Well, UVs are like ummmm x-rays. They always work, but they are more powerful (dangerous) the longer one is exposed, if one is exposed. So, they will work. How well they 'kill' depends on dwell time (how long the plant or animal is exposed), turbidity (how clear the water is) and power rating (how much of the nominal rating of 13 watts is still there, as UV power degrades over time.) Type of design affects this as well, but I'll skip that. And they only work in things that actually go through the exposure.
A 13 watt on a 180 gallon, if the water runs through at the correct speed will kill off most if not all free floating algae and some bacteria. At slower speeds it will kill off more bacteria and probably some viruses. However, at too slow a speed, bacteria and viruses may multiply faster than the UV kills them...so that's at least one good reason to consider a bigger UVs. Some sites claim that parasites can be killed as well, but I haven't seen any studies on that. Personally, I wouldn't get a UV for parasites, as the bulb would need to be quite large, and many parasites aren't free floating, plus there are less expensive and simpler means (quarantine, broad spectrums, etc.) to address that.
If you want to work for a clear looking tank (free floating algae) and also opt to kill of some, but not all bacteria, a 13 watt will work. Be sure to keep the water clear, to set the water flow correctly and to replace it when it's rating gets too low (iirc this seems to be every 6 months.) There are scores of posts out there from people who had water clouded with algae that reported phenomenal success.
Many people opt to have only recently filtered water go through the UV as it is most effective with clear water.