The news on the renewable energy front continues to be good, with the costs of wind and solar generation falling steadily.

By some measures, large-scale renewable energy is now cost-competitive (or close) with the most efficient natural-gas electric plants.

Moreover, the amounts of clean generating capacity installed continue to rise, both in North America and the rest of the world.

DON'T MISS: More solar energy was added in 2016 than natural gas or wind

That's undoubtedly a good thing, but as a recent post from the Environmental Defense Fund pointed out almost two years ago, there's more to the story.

And it bears repeating, to ensure that clean-energy advocates focus on the correct metrics when lauding progress in the field.

In May 2015, the EDF reacted to a breathless story published two weeks earlier by Bloomberg that carried a somewhat click-baiting headline: "Fossil Fuels Just Lost the Race Against Renewables."

wind farm

The second and third sentences of its opening paragraph laid out the salient point: "The world is now adding more capacity for renewable power each year than coal, natural gas, and oil combined. And there's no going back."

Hold on, responded the EDF in a rebuttal titled "Fossil fuels haven't lost the race, yet. Here's the full story."

Sure, said the advocacy group, the amount of capacity has increased. But a plant's theoretical capacity says little about how much output it actually generates.

While coal plants typically operate at 80 percent of capacity, the numbers for solar and wind generation are typically only 20 percent to 40 percent.

In other words, don't look at total capacity, look at how much energy each source actually puts back into the grid.

It's also necessary to look at net changes over time, taking into account which types of generation are retired (or, in the case of coal plants, retrofitted to natural gas) and which come online.

The EDF's accompanying chart of projected net generation-source changes from 2012 through 2040, above, is far from indicating that clean energy will supplant coal, gas, and oil.

Projected net changes in electricity generation 2012-2040, based on U.S. EIA data [via EDF]

It's based on U.S. Energy Information Agency projections, which have historically proven to have a wide range of accuracy over time.

"While we might still expect future generation in renewables to exceed fossil fuels," the EDF concludes, "this shows there isn’t necessarily a clearly diverging trend – yet."

The same principle applies to other statistics that may be touted in the absence of context.

CHECK OUT: We're there! Renewables now cheapest unsubsidized electricity in U.S.

If you look at U.S. plug-in electric car sales last year versus 2011, for instance, you can accurately say that 2016's total sales were more than seven times as high. That sounds good.

But in a U.S. market where more than 17 million vehicles were sold, the actual numbers (133,000 last year) remain very low, a crucial part of the full context that's also known as the denominator problem.

In other words: make sure you're using the right metrics and understand the full context when you see headline numbers.

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