The electricity only is used as needed. As mentioned before, you can put a 2 watt nightlight on a 20 amp line. the night light happily draws 2 watts. No problem. Ditto your 20 watt electronic device. You may have 200 amp service to a big house, and at night while asleep, you may use as little as zero watts. the powerline is NOT exploding due to your lack of use. Like water in a pipe. it is happy to sit there until you turn on the faucet. There is no reason NOT to add a 20 amp line and use it for 20 watts. or for nothing. The line does not care if it gets used in any way at all. Think of it as a future use, maybe...