[michaelt]

  Why do you think "essentially free" marginal
  renewable cost is a falsehood?

Unless you can control the sun, the cost for one extra kilowatt of solar power is the cost of a 1kw solar panel?

[Dylan16807]

That's the capital cost. You pay it up front and never again for the lifetime of the device.

The marginal cost is the price of producing one extra kilowatt-hour on your already-purchased panel.