Wind and solar power are transitioning from marginal to central players in the grid energy landscape. But wind and solar are also subject to the vagaries of daily and seasonal weather patterns, which means they sometimes can't be relied on to meet the grid’s critical needs.

That’s particularly true when energy demand is peaking, and the grid turns to rarely used yet critical natural-gas-fired “peaker plants” as its resource of last resort. But could energy storage, demand response and other next-generation grid technologies turn renewable power into both a stable and a peaking resource?

This proposition is fueling debates over the future of the grid around the world.

Take Germany, where gas-fired plants are losing money to solar and wind power. Or look at Southern California, where plans for new resources to make up for power plant closures are pitting traditionalists, who say gas-powered peaker plants need to be part of the mix, against green advocates, who say that renewables plus new technologies can fill the gap without gas.

So who’s right? The answer, of course, is complicated, but one useful measure to start getting at the core of the issue is “capacity factor.” That’s a measure of how much energy a generation resource actually creates over the course of a day or a year, compared to how much it’s capable of generating, or its “nameplate” capacity.

No power plant ever runs at all-out 100 percent capacity all the time, because we don’t use electricity that way. But some plants are better suited to “baseload,” or always-on, generation than others, as this chart from the U.S. Department of Energy’s Energy Information Administration (EIA) indicates:

As you can see, nuclear and coal-fired power plants run at high capacity factors as baseload resources. Natural gas combined-cycle plants, which have lower capacity factors, have traditionally been an “intermediate” resource -- running during the day and shutting down at night, for instance -- although they’re increasingly being used as a baseload resource as well.

But natural gas combustion turbines, which are less efficient but cheaper than combined-cycle plants, run at capacity factors that average out to less than 10 percent through the course of the year. For a few hundred hours per year, however, those “peaker plants” ramp up to nearly full production to cover the peaks in electricity demand -- and because that’s what they’re built to do, they’re compensated for that role, both by market mechanisms to secure them revenue streams so they can be built, and by the high energy prices they get when the grid’s stressed enough to call on them.

Could green power be called upon to serve a similar role? As this EIA chart indicates, hydropower, wind and solar power have much lower capacity factors, largely because of the variability of the wind, sunlight and water available to make them run:

At the same time, there are reasons why renewables could be seen as a peak demand replacement. In particular, solar PV and solar-thermal technologies tend to produce the most energy and reach their highest capacity factors during the same hot, sunny periods that drive peak electricity demand. That could make them a suitable replacement for peaker plants – particularly if their variability could be bolstered by energy storage, or mitigated by demand-side energy reductions, to align supply with demand in a more reliable manner.

There’s a big catch to this argument, however; the average capacity factors in the charts above don’t tell the whole story. Just as important is the range of day-to-day capacities that different resources can be called upon to provide.

Simply put, natural gas combustion turbines have much, much more range in their moment-to-moment capacity factor, while wind and solar capacity factors are much more limited by the nature of their weather-dependent generation profiles, as this chart from the DOE’s National Renewable Energy Laboratory (NREL) indicates:

Focus on that huge, fat line on the right side of the chart -- the bar that indicates just how much capacity factor range combustion turbines provide today -- and compare it to the relatively tiny ranges that wind and solar provide.

That gap, between peaker plants’ huge range in capacity factors and wind's and solar’s much narrower ranges, is the difference that any renewable-based peak power scheme will have to make up to solve the problems that peaker plants solve today.

Indeed, the argument could even be made the other way around: solar and wind power have to find ways to mitigate their own intermittency, so that they don't force even more gas-fired power plants to start revving up and down to cover their own unpredictable generation patterns.

As for where we’ll see this being attempted first, California and Germany are two places to watch on a macro-grid scale. At the same time, island grids like Hawaii and Puerto Rico will be interesting tests of the ability of smaller-scale grids to find ways to use storage, demand response and smart grid tools to turn intermittent wind and solar into firmer, more reliable peak resources.