UV systems for well water use a germicidal lamp that emits light at two hundred fifty-four nanometers. As water flows through a stainless chamber around a quartz sleeve, that light scrambles the DNA of bacteria, viruses, and parasites like Giardia or Cryptosporidium so they can no longer reproduce. It kills up to ninety-nine point nine nine percent of them without adding any chemicals or changing taste.
It is not a filter. It does nothing for iron, sediment, hardness, or chemicals. Those have to be removed first, or they coat the sleeve and block the light. Most setups put a five-micron sediment filter ahead of the UV, plus an iron filter or softener if your water needs it. The UV goes last, right before the water reaches the house.
For a three-thousand-square-foot home, size it to your peak flow—usually eight to twelve gallons per minute for a typical two-to-three-bath house. Look for NSF/ANSI fifty-five Class A certification; that is the one validated for water that may actually be contaminated. Class B is only for already-safe water.
Installed cost runs five hundred to fifteen hundred dollars. The main ongoing expense is the lamp, replaced once a year for fifty to one hundred fifty dollars. Electricity is minor. It needs power, so during an outage you lose protection until the grid comes back.
Start with a lab test for coliform and E. coli. If either shows up, a Class A unit is the standard fix. If the water is already clean, it is still a solid preventive layer for a private well.