mAh and Wh differ in their meaning: mAh measures electric charge, while Wh measures energy stored. mAh expresses battery capacity, while Wh shows the amount of energy in a battery. To convert between them, use the formula: Wh = (mAh * V) / 1000, where V is battery voltage in volts. For instance, a 2000 mAh battery at 3.7V has a 7.4 Wh capacity.