The following research should provide you with a solid understanding of the concept and priorities with ultra violet water treatment.

Ultra violet water treatment sterilizes the bacteria in the water. Not actually killing the existing individual bacteria cells but preventing them from multiplying.
Most manufacturers do not clarify specifically, but UVA lamps are the most widely used.

In regards to the question of "power wattage per cm", most specifications are relative to the dwell or contact time of the light to the water.

Instead of an ultraviolet lamp physically resting in a water tank, these applicators place the UV lamp in a tube which the water is then pumped / circulated through.
The UV sterilizer lamp is usually located after the water source, RO, or softening system and treats through recirculation.

Cooler water temperature (60-63f) minimizes bacteria growth. Many new pressrooms have water chilling equipment or utilize existing building chill towers to keep their water cold.

It is recommended that the UV sterilizer be installed as the final step of treatment and located as close as possible but just prior to the fountain solution mixer.
Most of the water is treated prior to the mixing of fountain solution concentrate.
Once the water is clean at the source, it can be proportioned with the fountain solution concentrate.

Biocide additives within the fountain solution concentrate minimize the growth of bacteria in the press dampening supply lines.

This being said, ultra violet is not the perfect treatment for all applications. Minimum water characteristics must be met prior to the successful operation of the ultra violet appliance.

It really depends on the source and condition of the water. Once the quality of the water source has been determined, you will need to look at things that will inhibit the UV from functioning properly such as excessive iron and manganese, dissolved solids, suspended solids, and turbidity of the water.

Iron and Manganese will cause staining on the quartz sleeve and prevent UV energy from transmitting into the water levels as low as 0.03 ppm of iron and 0.05 ppm of manganese. Proper pre-treatment is required to eliminates this staining problem. Total iron count less than 0.3 ppm and Manganese less than 0.05 ppm.

Total Dissolved Solids (TDS) should not exceed approximately 500ppm. There are many factors that make up this equation, such as the particular make up of the dissolved solids and how fast they absorb the available UV energy. Calcium and magnesium, in high amounts, has a tendency to build up on the quartz sleeve, again impeding the UV energy from penetrating the water.

Turbidity: is the inability of light to travel through water. Turbidity makes water cloudy and aesthetically unpleasant. In the case of UV, light levels over 1NTU can shield micro-organisms from UV energy, making the process ineffective.

Suspended Solids: less than 10 ppm and need to be reduced to a maximum of 5 microns in size. Larger solids have the potential of harboring or encompassing the micro organisms and preventing the necessary UV exposure. Pre-filtering is a must on all UV applications to effectively destroy micro organisms to a 99.9% kill rate. Hydrogen sulfide less than 0.05 ppm

Hardness level: not to exceed 7 grains per gallon.