To calculate how many kilowatt-hours a solar panel can generate when exposed to the sun for an hour, you first need to know the rated power of the solar panel (ie 400W) and the lighting conditions. Normally, the rated power of solar panels is obtained under standard test conditions, that is, under the conditions of a light intensity of 1000W/m² and a temperature of 25 degrees Celsius.

However, in reality, the intensity of sunlight will vary with geographical location, season, weather and other factors. Therefore, we need to first estimate the intensity of sunlight at a given location and time.

Assume that in a specific location, the sunlight intensity is 800W/m² (this is a simple assumption, the actual light intensity will vary according to the location and time). Then, the power generated by the solar panel exposed to the sun for one hour under this light intensity can be calculated as follows:

Power (kWh) = Rated power (kW) × Lighting time (hours)
Electricity (kWh) = 0.4 kW × 1 hour = 0.4 kWh

Thus, a solar panel rated at 400W can generate 0.4 kWh of electricity when exposed to the sun for one hour at a light intensity of 800W/m². Note that this is an estimate only and actual lighting conditions may vary.