The PIR on the STV610 is very sensitive (which is good) but things like nearby plants moving in the wind will also activate it, so you might want to take that into account if you're planning on using batteries. However it makes much more sense than batteries if you have a high traffic area to cover, & being able to buy BOTH the STV610 & the mains adaptor- for less than some places are selling JUST the STV610, is a real bonus. There is also a 5mtr cable supplied with the adaptor which is handy but it does run out rather quickly. 10mtrs would have been even better, but for the money its a fair deal.