When we’re figuring the costs of achieving a carbon-reduced economy, among the most important numbers are

• current energy produced by fossil fuels
• cost of transition to solar, wind, hydro, nuclear, and other low-carbon means of energy production

When doing that math, though, we need to confront an often-unstated assumption: that a watt-hour of energy from burning hydrocarbons needs to be replaced by a watt-hour from an alternate source. Resulting numbers for square miles of solar panel, or number of wind turbines needing to be built and maintained, may seem overwhelming – but there’s a crucial error embedded in some carelessly calculated ratios. Put simply, the journey to electrification won’t need to carry fossil fuels’ baggage of waste.

I originally typed, “thermodynamic waste,” but that’s too many syllables for a friendly conversation – so let’s put this in short words. Burning fuel to move a car turns 100 units of gasoline energy content into 20 units of motion – with most of the other energy going out the tailpipe as hot gas, or shimmering off the radiator to keep the engine from melting. Compare that with an electric car, in which 100 units of electrical energy yield 67 units of motion – of which 22 units come back to the battery through regenerative braking. In effect, then, an electric car is only wasting 11% of its battery’s energy, compared to the 80% waste of gasoline’s get-go. “Burning fuel to create motion tends to be an energy-wasting proposition,” observes one geologist who’s noted for her effective data visualizations.

You can get an alternative (but compatible) view from the U.S. Department of Energy, often a promoter of fossil fuels, which apparently concedes the correctness of the math above: “Only about 12%–30% of the energy from the fuel you put in a conventional vehicle is used to move it down the road, depending on the drive cycle. The rest of the energy is lost to engine and driveline inefficiencies or used to power accessories.” Another DoE web site acknowledges that “An EV electric drive system is only responsible for a 15% to 20% energy loss compared to 64% to 75% for a gasoline engine.”

Another crunch of the numbers puts the difference in practical terms of “how far will this much energy move us down the road?” Architect and educator Lloyd Alter comes up with the result that “6 kWh of energy in electricity would push a typical electric car about 18 miles… [I’ve checked his math, and the ten most efficient electric vehicles actually get a slightly higher ratio of 3.3 to 4.3 miles per kWh] As energy stored in gasoline is 33.7 kWh/gallon, 6 kWh worth of gasoline would push a car that gets 25 miles/gallon a total of 4.44 miles.” He also observes that it’s generous to ignore the energy cost of refining that gasoline, estimating that “A full 15% of all the energy consumed for industrial purposes is used for refining petroleum products.”

It’s not just about transportation. Other ratios can be even more compelling. Burning fuel to heat a room delivers, at most, 100% of the fuel’s energy content as useful comfort; a typical gas furnace runs at 95% to 98% efficiency, according to one manufacturer, which might sound pretty good. An electrically driven heat pump, though, moves heat rather than producing it, delivering three units of heat for every unit of power consumed – for (what might intuitively seem like an impossible) efficiency of 300%.

Failure to appreciate this massive, absolutely baked-in difference of true “efficiency”—between old and new models of energy production—is an error that Oxford professor Jan Rosenow sums up concisely:

We consistently overstate how much energy the transition has to replace, because we measure the wrong thing. Conventional primary energy accounting attributes the thermodynamic losses of thermal generation, internal combustion and fossil-fuelled heating to useful demand. It counts the waste as if it were the service.

If you’d like that in more quantitative terms, we probably can’t do better than last week’s release of the International Energy Agency’s special pre-COP31 report Electrification. On the side of simple cost-effectiveness, the report opens with: “around three-quarters of the energy consumption in the transport, mostly oil, sector is lost as waste energy.” On the side of climate-change mitigation, the report estimates that “Electrification lowers total energy sector carbon dioxide emissions in all the cases analysed in this report. Paired with an accelerated transition to low-emissions sources, the High Electrification Scenario reduces end-use emissions by 40% by 2035, and total energy sector emissions by more than half from their level in 2025.”

Yes, there’s a “but” at the end of that last quotation: “This requires, however, a substantial scale up of low emissions electricity generation.” So, no surprise, the math always wins – but we can decide to do the math that wins the most. That means knowing what to carry – and what to subtract.