Question

How efficient is your cell-phone charger if left plugged in without a phone attached?

Answer 1

Inside virtually every phone charger is a transformer. Transformers have a finite resistance, and hence there will always be current flowing through them if they are plugged in, even if there is no load (i.e. nothing charging). That's basic physics.

But the obvious follow-up question is: how much energy does it use?

Estimates vary, but its certainly not much. some claims (without reference) that its 1-5 watts. others claims that it's less than half of a watt. And further claims that this represents one hundredth of a percent of a typical person's usage. We can get more figures for different standbys, and gives that for a phone charger at about 0.4W. You can consume that power for 380 days to get the same amount of energy as a hot bath. Of course with very many people using phone chargers, that can add up to a very large amount of electricity.

With no phone/device attached to a charger, it is still possible for the charger to consume some energy. How much energy is dependent on what exactly is inside the charger.

The simplest charger could potentially be a transformer and a linear regulator. In this case, the transformer would step down the wall voltage and the linear regulator will pump out a relatively solid voltage to charge your device. In this case, the charger itself is always drawing some energy. A linear regulator converts voltages, say 9V to 5V, and dissipates the rest of the power as heat. As a result linear regulators can be fairly inefficient. The upside however, is that they are generally much cheaper than most alternatives. A simple rule here would be that if the charger is warm to the touch with no phone/device attached, you're wasting energy.

Charger technologies have certainly changed and adapted, especially with the growing concern for being green. As a result, many manufacturers have opted for more energy efficient means of charging devices. The message you receive from your cell phone telling you that it is fully charged is an example. Modern devices are usually smart enough to limit their power consumption once they are fully charged.