When people think of solar power, they tend to think of panels on rooftops. That kind of small-scale, distributed solar power is the most visible, gets the most press, and, from the consumer perspective, has the most sex appeal.

But the humble workhorse of solar power is the utility-scale solar power plant, usually defined as a solar array larger than 5 megawatts.

Solar power plants can consist in either photovoltaic (PV) panels or mirrors that focus sunlight on a fluid that boils and turns a turbine ("concentrating solar power," or CSP). In practice, most new solar plants these days use PV, which has gotten so cheap so fast that it's outcompeted CSP and every other solar segment, at least for now.

In 2007, there were zero utility-scale solar power plants in the US. Today there are hundreds, ranging from the 579 MW Solar Star project (the world's largest solar farm) in California down to dozens upon dozens of 10, 20, and 50 MW projects in communities across the country. (SEIA counts 2,100 solar PV projects over 1 MW.)

Big solar power plants still provide a measly 0.6 percent of overall US electricity. But they are headed up a steep growth curve.

Residential rooftop solar is the fastest growing solar segment, but utility-scale solar is bigger. There's more installed, so even with its slower growth rate it adds more capacity each year — in 2015, it accounted for 57 percent of all new installed solar capacity.

("Non-residential" in this graph refers to rooftop solar on commercial buildings — parking garages, big-box stores, etc.)

What's more, there's a ton of utility solar in the pipeline. According to the Energy Information Administration, 9.5 GW of utility solar is scheduled for installation in 2016 — more than from any other single energy source, including natural gas.

That would make 2016 a banner year, with utility solar accounting for more than three-quarters of installed solar capacity, installing more in a year than in the past three combined.

That's serious growth. A new report from GTM Research is also optimistic about utility-scale solar passing something of a milestone in 2016.

For years, the growth of big solar was driven by state-level renewable energy mandates; utilities had to build these plants. This coming year, GTM expects more than half the growth in big solar to come outside those mandates.

In other words, utilities are beginning to voluntarily opt for big solar.

Why is that? Utility solar is being boosted by three strengths — and it's making progress against its one weakness.

Strength #1: Price

The total installed cost of big solar, per watt of capacity, is rapidly getting cheaper:

As you can see, most of the cost drop has been due to the falling cost of solar modules — and panels are only expected to get cheaper.

According to this 2015 Lawrence Berkeley National Laboratory report (the latest comprehensive data I could find), the installed cost of big solar has fallen 50 percent since 2009, from $6.30/W to around $3.10/W at the end of 2014.

Some projects were down as low as $2/W when LBNL released its report. And these days, solar projects in North Carolina and Georgia regularly report costs as low as $1.15/W.

The stretch goal of the Department of Energy's 2011 Sunshot Initiative is to drive installation costs down to $1/W, which it says "would make solar without additional subsidies competitive with the wholesale rate of electricity, nearly everywhere in the US."

The CEO of First Solar recently said that "by 2017, we'll be under $1.00 per watt fully installed on a tracker in the western United States." (More on trackers later.) It appears costs are falling faster than almost anyone predicted.

Strength #2: Predictability

The thing about solar power is you know exactly how much it's going to cost, forever. (At least for the life of a solar PV installation.)

Coal and natural gas have highly volatile prices. The price of sunlight is zero. There is no fuel cost.

That means the costs of the produced electricity can be calculated in advance, based on capital and operation and maintenance costs. It's more like building infrastructure than like operating a commodity-based asset.

This allows solar developers to offer utilities extremely stable, long-term power purchase agreements, or PPAs. For a utility in these turbulent times, knowing exactly how much power is going to cost for the next 20 (or more) years is a great comfort. That kind of risk hedging is worth money.

Here's a plot of PPA prices over the past several years:

This graph makes two things clear. First, almost all new big solar plants in the past few years are PV. And second, power prices are getting extremely low, now regularly falling below $0.05/kWh, or $50/MWh, sometimes as low as $40/MWh.

The city of Palo Alto, California, is on the verge of signing a 25-year solar PPA at about $37/MWh, which could well be the cheapest PPA ever signed for solar. The city has the option to extend the PPA to 40 years, which might also make it the longest PPA ever signed.

Imagine the peace of mind that comes with knowing you've locked in extremely cheap wholesale electricity, at $0.037/kWh, for 40 years. With a stroke, a big source of volatility is eliminated from the budget.

One reason big solar PPAs are so cheap is that the federal solar investment tax credit (the ITC) is still in effect and was recently extended, set to phase out over five years. That gives the industry a clear runway to grow past the need for subsidies.

And it's already looking past them. First Solar's CEO also said, "I fully believe that within 10 years we'll be talking about low-3-cent power on a peak basis," without subsidies. That would make big solar the cheapest source of power, period.

Strength #3: More states have plants

The market for big solar used to be fairly concentrated in a few states, particularly states where it was required by mandates. But now that growth is expanding outside those mandates, the industry is starting to spread out.

"To date," writes GTM, "19 state markets have at least 50 megawatts of non-RPS utility-scale solar in development."

Why is this geographic spread a strength? Politics.

The more states where large-scale solar is a business interest, the more respect and assistance it will get from state politicians.

As Jack Fitzpatrick notes in Morning Consult, "The number of U.S. House Republicans representing districts where there are utility-scale solar facilities increased from a measly 12 in 2008 to 88 in 2016."

There are now as many Republicans as Democrats with solar power plants in their districts (and more Republicans with wind farms). The reason red districts are catching up (and pulling ahead) so fast is that they are more likely to be rural and have open, cheap land to attract developers. (Rural North Carolina, for instance, is going nuts over solar, in all kinds of ways.)

As Fitzpatrick says, this geographic spread helps explain why (to everyone's surprise) the renewable energy tax credits were extended last year. Despite the GOP's notional opposition to energy subsidies, and a state-level fight against solar funded by the Koch brothers and carried out by state-level organizations like the American Legislative Exchange Council, local and state-level Republicans welcome the economic benefits that renewable energy brings.

This is especially true for big solar, which dodges many of the thorny issues being raised by customer-owned rooftop solar.

Weakness: capacity factor

Here's an important distinction to understand: The capacity of a power plant refers to how much power it is theoretically capable of producing if it generated maximum output, around the clock. The capacity factor (CF) of a power plant refers to how often it actually runs, producing power.

Here's a hypothetical example. Say you build two power plants, one nuclear, one solar. They both have 1 GW of capacity, which means if they both produced maximum output, they would generate a GWh in an hour.

But they don't always produce maximum output, or run year-round.

Averaged over time, the nuclear plant's output is about 90 percent of its maximum capability, i.e., has a CF of 90 percent. It will generate 900 MWh (90 percent of its capacity) in an average hour.

The solar plant only "runs" when the sun is shining and only reaches maximum output at the sun's peak. Averaged over time, its output is about 20 percent of its capacity; it has a CF of 20 percent. It will generate 200 MWh (20 percent of its capacity) in an average hour.

In other words, to get the same amount of MWh, you need to build four to five times as much solar capacity as nuclear capacity.

So how are big solar plants doing in terms of CF? Let's put it in context.

Based on EIA data (thanks to Jesse Jenkins for sorting through it), the current CF of the US nuclear fleet is around 88 percent — nuclear plants are typically expensive and unwieldy to shut down and restart, so they are almost always running, except when they are offline for maintenance.

The current CF of the US coal fleet is around 55 percent. Individual coal plants are capable of much higher, but old coal plants are being idled more and more often.

The US wind fleet averages out at about 33 percent CF, where it's been more or less stuck for years. Turbines are getting taller, which improves CF, but they're being installed in more and more suboptimal sites, which lowers CF; for now, it's balancing out.

The natural gas fleet is at about 26 percent CF. Note that this lumps together combined-cycle plants, which tend to have CFs up toward 57 percent, and open-cycle plants that are used primarily for "peaking," which have CFs down around 7 percent.

The average CF of all utility-scale solar (including PV and CSP) is about 19 percent.

Here's an EIA graph showing capacity factors over time:

And here's another that focuses on renewable energy capacity factors:

Not exactly the upward sloping lines one might like.

A new report from LBNL takes a deep dive into the CFs of new utility-scale solar PV plants (installed in 2014). The report finds that the average CF across all new PV plants is 27.5 percent, but the range among individual projects is crazy wide, from 14.8 to 34.9 percent. (Though they did find that most solar PV plants produce roughly what investors forecast.)

The high end of that range, flirting with 35 percent, is really impressive — way better than what was possible even a few years ago.

What makes for higher CFs for solar PV plants? The intensity of sunlight is the biggest factor, obviously. Beyond that, trackers, which change the orientation of panels throughout the day to follow the sun, are a big boost. Keeping the panels clean is important. And "inverter load" is important, though I won't bore you by explaining it.

Long story short, solar PV plants are performing predictably and improving their capacity factors over time.

Big solar is about to get unstoppable

Big solar used to be almost entirely driven by policy, mainly state renewable energy standards and federal tax credits. It has all but outgrown the first and will outgrow the latter over the next five years.

It's about to stand on its own two feet, outcompeting even rivals that are allowed to dump carbon emissions into the atmosphere for free. It won't be long before the discussion about environmental benefits is moot — utilities will demand solar because it's the cheapest power available.