I removed the 60 feet of cable, and re routed it to fill the back of the upper panel.
The cable dissipates 270W, so I figure I have at least 200W under there.
To reduce the heat loss out the back, I put two layers of 1" thick Styrofoam.
The snow melted to slush and then flowed away as water, but the wet heavy snow did not simply slide off in a chunk as hoped. I put a big handfull of snow on the heated panel. It slid about 4 inches, and sat for 5 minutes before sliding to the bottom lip.
In all I ran the heater for 1 hour, and only mostly melted a 3" X 14" X 12" area
Need to get the snow to slide off instead of trying to fully melt it.
It looks like the panels surface has quite a bit of friction at my 26.5 degrees when the snow is very heavy and wet. Will try some rainx to see if the snow slides better on that.
Did some further test and found that if the panel is heated to over 100F, that a hand full of snow will melt immediately and slide right off, due to the thermal mass of the heated panel. With snow on the panel when the heaters are turned on,a much higher wattage would be required, and that will not be a practical, reliable, or economical approach to clearing the panels. A mechanical system may be the best overall solution, as once the sun hits the cleared panel, it will melt off the small amount of snow that remains.