2.2 Lighting System Costs Impacted by Photocontrol Choice

2.2.1 Energy usage

Photocontrols impact burning hours in three major ways: 1) On/off settings, 2) failure rate (higher means more dayburners), and 3) susceptibility to drift. Quality photocontrols (e.g., with failure rates under 1%) equate to fewer dayburners and fewer burning hours. Low On/Off settings (within safety guidelines) mean lower burning hours. Finally, a photocell in a circuit that does not stress a CdS cell leads to less drift, the tendency for a photocontrol over time to turn-on earlier and turn-off later, adding to burning hours.

Sunrise, sunset - when do they occur? How long is the day? The night? The length of the night and day vary by latitude, about 1 hour to be more precise, at adjacent latitudes. The longest days are in the north and the shortest closest to the equator. Data for each latitude are well recorded in the Astronomical Tables of the Naval Observatory. In brief, a photocontrol that provides 4,175 burning hours at latitude 35E will, all else being equal, provide about 4,176 burning hours per year at latitude 34E.

1) Photocontrol On/off settings and the Daylight curve The Daylight curve below depicts the relationship between light levels and time near sunrise and sunset. This chart is suitable for practical application for most of North America. As you can see, 1 ftc (footcandle) of light occurs about 18 minutes after sunset or 18 minutes before sunrise. Similarly, 3 ftc of light occurs about 11 minutes after sunset or 11 minutes before sunrise. Thus a photocontrol set to go on at 3 ftc should burn lights about 7 minutes per day more than a photocontrol set to go on at 1 ftc.

The next chart shows burning hours at latitude 35E (Los Angeles, CA or Charlotte, NC) for various photocontrol on/off settings, taking into account: dayburners; drift; and, about 5 minutes per day for clouds and fog. More detail about the assumptions can be found in the draft IES Guide.

Because electronic controls typically use less energy than conventional controls, they save energy related costs.

2) Failure rates Failure rates for electronic controls are claimed by manufacturer literature to be less than 1% per year; in general, warrantees are for 4 years or more. Failure rates for electromechanical (conventional) controls are generally found to be higher (worse) and have lower warrantees. In one large survey of utilities, failure rate data were collected; they are summarized in the chart shown.

For a lighting system with 20,000 lights, the difference between a 1% failure rate and a 10% failure rate would be 1,800 trouble tickets. Because utility labor is a higher cost than the material cost, it doesn't take very long for more reliable photocontrols to make up the cost difference and reduce overall maintenance costs.


Conventional Photocontrol Failure % Distribution

3) Drift When overheated (above 70°C (158°F)), CdS cells start to drift. Drift means that the cell suffers an irreversible change toward lower sensitivity and higher control turn-ON and OFF. This overheating can come from three places: 1) self heating due to power dissipation in the cell in conventional controls - this is inherent in the design of conventional controls; 2) heat from the fixture; 3) sun loading.

Discussions about how much drift occurs in conventional controls can be lively. Competitive literature suggests that conventional controls can drift to 75 or more extra burning hours per year in under 3 years of operation. While this sounds high, it is only 12 minutes per day, a few minutes in the evening (due to earlier turn-ON) but mostly in the early morning (due to late turn-OFF). This would occur if a control's turn-ON level drifted by just 1.5 ftc.