Nearly all photosensors are used to decrease the electric power demand for lighting. In addition to lowering the electric power demand, dimming the lights also reduces the thermal load on a building's cooling system when the building is running its chillers, adding to the energy savings. When considering new building designs, the added solar heat gain that occurs when substantial amounts of daylight enters a space must be taken into account for a whole building energy usage analysis.
When considering the energy savings potential from dimming fluorescent lamp luminaires, it is important to realize that fluorescent lamp systems have lower efficacy when dimmed. This loss of efficacy leads to diminished
energy savings as lamps are dimmed to lower and lower levels. The figure below shows relative light output (dim level) as a function of power.
At 20% dim level, the energy savings is approximately 60% compared to operating the lamp at full power. Ballasts that dim lamps down to less than 5% light output have a maximum energy savings of about 80% compared
to full light output operation.