In this new series, we ask our readers to submit questions for Cyrus Wadia, an expert on energy matters. Question: We hear about new little technical breakthroughs in solar all the time. Why aren't we using them yet?Answer: Solar installations have grown at more than 40 percent annually for the last six years, so we are using it more and more every day. But it still represents an embarrassingly low percentage of the total electricity supply.At a high level, I would say the hurdles for broader adoption are:(1) the cost is still too high for most geographic regions(2) issues of scale(3) the sun sets every dayFirst, cost. The number that matters to both you, a farmer in India, and PG&E is cents per kilowatt hour (c/kWh). Take a look at your electricity bill and you'll see exactly how much you pay. In Berkeley, California, you'll see a number like 11 c/kWh-which is not very much considering that this is for all the electricity that powers your lights, appliances, TV, and, even, in some cases, heat. Solar must compete with this number to be viable, yet today the best solar cell operating in the sunniest environment for 25 years still has an average cost of more than 20 c/kWh. Even if the silicon in the cell was free and the actual solar module cost 0 c/kWh, you still would have a baseline of roughly 10c/kWh for the entire balance of systems-all the components needed to support a solar installation, not including the panel itself. To push that number lower, we need further innovation in the technology, manufacturing, and balance of systems before solar is as cheap as coal or nuclear.Secondly, solving our most pressing energy problems will put a strain on our natural resources in an unprecedented way. The same amount of silicon that can produce 500 microprocessors will power just one compact fluorescent lightbulb. It's a staggering contrast. We are still figuring out how to generate solar power at scale, and there is still a lot to learn about procuring large volumes of solar material at low cost and then being able to rapidly manufacture that material into actual panels.The third problem is with intermittency. A nuclear plant can operate 24/7. But the sun sets every day. If you add up all the energy that hits a solar panel daily, it only reaches peak performance for 20 percent of the day. Even if we solve the cost issue, the solution gets more complicated because we need a way to cost effectively store energy (possible solutions include batteries, pumped water, or compressed air). We need a new grid system that can handle large percentages of electricity that come from intermittent supply sources. Before we can decommission a polluting coal plant we need a reliable replacement. Large solar projects that might solve this problem are happening, but happening slowly because of the grid interconnection, storage requirements for utilities, and other issues. That said, I am extremely encouraged by the technology and manufacturing progress we've seen over the last 10 years, and I fully expect that we will get there in the foreseeable future.Have a question for the energy expert? Paste it in the comments or hit us up on @GOOD.