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Bureau of Energy, Ministry of Economic Affairs, R.O.C.

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Does turning off a light really save energy?

The inrush current of fluorescent lamps during turning-on procedure could be as high as 5 to 40 times the current during normal operation. Taking into account the power consumption of the ballast and the lamp, a typical 40W fluorescent lamp consumes approximately 50W. Assuming the turning-on procedure takes 0.4 second, which is the norm of electronic ballasts, the additional power consumption due to the extraordinary high current during turning on the lamp is approximately 0.000028~0.000224 kw.hr, equivalent to the power consumption of lighting the lamp for 2~16 seconds. Frequently turning on and off a fluorescent lamp will reduce its lifetime and increase the maintenance cost in lamp replacement. Assuming the cost of replacing a 40W lamp is 60 NTD and an electricity price of 2.59NTD per kW.hr, turning off the lamp when it is not in use will result in energy-saving and money-saving, as long as the time to the next turning-on is longer than 2.94 minute. In general, the period between turning off and on a lamp in our daily life is longer than 3 minutes, and turning off the not-in-use lamps does contribute toward energy-saving.

Update: 2018-04-23
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