Another obstacle to solar electricity is photovoltaic efficiency. If you live in the desert, a single square meter of solar panel could receive the equivalent of more than 6 kilowatt-hours of energy in the course of a single day. But a solar panel cannot convert that entire amount of energy to electricity. The efficiency of a solar panel determines how much of that power is usable, and most commercial solar panels on the market in 2013 have efficiency ratings of less than 25 percent. The more efficient a panel is, the more expensive it is to produce. Barring any major leaps in technology, efficiency ratings beyond 33 percent are unlikely in the near future.
One major problem with solar power is reliability. At best, a solar panel can produce electricity for 12 hours a day, and a panel will only reach peak output for a short period around midday. Tracking panels that follow the sun can extend this prime generation period somewhat, but it still means that panels spend very little of the day producing at maximum capacity. Storage batteries can charge during peak generation and provide a trickle of power at night, but they can be expensive, contain toxic materials and wear out quickly due to repeated charge and discharge cycles.
While solar generation is emission-free, the manufacture of solar panels and related technologies can involve some environmentally unfriendly substances. Nitrogen trifluoride is a common byproduct of electronics manufacture, including those used in solar cells, and it is a greenhouse gas 17,000 times more potent than carbon dioxide. In addition, many solar cells include small amounts of the toxic metal cadmium, and the batteries required to store generated electricity can contain a host of other heavy metals and dangerous substances. As solar technology improves, manufacturers may be able to move away from these potentially dangerous substances, but for now, they mar the otherwise impressive ecological benefits solar power offers.