Convert lumens to watts with the following formula: Watts = Lumens / (Lumens per Watt), or W = lm / (lm/W). The number of lumens per watt is dependent on the type of light and the manufacturer. Incandescent light bulbs are typically between 12.5 and 17.5 lm/W.
A light's lumens per watt value is generally specified by its manufacturer. If you know the brand and type of light, the specifications can be obtained from the manufacturer, which often lists such information on its website. If these values cannot be obtained from the documentation, you can, conceivably, measure both of them. However, that requires measuring equipment, at least an electrical multimeter and light meter, and the know-how to use those tools effectively in the given situation.