[dragontamer]

I don't know the answer to your question, but lets assume 20%.

If PVs are 20% less efficient in the desert, you'll need 25% more PVs. (Ex: Instead of buying 1MW worth of panels, you need 1.25MW of panels to generate only 1MW of power)

If PVs are lol 90% less efficient in the desert, you'll need 900% more PVs (instead of buying 1MW worth of panels, you need 10MW worth of panels to make 1MW)

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Because PVs are the "expensive part" of solar power (and land is really, really cheap, even German Farmland), we're more interested in reducing the number of PV-panels to buy, rather than reducing the cost of land acquisition.