This comes down to the efficiency of the solar cell, which "collects" the light energy, and it’s a simple exercise in scale. If you have a collector that is 20% efficient, which is on the high side for solar cells available on today’s market, it will absorb about 20% of the light that hits it. If you shine a lot of light on it (direct sunlight), the total amount of useful light for the production of energy will be much higher than if you shine a very little bit of light on it (ambient light). So, while "ambient light" may be able to generate a small amount of electricity from a solar PV system, it will greatly reduce the actual capacity factor of that system.
