The bulb that first came to be known as zero watt bulb was actually of 15 watt. Conventional energy meters did not record consumption if the load was very small. As a result, the indicator did not move when the load was only 15 watts. That is the reason these bulbs came to be known as zero watt bulbs. At that time there were no bulbs with wattage less than 15 watts, and such bulbs were wrongly presumed to be zero watt bulbs.
Today the bulbs consuming power even less than 15 watts are available. But the convention of calling them “zero watt bulb” still continues.