Calculation of PV daily energy yield
Irradiance is defined as the power of electromagnetic radiation per unit area incident on a surface.  In the example you've given, that should be kW/m², not kWh/m²/day.

Energy is defined as power expended over time, with the kilowatt-hour (kWh) being a unit of energy equivalent to one kilowatt of power expended for one hour.  Consequently, one kilowatt expended for 30 minutes would be 0.5kWh, ( as would 500 watts expended for one hour) and 1kW expended for one day would be 24kWh.

Then, being irradiated by 1kW/m² falling on, and normal to the surface of a perfect 1 square meter PV array, it could supply 1000 watts of powwr to a load for as long as the irradiation persisted.