outweigh the capacitive ones, and the grid as a whole has a inductive load on it.

You don't say where this is, but perhaps in your local area once whatever industry you have shuts down for the day, the overall grid load becomes capacitive.

If you are going to dump power onto the grid, your equipment has to be able to source current that is somewhat out of phase with the voltage.  Again, generally the current lags the voltage, but for robustness you should be able to tolerate a significant power factor either way.