In the short list of climate actions that will work, why is overbuilding renewables positioned so highly? Why not build exactly the right amount, or build a lot of nuclear to keep the lights on, or wire everybody’s home to turn the heat off when there isn’t enough electricity?

The first reason is the same reason as why electrify everything is top of the list, because renewables don’t waste nearly as much energy. Most economies throw away about two-thirds of the primary energy that enters the economy in the form of coal, oil, gas, nuclear generation and renewables. Almost all of that is waste heat from burning the coal, oil and gas for transportation and electrical generation, about 80% and 60% respectively.

And while the energy in coal, oil and gas turns into heat on about a one-for-one basis when we just need heat, heat pumps turn electricity into three times as much heat, so it makes more sense to use them instead.

What does that have to do with renewable electricity?

Wasted energy applies to fuels we have to work for, not energy that we get for free. Every ton of coal, barrel of oil or cubic meter of natural gas is something we have to work for. We have to take it out of the ground. We have to process it to remove impurities like too much water or sulfur. We have to refine it and separate out the most valuable light fractions of it. We have to transport it, often thousands of kilometers in pipelines, ships, rail cars and trucks.

About 11% of all energy globally is used to extract, process, refine and distribute coal, oil and gas. Use renewables and electricity instead of fossil fuels, and our global energy demand drops by over 10%.

Then there’s something something called auxiliary power, which is how much energy is required to run an electrical generation plant. Wind and solar farms just sit there and generate electricity when the wind blows or the sun shines. Wind farms need a little bit of energy to point into the wind and shut down when there’s too much wind, but that’s about 0.2% of the electricity that they generate. Solar farms use even less to run the power management bits that connect them to the grid in a well-behaved manner.

But coal and gas plants move a lot of mass around. Gas plants have to pump the natural gas into the turbines and there are a bunch of other mechanical and electronic components that need power. A natural gas combined cycle generation plant uses about 4% of the electricity it generates. Natural gas plants often bid on day ahead reserve markets to be the backup necessary for big coal, nuclear and hydro plants that might go offline abruptly. They burn a bit of gas all day long in case they have to burn a lot of gas suddenly, which is like an internal combustion car idling in a traffic jam, going nowhere.

Coal plants are even more problematic. Coal is carbon rich dirt that’s been dug up, delivered to coal plants and piled up. The coal has to be put onto a conveyor belt that’s constantly running. Frequently the coal needs to be crushed to a finer consistency. When it gets burned it turns into gases and lightweight ash which needs to be filtered to keep too much air pollution from landing downwind. And it turns into heavier ash which needs to be pulled out of the furnace and piled up in big piles of its own, then moved off the coal generation facility with more energy. That’s why coal plants use about 7% of the electricity they generate just to run themselves.

Those percentages would go way up if the gas and coal plants were required to capture and dispose of most of the carbon dioxide they produce while burning their fuels.

All that waste doesn’t happen with wind and solar plants, which is part of why they are cheap forms of electrical generation. Or with hydroelectric plants or geothermal for that matter. Efficiency really doesn’t matter when the fuel is free, but it really does when the fuel isn’t free.

We have overbuilt every other form of electrical generation and always run it under its potential capacity, with the exception of nuclear which has to be run as much as possible to make any economic sense. Natural gas plants often run as little as 10% of a year, even though they could run 90% of the year. Coal plants, because their electricity is so dirty, are run as little as possible in pretty much every country in the world. The USA’s coal plants were built to run as much of the year as they could, but these days run under 50% of the time. We spill water through dams when demand is low and water is high. Even well-operated nuclear plants only average about 90% of the year on average because of maintenance, refueling and refurbishment.

Our electrical grids have always had multiple forms of generation that were overbuilt to provide resiliency, backup and to deal with peaks. And we have been building grid storage in the form of pumped hydro since 1907 to provide some of those same benefits.

While wind and solar don’t have the potential to run full tilt 90% of the year, they do operate a lot more than most people realize. Wind farms are providing electricity about 85% of the time, just less than their maximum because the wind is light. And solar farms provide electricity for up to 14 hours a day in the summertime, just not that much in the morning or evening.

It’s easy to change the pitch of wind turbine blades remotely and automatically so that the turbine doesn’t generate as much electricity as it could. It’s easy to turn off strings of solar panels in a solar farm in a similar way. Just because the wind is blowing or the sun is shining doesn’t mean we have to harvest the energy. We curtail a lot of wind and solar farms because inflexible generation like nuclear power is running strongly but demand is low. And it’s just and easy and fast to turn it up again.

What this means is that if we overbuild renewables we can use it as backup and reserve instead of natural gas a lot of the time. We can use them for other grid stability purposes too. And because they are so cheap per kilowatt hour to build and operate, because they don’t waste any energy as coal, oil or gas do, and because they are environmentally benign when running or curtailed, it makes sense to overbuild them.

Overbuilding renewables is a sensible and cheap climate action lever, especially when combined with electrification. When combined with other short list actions, it means real climate action is much easier than most imagine.