I would like to add several details to David Kramer’s item “The cost of solar energy production has plunged, but it needs to fall further” in the June 2021 issue of Physics Today (page 27).

First, while the monetary costs are important for investment decisions, they are less relevant in decisions regarding sustainability and decarbonization because they depend not only on where and with what energy source panels are made but also on financing, depreciation, taxes, and government subsidies. Costs should be accrued in energy units and compared with energy outputs in the same units. This ratio is typically referred to as energy return on energy invested (EROEI), which is a useful figure of merit for an energy source. Carbon saved over carbon invested would be another useful figure of merit.

I presume that the costs of solar photovoltaics (PV), particularly for the energy-intensive production of silicon crystals and aluminum frames, have not fallen in energy units as much as they have in dollars. I also presume that the EROEI for solar PV is still rather poor, and storage further reduces EROEI. In addition, one must consider the low duty factor (around 20% in my area) for solar energy.

Second, while solar PV is locally “clean,” one must look at the whole production cycle—including mining of materials, fabrication (particularly of crystalline silicon and the aluminum frames), and transportation—and the energy involved and carbon produced in those processes. All these energy and carbon costs are up-front, and they are only recovered over some fraction of the life expectancy of the facility, which is about 25 years. The disposal process must be considered as well.

Third, I have found the transparency about the costs of solar PV to be abysmal, and I have found it nearly impossible to get detailed information about projects in my region. When public support, in the form of subsidies, tax advantages, or soft costs such as government reviews, goes to such projects, the public has a right to know about them. If the economics and the sustainability of the projects were as good as we are led to believe, I expect that this information would not be hidden behind corporate and government curtains.

David Kramer is right that the costs of solar PV need to fall further, but I would modify that to read as follows: The costs of solar PV measured in energy units need to fall a lot.

Physics Today