Air con cuts out engine

I got this weird problem. Put in a new alternator recently, now every time I turn on the air condition unit it cuts out my engine!
Engine idling smooth and perfect, turn on air con, the Rev will drop like a stone. Rev it up to 2000 RPM turn on air con, it will drop about 1000 RPM. My local garage checked all the wiring, switches etc. thinks it's an air con issue. when to air con specialist. they say maybe the new alternator not powerful enough. I'm not sure, everything else working fine.
Any advice from you experts out there will be much appreciated.