Those hot plates, in my experience, are useless. The only reason we had one on our PA34 cargo plane was to make it legal for flight into known icing. As far as I'm concerned, the whole and entire point of those external hot plates is to check a regulatory box to make the plane legal for FIKI if so certified.
It never once actually kept ice from accumulating and the entire windshield, hot plate included, frosted over on more than one occasion. I recall hoping for strong right crosswinds so I could crab down to the runway using the side window. We checked that the circuit was powered so we knew the plate had power, it was just totally ineffective. Cutting a new pilot loose in that thing for their first low IFR icing encounter was a serious decision.
Now, proper heated windshields with integrated elements work wonders on ice. They also cost $25k per side (twin Cessnas), so they'd better work. The plates bonded to the windshields (on Barons, for example) seem to work OK too though I've never used one. I think the cold air flowing over and under the stand-off hot plates simply overcomes the capabilities of the unit except in the most mild conditions.