To expand on this 21% yearly rate, my notes say that in July 02014 the price on the Solarserver page was €0.55/Wp, presumably for the "low cost" category, which is now (February) at €0.070/Wp. That drop by a factor of 7.86 over 10 years and 8 months (128 months) works out to 1.62% per month (√√√√√√√7.86 ≈ 1.0162) which is coincidentally 21.3% yearly growth in watts per euro (a 17.6% cost reduction per year).

This is staggering, even at its current level. €0.070/Wp at a nominal 15% capacity factor is €0.46/W; at a 5% interest rate, assuming no aging, that's €0.74 per gigajoule, or, in the quaint non-SI units more commonly used for trading energy, €0.0027/kWh†, €0.029 per liter of diesel, 10¢ per gallon of gasoline, or US$4.60 per barrel of oil. And it's pure, undiluted exergy; you incur no Carnot losses to use it to drive motors or train neural networks.

The current WTI oil price is US$68.20 per barrel of oil: https://markets.businessinsider.com/commodities/oil-price?ty.... That makes solar energy fourteen times cheaper than oil, or more than thirty times cheaper if you're using it for transport or electricity.

The US's current policy of imposing prohibitive import tariffs on solar panels is similar to the Arab oil embargo of 01973, but self-imposed, attempting to prolong the energy crisis that began at that time.

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† Not €0.27/kWh or even €0.027/kWh. €0.0027/kWh. 0.28¢/kWh.