>even when idle, a UPS wastes energy
A UPS isn't just a battery; it's an AC > DC > AC converter running all the time. This conversion is never 100% efficient. Typical small UPS units run at 85-95% efficiency in "online" mode. That means if you plug in a device that draws 100 W, the UPS might draw 105-115 W from the wall just to feed it. That extra 5-15% is pure heat loss, and you’re paying for it.