Solar power has begun to transform the world’s energy system

> ...people are now putting up a gigawatt’s worth of solar panels, the rough equivalent of the power generated by one coal-fired plant, every fifteen hours.

This is amazing! Whether you believe photovoltaics are the most efficient form of green energy production or not, you cannot argue the impressive economics behind them. Successful engineering has to meet the market at the end of the day.

> are the most efficient form

What does this even mean?

being a sentence fragment, not much! It helps to zoom out to the context of the entire sentence, where the GP says: "Whether you believe photovoltaics are the most efficient form of green energy production or not, you cannot argue the impressive economics behind them"

It's definitely impressive that the cost per watt of a PV panel is roughly 13% of where it was just 15 years ago.

https://ourworldindata.org/grapher/solar-pv-prices

So, if I zoom out as you suggest, this means efficient doesn't mean economically efficient. What does it mean here, then? Engineering efficiency? That would make very little sense, since PV and other generating technologies have different inputs. It makes no sense to compare the efficiency of PV modules converting light to electrical energy vs. the efficiency of combustion turbines converting chemical energy to mechanical kinetic energy.