Matthew's Journal

I assume it's a modern dimmer that uses a TRIAC circuit to switch the light on and off rapidly. I think they contain capacitors to smooth out the choppiness of the current too.

1. So up to the point that you come to turn the light off, energy use is the same for both cases. If you then turn the switch off, energy use stops, whereas when you turn the dial down a small amount of energy is used until you turn off the light. Therefore dimming and turning off uses fractionally more energy. However, when you come to turn it on again you save the same amount of energy in the dimmed down state (provided you dim up and down at the same rate). But the circuit will use a very small amount of energy to drive the transistors in the dimmer. So the answer is turning it off immediately saves a tiny fraction of energy, namely the energy consumed by the during the period of dimming down and up. Dimming the switch to minimum before turning it on and off uses a minimal amount of extra energy.

2. Light bulbs burn out when the tungsten in a section of the filament evaporates away so much that there's a break. Switching on from cold means more current is drawn because cold filaments have more resistance, but the warming up happens very quickly. This surge can cause the bulb to blow (it's why bulbs often blow when you turn the light on). You would think that turning up the dimmer slowly is like a soft-start device such as a thermister, which will extend the life of the bulb, but we're chopping the current, rather than reducing the current draw, so a dimmer is a very poor soft-start device. And good slow-start thermistors only minimally extend the life of bulbs anyway. So dimming the switch to minimum before turning it on and off will result in a minimal life extention for the bulb.

That's what I think, anyway.