Estimate charging losses.
A charger delivers less energy to the battery than it draws from the wall, and the difference leaves as heat. Dividing the battery capacity by efficiency gives the wall-side energy, and subtracting the capacity isolates the loss. At eighty-seven percent efficiency a 58 Wh laptop battery wastes roughly 9 Wh per charge — a real loss, but one that annualises to a trivial amount of money, which is the useful conclusion.
Wall-side energy
Energy drawn = battery capacity Wh / charger efficiency
Annual waste
Annual wasted kWh = (energy drawn - battery capacity) x charges per week x 52 / 1000
Modern gallium nitride chargers meeting current efficiency standards typically run 88 to 94 percent at moderate load. Old unbranded adapters can fall below 70 percent and run noticeably hot.
Very little now. Standby draw on a certified charger is typically well under a tenth of a watt, which is a few pennies a year.