The time of swimming pools, sweat and sangria is almost at an end — and with it will end the year’s peak electricity use. Although the data on exactly how much electricity Americans consumed in July and August isn’t yet available, it’s all but guaranteed to be the peak. It always is. You can see it in the data from 2015, and you can see it in the past 15 years’ worth of data, too.

That pattern — summer peak, fall lull, smaller winter bump, spring respite — is the energy data equivalent of peeping on your neighbor’s bubble bath. It exposes how humans in the U.S. make themselves comfortable. The biggest seasonal fluctuations in energy happen in homes. What you’re seeing in these charts is, largely, the cool blast of millions of air conditioners as they quell marital disputes, soothe sleeping babies and save lives that might otherwise be lost to heatstroke.

As you enjoy the last of those cool artificial breezes before fall sets in, I suggest taking a moment to get a little more familiar with electricity generation — since it’s already on such familiar terms with you. One of my favorite energy charts for laypeople is produced by Lawrence Livermore National Laboratory — it shows you exactly where our energy comes from, what we use it for and how much goes to waste. And herein hangs a tale.

If you look at this chart, you can follow how energy goes from source on the left to use on the right. The electricity you’re now enjoying is the biggest single use of energy in the country (that orange box at the top). We burn more energy to make electricity than we do to drive cars (the pink box at the bottom).

But you’ll also see that we never get to use most of that energy. Thirty-eight quadrillion British thermal units of energy — mostly from burning coal and natural gas — went into electricity production in 2015. Almost 13 quadrillion Btu of useful energy came buzzing out across our power lines. The remaining 25 quadrillion Btu? It was just lost — mostly in the form of heat that rises and shimmers off the motors and machinery of the nation’s electric power plants.

Now, you might think that’s a big opportunity to make this system more energy efficient. Capture some of that lost energy, and we wouldn’t have to burn so much fossil fuel up front. (A big deal, since electricity generation is also the U.S.’s No. 1 source of carbon emissions.) But there’s a catch. And the catch is physics. Well, physics and the long life spans of expensive bits of infrastructure.

Most power plants built from the 1950s through the 1980s are about 35 percent efficient, said A.J. Simon, an energy systems scientist at the Lawrence Livermore National Laboratory. That category includes most of our coal-powered energy generation. Low efficiency is baked into the being of those plants. Just as a car engine burns gas more efficiently on the highway than at slower street speeds, it takes less coal to turn water into steam (which makes the turbines go) at higher temperatures and pressures than at lower ones.

And the old power plants are made of old materials that don’t allow them to be hot enough or pressurized enough to reach peak efficiency. Many new power plants — mostly powered by natural gas — have been built in the past 10 years, and they do have higher efficiencies — up to 60 percent on their best days. There’s also a new generation of “ultra-supercritical” coal plant, made with more heat-durable materials. The efficiency of those plants tops out at about 42 percent, Simon said. The first of these in the U.S. started operating in 2012, and it was hotly contested by environmental advocates.

Why? Because it’s still coal, and philosophy matters here as much as engineering. Do we reduce fossil fuel use by building as much renewable generation as is technologically feasible and rely on old, less-efficient fossil fuel plants for backup? Or do we invest in more efficient fossil fuel plants even if that choice prolongs our dependence? Everybody enjoys a cold drink in a chill room on a summer day, but the answers to our energy-money conundrums are a lot harder to agree on.