Hey man!!! Have a 5x5 in action for about a year. If you are only gonna use it for veg, one light will be fine. If you are gonna use it for flower at all, I would suggest two lights. Not to harsh your mellow, but you'll have issues in the corners with light strength. I have a Photontek 1000W and still have corner issues. Others I know that have a 5x5 have the same issue. Would suggest two light designed for a 3x5. Also, if you go with two lights, you can adjust for different plant height and/or stages of growth. Just my 2 cents...
Also, someone just the other day on here was having an issue with a AC light and seemed to think it was a design flaw. Can't remember who it was at the moment
, but maybe they will see it and chime in.