When people think of solar power, they tend to think of panels on rooftops.

Weakness: capacity factor Here’s an important distinction to understand: The capacity of a power plant refers to how much power it is theoretically capable of producing if it generated maximum output, around the clock. The capacity factor (CF) of a power plant refers to how often it actually runs, producing power. Here’s a hypothetical example. Say you build two power plants, one nuclear, one solar. They both have 1 GW of capacity, which means if they both produced maximum output, they would generate a GWh in an hour. But they don’t always produce maximum output, or run year-round. Averaged over time, the nuclear plant’s output is about 90 percent of its maximum capability, i.e., has a CF of 90 percent. It will generate 900 MWh (90 percent of its capacity) in an average hour. The solar plant only “runs” when the sun is shining and only reaches maximum output at the sun’s peak. Averaged over time, its output is about 20 percent of its capacity; it has a CF of 20 percent. It will generate 200 MWh (20 percent of its capacity) in an average hour. In other words, to get the same amount of MWh, you need to build four to five times as much solar capacity as nuclear capacity. So how are big solar plants doing in terms of CF? Let’s put it in context. Based on EIA data (thanks to Jesse Jenkins for sorting through it), the current CF of the US nuclear fleet is around 88 percent — nuclear plants are typically expensive and unwieldy to shut down and restart, so they are almost always running, except when they are offline for maintenance. The current CF of the US coal fleet is around 55 percent. Individual coal plants are capable of much higher, but old coal plants are being idled more and more often....