It's just a coulomb counter you can read from an MSR. But yes monitoring it inevitably consumes some amount of energy. It won't cost anything on a busy system but waking up an idle system to read it will be more noticeable. This is why I no longer use background metrics monitors like atop or netdata. An Intel client CPU can idle below 100mw if you leave it be, but something like netdata will raise that to 5W or worse.
