Do DC adapters consume energy when no device is drawing DC current?

I'm not finding the UL specifications, but the U.S. Energy Star specifications require less than 0.5 W for up to 10 W chargers and less than 0.75 W for 10-250 W chargers.

I forget where, but I think 80 plus or similar has requirements for idle desktop power supply usage (with a small amount more allowed for WOL).

I am not finding it, but Energy Star again has a rating of less than 2 watts (or some complicated equivalent).

The thing that I've noticed really draws power is the cable modem for TV's. Unplugging that is the same as all the phone chargers + laptop chargers combined.

Yes, don't be fooled... they consume a "quiescent current" which although very low, it is a value that can be measured. This is to power up the sensing circuitry so they know that something is or isn't plugged. It is usually negligible but add many of them and things start to escalate... not a lot, but still something if you are really worried about that extra couple dollars on your bill. Once you plug in a device, then the power consumption will drastically increase to supply the required charge.

I hooked up a Lenovo 65-watt adapter to a good Wattmeter and it drew less than 1 watt after the first "surge" of plugging in (2 watts). I tried an Apple adapter and it did not even register.

Ordinary good quality adapters do not draw appreciable current and can be left plugged in without any concern.

I trust this helps.

All power adapters have some parasitic loss inside them such as:

• switching loss from hysteresis of the transformer
• switching loss from transistors (leakage current)
• Partial conduction from movs/x/y caps (age/wear)

However like Mario has advised, they do ramp up/down subject to demand, but only for smart chargers like your laptop and not so much for basic devices such as wall warts.

This really depends on the inner workings of the individual adapter. A cheap one might just continue running, while a more elaborate one will turn itself (almost completely) off.

For example, Nintendo's power adapters for the Wii U and 3DS are basically dormant when not connected (the current is simply too low for measurement; at least in my case).

If you're curious, you can buy an energy meter, which is like a plug you put between the outlet and the device you want to measure. It will tell you the current amount of power consumed as well as the consumption over time.