A dimmer works so that it pulses out current to the lamp. Each time the current drops to zero, ie when the alternating current changes direction. (50Hz AC passes through the zero point 100 times per second, once up once and once down for each period then the dimmer restricts the current so that it remains zero for a moment, how long is determined by how much you dim.
Then the dimmer releases at full power for the rest of the period. This is repeated for each of the 100 periods every second.
The finesse of this is that as long as the current is throttled to zero, there will also be no power loss (power P = U x I, I is zero, the power will be zero). And when the dimmer opens fully, current flows through the dimmer, but then the voltage across the dimmer is zero (or almost 0), the entire voltage settles over the lamp.
The power loss in the dimmer is then again in principle 0 acc. same formula as above. There are dimmers that work just the opposite, they start each period by conducting current, but throttle the current towards the end of the period.
In practice, there is a small loss effect in the dimmer, the voltage does not really become 0 when it conducts, and it consumes a weak current for its regulation even during the off part of the period. But this power loss is in principle negligible.
So a dimmer consumes no, or very little energy.
On the other hand, a lamp that is dimmed is not as effective as one that shines at its full effect. A 10W lamp that is dimmed consumes 10w. A 100w lamp that is dimmed to the same brightness as the 10W lamp consumes significantly more than 10w, the difference goes away as heat. I do not know exactly how big the difference in efficiency is.
Do you save power by reducing the lighting with a dimmer?
A dimmer uses a thyristor, a circuit that only lets the current through when the voltage exceeds a certain level, which is set with the knob on the dimmer. Instead of letting the current through during the entire oscillation of the alternating voltage, you only let it through during the time when it is at its highest, the rest of the time no current flows at all.
Since the filament still does not have time to cool down as fast as the voltage fluctuates, it simply radiates the energy that is supplied on average. The leakage current in the electronics is negligible.
There are two types of dimmers. One is a pulse dimmer and the other is the more common type with varying resistance.
A pulse dimmer sends current in pulses, hence the name. This type of dimmer will save energy and is the only type that works on LED lights.
A dimmer with varying resistors, on the other hand, uses a resistor to reduce the amount of current that reaches the lamp. But that current is used and becomes heat in the resistor. Therefore, the same amount of energy is drawn all the time, regardless of what the dimmer is set to.
Is it true that a dimmer consumes about the same amount of current, even if it has been turned down?
This is partly true for the most common type of dimmer with a near-commutated thyristor. There you regulate the effect by lighting at different points in the halfway point, the later the weaker light. However, this means both an inductive phase shift between voltage and current and harmonics, both get worse the later in the half-wave you turn on
This means that the current does not decrease in proportion to the effect when you dim down and get an increasing reactive proportion in both the fundamental tone and the harmonics. An electricity meter should measure active power, not electricity, so the cost of electricity is reduced when you dim down. But you increase the pollution of the electricity grid with harmonics and reactive effect.
The lamp definitely draws less power if you dim it. However, the efficiency decreases significantly. Two lamps that are dimmed to 50 percent brightness draw much more power than one, but at the same time noticeably less than two.
Dimmable fluorescent lamps do not suffer from this problem. On the other hand, they do not change color either. Most people who dim the light are looking for the redder light rather than poorer lighting.