Ever felt like you're comparing apples to spaceships when looking at gas cars versus electric ones? You aren't alone. Gas is measured in gallons, obviously. But electricity? That’s measured in kilowatt-hours (kWh). It’s confusing. Most people just want to know how much "juice" is actually inside a gallon of regular unleaded.
The short answer is 33.7 kWh.
That is the magic number. The U.S. Environmental Protection Agency (EPA) uses this figure to calculate MPGe, or miles per gallon equivalent. Basically, if you took all the heat energy sitting in a single gallon of gasoline and turned it perfectly into electricity, you’d have 33.7 kWh of energy.
But numbers on paper rarely tell the whole story. Honestly, the way your car uses those kWh is where things get weird.
Why 33.7 kWh per gallon of gas isn't what it seems
So, a gallon of gas has 33.7 kWh. A Tesla Model 3 Long Range has roughly an 82 kWh battery. Does that mean a Tesla only carries about 2.4 gallons of "gas" energy?
Yes. Exactly.
It sounds crazy because a gas car with a 2.4-gallon tank wouldn't get you out of the driveway. Yet, that Tesla can go 300+ miles. This happens because internal combustion engines (ICE) are, frankly, quite terrible at their jobs. Most of the energy in gas—about 70% to 80%—is wasted as heat. You’re driving a furnace that happens to move.
Electric motors are different. They convert over 85% of their electrical energy into actual motion. When you look at kWh per gallon of gas, you’re seeing the raw potential energy, but the "burn" is what matters.
The physics of the burn
Gasoline is incredibly energy-dense. It’s one of the best ways to store energy we’ve ever found. To match the energy in a 15-gallon tank of gas, you’d need a battery weighing several thousand pounds.
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The Oak Ridge National Laboratory has done extensive research on this. They point out that while gasoline has high energy density, the efficiency of the "prime mover" (the engine) is the bottleneck. In a standard gas car, you lose energy through:
- Exhaust heat (massive loss)
- The cooling system (radiator)
- Mechanical friction in the pistons and gears
- Idling (consuming energy while standing still)
EVs don't idle. They don't have radiators in the traditional sense. They use regenerative braking to put energy back into the "tank." This is why that 33.7 kWh figure is a bit of a trick; it shows how much energy gas has, but underscores how much of it we actually waste.
Breaking down the math for your wallet
If you want to know if you're saving money, you have to look at the cost per kWh versus the cost per gallon.
Let's do some quick math. Say gas is $3.50 a gallon.
To get the same energy from a wall outlet, you’d look at your local utility rate. The national average is around $0.16 per kWh.
$0.16 x 33.7 kWh = $5.39.
Wait. Does that mean electricity is more expensive than gas?
Nope.
Because an EV is roughly three to four times more efficient at using those kWh. You don't need 33.7 kWh to go the same distance a gas car goes on one gallon. A typical EV might use only 25 to 35 kWh to travel 100 miles. A gas car getting 25 MPG would need four gallons of gas—or 134.8 kWh of energy—to travel that same 100 miles.
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It's a massive gap.
The MPGe trap and what to look for
The EPA created MPGe to help people compare cars, but it’s kinda flawed. It assumes that 33.7 kWh equals one gallon, then tells you how many miles the EV goes on that amount.
If a car is rated at 100 MPGe, it means it travels 100 miles using 33.7 kWh of electricity.
Most people don't think in kWh. We think in dollars. If you're looking at a window sticker, ignore the MPGe for a second and look at the "kWh per 100 miles" figure. This is the "Gallons per 100 miles" of the electric world. The lower this number, the better. A Hyundai Ioniq 6 might do 24 kWh/100 miles, while a massive Hummer EV might use 50+ kWh/100 miles.
Temperature, weight, and the "Hidden" losses
Gasoline is remarkably stable. Whether it’s 20 degrees or 90 degrees out, a gallon of gas still contains roughly 33.7 kWh of potential energy. Batteries are finicky.
When it gets cold, the chemical reactions inside a lithium-ion battery slow down. You also have to use those precious kWh to heat the cabin. Since an EV doesn't have a hot engine to provide "free" heat, it has to use the battery. This can drop your efficiency significantly—sometimes by 30%.
Also, consider weight.
Energy density is the Achilles' heel of the EV.
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- Gasoline: ~12,000 Wh/kg
- Lithium-ion battery: ~250-300 Wh/kg
This is why EVs are so heavy. They have to carry a lot of physical mass just to hold a fraction of the energy found in a gas tank. However, because they are so much better at converting that energy into wheel rotation, they still win the efficiency race.
Real-world scenarios: The cost of a "Fill up"
Let's look at a practical example.
You have a Toyota Camry (32 MPG) and a Tesla Model 3 (roughly 4 miles per kWh).
To drive 300 miles:
The Camry needs about 9.4 gallons of gas. At $3.50/gal, that’s **$32.90**.
The Tesla needs about 75 kWh. At $0.16/kWh, that’s **$12.00**.
Even though the "per unit" energy in gas might seem cheaper in some specific utility markets, the sheer waste of the internal combustion process makes it the more expensive way to travel.
The future of energy density
We are seeing a shift toward solid-state batteries. Companies like Toyota and QuantumScape are working on cells that could significantly increase the kWh per kilogram. We likely won't ever see a battery as energy-dense as gasoline in our lifetimes—it's just a matter of chemistry—but we don't need to.
If we can get batteries to 500 Wh/kg, the weight penalty of EVs disappears. At that point, the 33.7 kWh per gallon comparison becomes almost irrelevant because the efficiency gains will be so overwhelming.
Actionable steps for your next purchase
If you're trying to decide between staying with gas or going electric, don't just look at the MSRP.
- Check your electric bill. Look for the "Supply" and "Delivery" charges to find your true cost per kWh. Some people pay $0.10, others pay $0.45 (looking at you, California).
- Calculate your "Break Even." Use the 33.7 kWh figure to realize that an EV only needs about 1/3 of the energy of a gas car to do the same work.
- Factor in your climate. If you live in North Dakota, your "effective" kWh per gallon is lower in the winter. If you're in Florida, you're golden.
- Look at "kWh per 100 miles" on the sticker. This is the most honest metric for how much you will actually spend to drive the car.
Understanding the relationship between kWh and gasoline isn't just for scientists. It’s the only way to cut through the marketing noise and figure out what a car actually costs to run. Gas is packed with energy, but we're mostly just burning it for heat. Electricity is harder to store, but every bit of it goes toward getting you where you’re going.