Is it necessary to power-stretch when installing carpet?
According to CRI (Carpet and Rug Institute, a non-profit industry supported professional standards and certification organization) 


  The use of a Power Stretcher is MANDATORY.  Devices used as a substitute for, or an attachment to such devices that penetrate through the carpet backing may cause injury, damage carpet or substrates, or result in inadequate stretch.  Such devices are not acceptable.


They also publish installation standards (CRI-105 for residential carpet installation).

Now, the reality is that I have installed carpets using just a kicker, but I always had a helper with a makeshift "T" shaped stretcher/holder and this was in small rooms only.