Re: Not really
Just rectify the mains (inside the room, obvs..) onto a DC supply, put a soddin' great capacitor on it, then run an inverter off the stabilised DC. You might manage to exfiltrate at 1bit/hour off that arrangement :-)
In fact, a 12V car battery float charged feeding a cheapo 300W inverter from Maplins (oh, hang on) would do the job nicely.
But if an attacker has uncontrolled access to your building riser, you've got plenty of other things to worry about, too.