I received a new Inspiron 15z last night hooked it up to my S2340L. The monitor works fine, but refuses to go into power saving mode when the laptop display is turned off. When the laptop display is powered off, the S2340L just goes into a back-lit black screen instead of shutting down the back light all together. However, if I unplug the HDMI cable while the laptop screen is powered down and then plug it back in, the screen enters power saving mode as normal.
I've had this monitor hooked up to multiple laptops in the past, Dell and other brands. A M1730, M17x R4, U845W, 3830TG and they were all able to put the monitor into power saving mode when the display was turned off.
I've installed the monitor drivers and set PowerNap mode on, but it doesn't do anything at all.
My guess is the laptop sends a blank signal to the external monitor which keeps it from automatically entering power save.