
Before we move on to our discussion of 15-kilowatt (kW) installations, let’s take a moment to discuss something that’s not as cool or exciting as solar, but can save you some serious cash in the long run. Installing solar is a great financial decision, but did you know that energy efficiency measures are the most cost-effective. . Now on to solar installations. There are many ways to look at the size of a 15 kW solar installation: . The average installation cost for residential solar, according to a 2016 report from the National Renewable Energy Lab, is $2.93 per watt. So if you purchased a 15 kW.