How much energy do solar panels produce in a 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).