While a pool light is optional pool equipment, every pool should have one. The main responsibility for any pool owner is pool safety, and more precisely, to promote an environment that is conducive to safe swimming as well as safety in and around the pool. Adequate pool lighting is not just for ambiance and aesthetics; the lighting also makes swimming at night safe because of visibility under the night sky.
The illumination in the pool must be bright enough and sufficient to clearly gauge the depth of the water. While there is no universal code, most industry experts and safety advocates agree that an pool light should illuminate at ½ watt per every 1 square foot of the surface area of the perimeter of the pool.
For example, if your pool is 20' wide and 40' long, then the surface area is 800 square feet (20x40+800). Half of 800 is 400 so at least a 400-watt light would be recommended for the pool in this example. But, due to the depth of water, a higher watt output would be the true recommendation, perhaps a 500-watt light.
Since this discussion is about pool lighting as a safety feature, please keep in mind that water is a real conductor of electricity, therefore, electrical shock or electrocution can occur if live electrical current comes into contact with the water. Make sure that the underwater pool light is protected by ground fault circuit interruption (GFCI). If you have questions about electrical and/or GFCI protection for an underwater pool light, you are advised to speak to a LICENSED ELECTRICIAN in your area.
You can read the Pool Manual for additional information about pool lights, including how pool lights increase safety in the pool.