SOLAR power has become an unlovely adolescent. It used to be a sweet little thing, shiny and new and full of promise. One day it will doubtless grow into a solid citizen, quite possibly a person of substance. At the moment it is stuck in between; no longer a child to be coddled and pampered, but not yet able to pay its own way. This presents a challenge both for the governments who want to see it grow up big and strong, and the companies that have been making money out of its progress to date. No one doubts that it will continue to grow; the question is who will suffer most from the growing pains.

Solar energy is popular because it is clean and abundant. The problem is that it remains expensive. According to recent calculations by the International Energy Agency, power from photovoltaic systems (solar cells) costs $200-600 a megawatt-hour, depending on the efficiency of the installation and the discount rate applied to future output. That compares with $50-70 per MWh for onshore wind power in America, by the IEA's reckoning, and even lower prices for power from fossil fuels, unless taxes on greenhouse-gas emissions are included. The costs of solar are dropping; in some sunny places it may, in a few years, be possible to get solar electricity as cheaply from a set of panels as from the grid, and later on for solar to compete with conventional ways of putting electricity into the grid. But for the moment there would be no significant market for solar cells were it not for government subsidies.

Given that there are subsidies of various sorts in various places, some of which have been very generous, there is a market, and a fast-growing one. According to Bloomberg New Energy Finance, a research firm, there will be demand for 10.5 gigawatts of new photovoltaic-energy systems in 2010, up from just 1.7GW in 2006. The consistent engine of growth over those four years has been Germany's feed-in tariffs, a guaranteed price for solar power that makes every panel installed in the country a profitable investment, at the expense of electricity consumers. For a fair part of that time, global supply was only just keeping up with demand, and prices for solar modules—the assemblies of cells that you might put on a roof, in a field or on a patch of desert—stayed fairly stable.

Last year, though, prices began to drop. One reason for this is that the supply of silicon, from which most photovoltaic cells are made, has increased. Another is that more and more Chinese companies with low costs are coming into the market. When the California Solar Initiative, a scheme for getting solar panels onto roofs, got under way in 2007, just 2% of the modules used were Chinese. In the fourth quarter of 2009, according to Nathaniel Bullard of New Energy Finance, the figure was 46%. Yingli Solar, a Chinese manufacturer which has been making cells for less than a decade, expects to ship a gigawatt's worth in the coming year.

The lower the price of a module, the more attractive a feed-in tariff looks. As prices came down, installations of solar cells in Germany shot up. The country accounted for roughly half of new installations around the world last year. As a result, the feed-in tariff has been reduced, and will be reduced again this summer, dampening demand and favouring low-cost producers. Q-Cells, Germany's largest wafer manufacturer (the wafer is the step between the refined silicon and the finished solar cell) has seen its market value drop by half over the past year. Margins for cell producers seem to have fallen even more dramatically. An increasing number are outsourcing production to China.

Some companies, such as Germany's SolarWorld, which does everything from refining silicon to installing panels, have respected brands that allow a bit of premium pricing. But for most, solar panels are more or less a commodity, which favours big producers with access to cheap capital. “Cost of capital is now king,” says B.J. Stanbery, an industry veteran who founded a novel solar-cell manufacturer called HelioVolt.

The king seems to live in China. According to Mark Pinto, corporate technology officer of Applied Materials, which is the biggest supplier of machines to make solar cells, half of the world's production capacity is already in China. Manufacturing capacity, he says, will grow faster than his company had expected this year, and two-thirds of the growth will be in China. The company has just opened a new research and development centre in Xi'an.

Cheaper panels have led to a renewed interest in power-plant-sized installations. When Spain introduced a generous feed-in tariff in 2007 it triggered the first boom in these “utility-scale” projects, with two gigawatts installed before the blindsided government capped the tariff. Since then the utility market has slumbered, but if cells are cheap it does not take much to make stacking them up in the sun look attractive. When that happens, as Spain's example shows, it can be done quickly.

The company with the largest utility-scale pipeline is First Solar, one of the pioneers of what are called “thin-film” cells. These use less silicon than their thicker cousins, or none at all, and can be cheaper to make, but they are often less efficient; better for deserts, or sunny lots on the edge of town, than for roofs where the area available is limited. First Solar's panels, which use cadmium telluride instead of silicon, cost less than a dollar a watt to make, which makes them cheaper than any silicon-based rivals. Its output is about a gigawatt a year. Last year it signed a deal to start manufacturing in China and entered Forbes magazine's list of fast-growing tech companies in the top slot.

But some hedge funds have been shorting First Solar on the basis that plummeting prices for normal cells will undermine its advantage. It is also possible that it will be leapfrogged. There are various start-ups looking to make even cheaper thin films, such as Nanosolar, in California, and HelioVolt, though they have yet to show that they can reliably produce efficient cells on a large scale. Mr Stanbery says that now First Solar has proved the disruptive capability of such approaches, larger companies looking to enter the solar business are interested—and leading makers of silicon cells, preferring to disrupt than be disrupted, are hoping to add similar technology to their portfolios.

The utility market also serves to highlight the flaws and expense of solar power. A typical utility-scale installation produces power at only a fifth of its maximum capacity, thanks to clouds, night-time, dirty panels and so on. To replace a one-gigawatt coal plant running at 70% of capacity with solar panels would require about half of the 6GW installed worldwide last year.

This is one of the arguments for looking instead at another solar technology, solar thermal, which uses mirrors to concentrate heat, produce steam and thus drive turbines. Efficient solar-thermal plants can in principle be built on the same sort of scale as gas-fired power stations, a few hundred megawatts at a time. Such big plants are harder to finance than small photovoltaic installations, and require more planning permissions and infrastructure, such as transmission lines. But they produce a lot of power. Brightsource Energy, based in California, recently received government loan guarantees for a project in the Mojave desert which, if completed, could deliver more power than all the photovoltaic cells installed in America last year.