How do you calculate the amount of solar energy the earth receives in one day?

Depends on how big the panels are and the duration (day length? weather?) of sunshine that strikes them.

Each solar panel has a wattage, for example, 200 watts. You can multiply that wattage by the number of hours in a day that the sun shines, for example, 6 hours, and the answer is in kWh (0.2 kW x 6 = 1.2 kWh).