Cost per watt is what you want when purchasing a panel to install, and you have sufficient space for what you are purchasing. A buyer's guide like that should also consider longevity and reliability.
The link is not a buyers guide. This is simply a table of efficiencies achieved by various technologies. There are plenty of non-buyer's guide uses for this table. Off the top of my head, this could be helpful in deciding which types of cells to invested in to see if a company could reduce cost and become best on a cost-per-watt basis.
Panel efficiency and manufacturing costs alone directly equate to cost per watt.
At 18 vs 22% efficiency means buying ~20% more land or buying 20% more expensive land in a better location resulting in less transmission losses etc. Higher efficiency also means spending less money on mounting and solar trackers for the same output.
In the end different technologies means different optimizations. Steel is vastly superior to wood in many ways, but we build skyscrapers with one and single family homes with the other.
There’s a few largely wooden structures proposed that most people would call skyscrapers, but they generally use quite a bit of steel.
PS: Some people argue for 100m because it’s a round number roughly in the right height range, but it’s common to use 150m in part because that’s just a little taller than the great pyramid of Great Pyramid of Giza.
I agree that vost per watt is the mist important for situations where the install is not space constrained.
On the other hand if space is a constraint, then watts per square metre (which correlates with effeciency) is more important.
Most small-scale installs are space limited. Think RVs, domestic residences, boats, and so on. However large-scale is likely to be capital constrained, not space constrained (large plots in the desert are cheap) so in that case cost per watt is the limiting factor.
It's not so simple to look at one constraint for a rooftop install. When planning an install, cost per watt and watts per square meter interact, as does module size, module weight, low illuminance performance, durability and degradation, appearance, and other factors.
I would argue that efficiency is more important only in offgrid applications where you absolutely need to get a specific amount of energy no matter what.
If you are on grid but space constricted the price per watt or rather LCOE is more relevant.
It's been the cheapest source of energy worldwide, at ~4 cents iirc, and falling. For several years now.
The question is no longer what to pick based on cost per watt: it's solar.
Cost per Watt is relevant as a historical curve.
Efficiency on the other hand provides an outlook. We can get 4x more efficient solar cells (theoretically). We are still getting manufacturing improvements and scaling effects. I would expect solar to break through the 1 cent per Watt barrier based on that.
The system may not produce enough energy to offset the cost of its manufacture, installation, and maintenance. This is more common with PVs than you'd expect.
