Solar panels now cost less to buy than they do to install.
Installing solar panels on roofs is significantly more expensive than putting them in fields. Solar panels have ended up on roofs of houses mostly because of subsidies, and partly because some people like the message that they send by having solar panels.
Putting solar panels over parking lots is also more expensive than putting them in fields, because people make holes in the asphalt for support poles and the poles need to be taller, but it's only a little bit more expensive and doesn't need extra land. Because shade over parking lots is a positive thing anyway, I think parking lots are a good place to put solar panels.
An important thing to note about solar panels is that because they have many cells in series, shading even a small part of them (eg with trees growing near houses) greatly reduces their power output.
With solar panel prices having come down so much, two-axis tracking is not cost-effective, but single-axis tracking is still generally worthwhile. 3x concentration of light with fresnel lenses could be viable, but the potential economic benefit would be fairly small, and highly concentrated photovoltaic power is highly impractical.
Inverters are now a significant fraction of the cost of solar panels. If possible, it's cheaper and more efficient to output DC than AC. Many things can already run on DC power; fixed-speed electric motors are the main thing that can't, and they're becoming less common.
But again, the most important thing at this point is the cost of supports and installation.
Photovoltaic power and wind power are unreliable. So, they're both best used with electricity production that can be easily turned on and off, which means natural gas turbines or (some) hydroelectric power.
Cost per average power seems to be something like:
solar-thermal > nuclear > open-field photovoltaic = coal > wind > (typical existing) hydroelectric
Natural gas turbines are relatively cheap, so fuel is most of the cost there.
Solar-thermal power is currently expensive, but heat can be stored with molten salt to produce power at night fairly easily. I also think that power tower solar thermal costs could be brought down significantly. On the other hand, storing electricity in batteries is currently much too expensive, and looking at the state of battery research and manufacturing, there are no signs of that changing soon. (Of course, some people disagree with me there.)
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