What cord length do I need?
Measure the run from the wet niche back to the junction box and buy at least that, because the cord cannot be extended. Pool light cord is a single unbroken run by design: there is no splice rated to sit in a wet niche, so a cord that lands short means pulling the light back out and buying another. Lengths here are 30, 50, 100 and 150 ft, and 100 is the most common for a reason. Too long is not a problem, since the surplus coils in the box, so when you are between two lengths take the longer one.
12V or 120V, and how do I tell which I have?
Look for a transformer, because that is what the difference means in practice. A 120V light runs straight off the circuit and there is nothing between it and the breaker but the junction box. A 12V light needs a transformer stepping the supply down, usually mounted near the pad or inside the panel, and if you have one you are on low voltage. They are not interchangeable: a 12V lamp on 120V fails immediately, and a 120V light on a transformer barely glows. Our shelf is close to an even split between them.
Can I put an LED in my old light housing?
Usually yes, and it is the cheapest way to change how a pool looks. Many LED bodies and lamps here are built specifically to drop into an existing niche and say which one they replace, naming Hayward Astrolite and Duralite bodies and Sta-Rite and SwimQuip niches among others. The gain is not subtle: an LED doing the same job as a 500 W incandescent draws a small fraction of it. Check the wet niche is sound and the cord will reach before ordering, because those are the two things a retrofit does not fix.
What does nicheless mean, and do I want it?
It means the light has no separate housing cast into the wall, so it fits a standard return fitting or a small opening instead of a full niche. That makes it the answer for a pool that never had a light, for a spa or a step where there is no room for a niche, and for a vinyl or fibreglass pool where cutting one in is a job. If you already have a working niche, a standard light in it is usually the simpler purchase. The products that need no niche say so on their own specification.
My light is out. Is it the bulb or the whole thing?
Work from cheapest outward and check the obvious first. A tripped GFCI or breaker accounts for more dead pool lights than anything inside the niche, so reset before you get wet. On an older incandescent body the lamp itself is genuinely replaceable and is the usual culprit. On a sealed LED unit there is often no serviceable lamp and the body is the part. A light that works intermittently, or that trips the GFCI when it runs, is water in the fixture or a damaged cord, and that one is not a bulb.
