Try it out and you will know
There are a bunch of free AV solutions and you will get a bunch of different answers.
The resource usage is really a moot point in regard to the question at hand as one person will say it affects their system and one will say it is not noticeable."Light" is a vague term and open to interpretation,what is light on my system might be not so light on yours.Unless it is a marked increase in resource usage from one program to another you are probably never going to notice the difference and detection & protection should be the deciding factor.I would rather sacrifice some extra resources than sacrifice on protection.
I used KIS for years and listened to how heavy it was on your system and the how the resource usage was so high.Never once did I have a problem running it on any PC I had it installed on.
Resource usage while important really depends on many factors as to how it is going to run on your computer.You will never really get a true answer to this question in this thread or anywhere else.The only way to know for sure is to try them.
There are the same type of threads already posted in the forums.Personally I would skim through them and pick a couple you might want to try and give them a go.
I have used many programs antyvirusowych but for me, the best is Avira Free is so install and forget, this added Zemana Anti-Malware Free Premium and Mbam and is well do not need anything more.
My voice is AviraFree.