the secret is if the output switching device is a relay,
thermal relay, or a solidstate device.
You can tell by the wiring, relay/thermal relay is operated
from the AC line and will have THREE wires, black(hot),
white(neutral), and red(lamp load). It will directly
connect the red to the black when "dark", you can hear a
relay "click" but the thermal relay will be silent and
delayed as a small heater heats a bimetal set of contacts.
Without being connected to anything, an ohmmeter will show
the black and red to be a direct short. the photo cell only
powers the relay while in daylight, the relay actually
"opens" the contacts, turning off the light. backwards from
what you might have expected. they also usually have a high
current rating like 15A.
Solid state devices are TWO wire, black and black, or black
and red. It is wired in SERIES with the hot line and the
lamp. This creates two problems, while "off" the cfl isn't
enough of a load to provide the few volts the ss device
needs to properly operate. This causes the output to pulse
on and off, beating the crap out of the cfl until it fails.
And even when "on" the cfl can't provide a stable voltage to
keep the ss device in "on" mode while operating, sometimes
only providing half wave DC to the cfl.
Cfl's only like fullwave AC power fully "on" or completely
'off'. Anything else and the two fight each other until the
cfl just fries itself. these devices will have a minimum
wattage(load) like 5-20 watts, and a maximum like 600 watts.
There are ways of making a "safe" ss device that switches
just like a relay, but that would require 12 components vs
the 4 used now, and there MUST be a neutral wire connection.
And no one would pay for them ;-)
-larry / dallas