Create dump load with a schmitt trigger and controller?

First:

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Then decide what you're going to do with the excess energy. If you are going to "harvest" 300 watt hours / 24 hours, I would tend to have 2nd through 3,000th thoughts about the viability issue.

There is -ahem- always the option of dumping the energy into an appropriate added battery. Sorta resolves more issues than it creates.