This is mistake to think this way!
240 volts with 50 watts light cost the same as 120 volts 50 watts cost the same as IF 12 volts with 50 watts!
Your electric meter measures how many watts you use REGARDLESS how many volts are used!
In other word, if you reduce watts you will reduce the cost of electricity.
As in OHM law, if you had 240 volts with 50 watts of power, and you reduce to 120 volts with same wattage, you'd increase the current twice, and if you reduce to 12 volts with same watts, you'd increase 20 times more curent than 240 volts, meaning you'd need thicker wire to handle the higher current. but the cost of using it is the same!
