The power, or strength, of a UV lamp is measured in millijoules per centimeter squared, or mj/cm2. According to the US Department of Health, the minimum power provided by a UV water filter is 30,000 mj/cm2. You can usually find how many mj/cm2 a UV system is from its product description. If a system is NSF Class B certified, that means it has met test requirements and exceeds the minimum required power of 30,000 mj/cm2.