Where2 - Thank you for your concern.
The manufacturer recommends using the standard 4500W element. This provides optimal power transfer since the circuit does not have a buck/boost stage like a normal PV controller. The controller provides power to the lower element. Leaving the upper element connected to mains power allows partial heating on cloudy days. The panels are wired in series. As the sum voltage goes up so does the power. Maximum working values are as follows:
current - 10A
Voltage - 200V
Panel nameplate watts - 1280W
He lists various approved combinations without going into detail as to why, though again, I think he is trying to prevent "Tim the Toolman" mentality and the resulting failures. The mounting plate doubles as a heat sink and is inadequate for much more than 700W. The manufacturer recommends a bigger box for higher wattages. He is adamant about what box to use, probably the result of bad prior experience with unknowing customers. I think a real heat sink or even a bigger slab of aluminum would be a better approach. He certainly deserves credit for avoiding cooling fans.
I was discussing this device with a tinkerer friend and fellow dumpster diver. He offered me two large solar collectors as an alternative. After considering the plumbing and various safety mechanisms that would be required I decided the PV approach would be cheaper and simpler. We get a lot of freezing weather here.
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