In article , Andy Burns wrote:
On 18/08/2007 18:59, Marsbar wrote:

Switch on only when required. What made you think that switching a
fluorescent light on and off wasn't a good idea?

It was either a common misapprehension, or used to be different with
older tubes. I certainly remember an "order" going around school in the
early 70's to NOT switch off lights during break-times as it took more
electricity to re-start them, than to leave them on for 20 minutes.

That is a common myth. Any extra power consumption surge during
starting amounts to the amount of energy consumed in a second or two of
steady operation - or less.

However, the cost of bulb wear from an extra start could require several
minutes of off-time in order for electricity savings to outweigh that.

How many minutes? This depends on the bulb cost, electricity cost,
starting method, and when the bulbs were made.

Modern fluorescents suffer less starting wear than older ones.

The "break even time" is less for ones 4 feet and longer than smaller
ones. Lower wattage bulbs cost even more than 4-footers, and lower
ratio of power consumption to bulb cost increases the "break even time".

The "break even time" varies with starting method because different
starting methods cause different amounts of starting-related wear:

"Program Start" - this is used in some CFLs. The bulb does not come on
at all until a fraction of a second to about a second after power is
applied, then turns on without blinking. It may have a "rapid fade-on"
during a fraction of a second.
This causes the least wear, and is often used in CFLs of Philips and
Sylvania brands (and some others but I can't remember who and I have not
tried them all).

"Rapid Start" - bulbs come on instantly very dim, usually slightly
flickery, stay dim for about half a second to a second, then quickly
brighten over a fraction of a second. This is next-best to "Program
Start" for minimizing wear from starting.
"Trigger Start" refers to a variation of "rapid start" used on bulbs
designed for "Preheat Start".

"Instant Start" - The bulbs are on instantly. Sometimes the brightness
makes a sudden slight upward jump a fraction of a second after starting
when the filaments achieve normal operating temperature. This is worse
than "program start" and "rapid start" for starting-related-wear.

"Preheat Start" - usually has a "glow switch starter" or "glow bottle
starter", rarely an electronic alternative. Bulbs usually blink a few
times before they start and stay started. Since each blink causes
starting-related-wear, this method is worst for starting-related-wear.
Fluorescent lamps using this starting method, especially with bulbs 22
watts or less, are likely to have break-even times in/near the 15 minute
to 1 hour ballpark, and should be left on rather than being turned off and
back on shortly later.
Electronic versions of starters that make the first starting attempt
successful greatly reduce the starting-related-wear. Electronic schemes
that make some determination when the filaments are properly preheated as
well as making the first starting attempt successful are at least
arghuably "program start" schemes.

===========================

Now, how bad is it to turn off and back on a fluorescent lamp?

Case 1: 4-footer, 32 watt T8, instant start costing $2. I am guessing
that a start with an instant start ballast takes 20 minutes off its 20,000
hour life. I am assuming also that this is with an electronic ballast
that improves energy efficiency and also mildly underpowers the lamp/bulb
(expect about 90% of "catalog" light output from the lamp/bulb), and
per-bulb power consumption could be about 30 watts, usually not exceeding
32 watts. Another assumption - electricity cost 11 cents per KWH, which I
believe is close to current USA average residential rate.

Starting wear taking 20 minutes off the life of a $2 20,000 hour bulb
costs .003333 cent. (Actually slightly less, since these bulbs are rated
to last 20,000 hours with one start using rapid-start-method every 3
hours, and will last slightly longer than 20,000 hours if used
continuously.)

To consume .003333 cent worth of electricity at 30 watts and 11 cents
per KWH (.33 cent per hour) only takes .0101 hour, about 36 seconds.

Case 2: 15 watt spiral CFL purchased at a higher-side price of $7,
instant-start. Assuming that the filament here is optimized a bit more
for enduring starts, so I guess 15 minutes of life lost per start. (It
could easily be 10 minutes or less.) Also, rated life expectancy 6,000
hours. (I know, now they make ones rated 7500 or 10,000 hours. But I
want to be a little conservative here!)
The life rating is with a start every 3 hours. So if it lasts 6,000
hours with a start every 3 hours and a start costs 15 minutes, then
continuous operation avoids 2,000 starts over 6,000 hours and would add
500 hours to the 6,000 hour figure and make it 6,500 hours.
Assuming that all my numbers here are good including ones that I am
halfway pulling out of a hat, a start costs about .027 cent. If you get
these bulbs in a $10 4-pack, then a start costs about .0096 cent. If you
get these bulbs in a promotional $10 6-pack, then a start costs about
..0063 cent.
Now, to balance against 15 watts of power consumption at 11 cents per
KWH (.165 cent per hour):
.027 cent per start ($7 bulb) means "break-even" at 10 minutes
.0096 cent per start ($2.50 bulb) breaks-even at about 3.5 minutes
.0063 cent per start ($1.67 bulb) breaks-even at about 2.35 minutes

Lower wattage CFLs will tend to have longer "break-even" times, higher
wattage ones will tend to have shorter "break-even" times.

Break-even time is also inversely proportional to electricity cost. It
will be a little shorter in the metro areas of Chicago, Philadelphia and
NYC.

================================================== =

Where starting wear on lower wattage CFLs is a big issue, consider cold
cathode ones. Those do not suffer any significant starting wear, and are
often even rated for heavy blinking duty. They also have longer life
expectancy than hot cathode ones of same wattage even without starting
wear.

Downsides:

1. Ones over 3 watts are mostly available from online lightbulb sellers
such as bulbs.com. Even there, they are largely limited to about 8 watts
or so as of last time I checked.
(A 3 watt cold-cathode model I have seen at Home Depot - of the N:Vision
brand. Look for it being rated for dimming, probably also blinking, and
life expectancy 20,000 or 24,000 hours or so.)

2. Cold cathode ones produce somewhat less light than hot cathode ones of
same wattage.
An 8 watt cold-cathode one only slightly outshines a 25 watt
incandescent, while a 9 watt hot cathode one is about as bright as most 40
watt incandescents.

- Don Klipstein )