Biomass, coal, oil, fuel cells, natural gas, nuclear - they all have their critics. If it's not greenhouse gases, it's air pollution or radioactive waste. Even wind power, as green as it is, comes under attack from those who worry about the impact on the natural scenery of 250-foot steel towers. So what's left? Solar power. Unfortunately. The most expensive of the renewable energy options, solar power has long been the light at the end of the tunnel for those who, for either economic or environmental reasons, dream- ed of an energy source that could replace fossil fuels. During the oil embargo of the 1970s, when the nation's vulnerability to foreign oil was made abundantly clear, a resolve formed to find a way to make solar power the ultimate solution to the energy crisis. But nearly four decades later, power derived from photovoltaics - the technology that converts sunlight to electricity - still costs, depending on the method used, from five to 10 times as much as power from the cheapest sources, such as natural gas and coal, unless heavily subsidized by the government. "Photovoltaics have a lot of advantages," said James F. Manwell, director of the Renewable Energy Research Laboratory at the University of Massachusetts in Amherst. "Fortunately, people don't seem to mind PV panels. They don't make any noise or stick up above the horizon. From that point of view, it's certainly good," said Manwell. "But," he added, "yes, it is an expensive technology. I think the price will keep going down, but it's starting at a high level." "My gut feeling is that it might come down in cost by a factor of two," said Jon G. McGowan, a UMass professor of mechanical and industrial engineering who specializes in wind energy. "It's $10 a watt now, so it might come down to $5. But it's going to be a long time," he said However, to become competitive with the least expensive power sources, it would have to fall to about $1 per watt, McGowan said. However, to become competitive with the least expensive power sources, it would have to fall to about $1 per watt, McGowan said. While government incentives can cut the price tag for a solar installation in half, solar power needs to be competitive in cost without government help to truly be a long-term solution to the energy problem, say energy analysts. The cost of the solar panels, as well as the cost to install them, are holding back the technology, McGowan said. "Even if the PV cells were free, the system would not be economical because of the cost of the installation and the batteries (to store the electricity) and the inverters and other power electronics," he said. However, fossil fuels are likely to rise in cost, cutting the cost gap, said E. Bryan Coughlin of the UMass Department of Polymer Science and Engineering. "It is a situation where the floor keeps coming up as research into solar pushes the ceiling down. At some point, they will meet," he said.