I did it again. I cut my building's lighting electric usage by yet another third.
Now I'm using a light-sensing photocell to automatically switch lights on and off in my building. These lights used to be
on 24x365. Now the lights switch off when there is adequate ambient light.
It would seem that a photocell would save about 50% in lighting costs, as the average 24 hour period is about 50% daylight. But it isn't that simple: For safety reasons, I want the lights to come
on before it gets dark, and I want the lights to turn off after
it gets bright out side. Furthermore, not every day is loaded with
full sunshine. After chugging through the numbers, I pessimistically
calculate the my lights will be on about 65% of the time.
When I started this lighting efficiency effort, I was already considered efficient: I was using
CFL bulbs in all my fixtures. Yet I was still able to reduce my lighting costs by about
85%.
Further savings are going to be a significant challenge as I already use low-wattage LEDs, and they've been extremely reliable. Considering that my current bulbs have been extremely reliable (zero failures), it is a good bet that I won't have a bulb failure over the next 5+ years. Perhaps by then, even more efficient bulbs will be available.