Americans love solar power, but most aren't in a position to get it. You can't install rooftop solar if you rent, own a condo, have poor credit, or have a rooftop that's shaded or faces the wrong way. In fact, it's a fairly small slice of Americans who can take advantage of rooftop solar:

That 15.8 million near the right end there is the current "addressable market" for residential rooftop solar. (Obviously that's not a fixed number — it can and will rise.) According to a new report by GTM Research, roughly one-fifth of that addressable market will have rooftop solar by 2020, and rates of installation will still be rising. One can see saturation of the market, if not in our immediate future, at least on the horizon.

So how can all those households that don't have suitable rooftops participate in the solar boom?

One answer is community solar. That's when a group of utility ratepayers get together and agree to purchase electricity from a solar PV project — they "subscribe" to its power — and pay a monthly charge that appears on their utility bill. That way they can help new solar projects get funded, feel good about consuming clean energy, and, sometimes, save money on their home power bills. It's a way to support solar PV power when you can't put it on your own roof.

There are a few different ways this can work. In most cases, a third-party solar developer contracts with the utility to sell it electricity, and then takes on the task of finding subscribers. Sometimes a utility runs the subscriber program; sometimes a utility owns the project. In some rare cases, the subscribers themselves own the project. But usually it's run by a third party like Clean Energy Collective or SunShare.

Today, community solar is a tiny market: just 74 megawatts installed through May 2015, compared with 8.1 gigawatts (or 8,100 megawatts) of total installed solar PV in the US. But there's already nearly 1 gigawatt in the pipeline. This is what GTM projects:

Those black bars are projected installations. The gray line shows the growing percentage of PV power represented by community solar — up to almost 5 percent by 2020.

(Note: GTM is only projecting the growth of community solar with on-bill financing. Community-owned solar projects — which face different market dynamics — are a separate thing. This NREL paper lays out the differences.)

The top states for community solar (including projects in the pipeline) are Minnesota, California, Massachusetts, and then Colorado. There are now 24 states with at least one community solar project, and at least 12 states with two or more. There are 10 states with community-solar legislation on the books (90 percent of community solar will be built in those states over the next two years); 10 more states have legislation under consideration.

Community solar is attractive to developers because it enables them to "incur commercial solar costs [thanks to the larger scale of these projects] while competing against residential retail electricity rates." It's also easier to find subscribers than it is to find rooftop solar customers, because of the lack of siting restrictions and credit problems.

Community solar rates are still higher than average retail electricity rates in some places, but as the market matures and solar PV prices continue to fall, that is likely to change.

If you don't have a roof fit for solar power, call your legislators and your utility. They might be working on a community solar program already, and if they're not, well, maybe your phone call will do the trick.