When someone says, “I own a hybrid,” we often imagine a Toyota Prius . . . or maybe its RAV4 Hybrid sibling. There’s a reason for that: Toyota has now sold the vast majority of the more than 25 million gasoline-electric hybrids on the world’s roads, most of which use its classic two-motor power-split parallel-hybrid system.

But what does that string of words actually mean? The most useful way to distinguish among hybrid-electric systems is to ask whether the hybrid system can power the vehicle on battery electricity alone, or whether the gasoline engine always has to be on. Both parallel and series hybrids can do that; so can plug-in hybrids. On the other hand, mild hybrids can’t.

Let’s break down the different types of hybrid systems used in vehicles people drive in their daily lives. Note these aren’t terms generally used by new-car shoppers (with the possible exception of “plug-in hybrid”), but technical differences in how the hardware works to propel the vehicle.

Parallel Hybrids

In a parallel hybrid system, both an engine and an electric motor can power the wheels, together or separately. Toyota’s system, first introduced in a Japan-market Prius in 1997, uses two motors between engine and front wheels, powered by a small battery pack (0.8 to 1.4 kilowatt-hours).

The “power split” part of the description means both sources come together to provide a single torque output to the wheels. Under light loads, at lower speeds, one or both motors can power the vehicle while the engine is off. The engine will kick on when more power is needed; it may power the car by itself, or power may be drawn from a combination of the engine and the motors.

The key to the system is that regenerative braking—using a motor as a generator when the car slows to recharge the battery—recaptures energy that would otherwise be wasted as brake heat. Under ideal circumstances, as much as 30 percent of the energy expended to power the car can be recaptured—leading to real-world fuel economy of 40 to 55 miles per gallon in the Toyota Prius or its Corolla Hybrid sibling (pictured at top). Efficiency is highest in stop-and-go traffic that allows the most electric-only running.

The two electric motors and engine are connected though a planetary gear set, which replaces a conventional transmission. This allows continuously adjustable gear ratios; many times each second, the vehicle’s control algorithms vary the power drawn from the battery and from the engine to maximize use of the electrical power and minimize fuel burned.

Under some circumstances, one motor may power the car while the other uses engine power or regenerative braking to recharge the battery. With 25 years of experience under its belt, Toyota has software algorithms that make all these transitions entirely seamless—and these days, drivers and passengers may not even notice the engine switching on and off.

Series Hybrids

A series hybrid is much simpler in concept. Whenever more power is needed than the battery can supply, a gasoline engine switches on to power a generator that recharges the pack. The key here is that the engine torque only powers the generator. It’s not mechanically connected to the wheels, which are powered only by electric motors.

This is the setup used by diesel-electric railroad locomotives. It’s best at steady vehicle speeds, where the engine can operate most efficiently at preset engine speeds. But the range of power needs of a passenger road vehicle spans more than an order of magnitude from steady-speed cruising to maximum acceleration, and that can be hard on batteries—meaning series hybrids usually require higher battery capacity than conventional hybrids.

Only two series hybrid passenger vehicles have been sold in the States. Both were plug-in hybrids (see below): the BMW i3 REx, which added a range-extending two-cylinder engine to the i3 battery-electric model, and the Fisker Karma and its successors the Karma Revero and GS-6, all sleek four-door luxury sedans. Neither remains on sale today, though a 2022 version of the GS-6, the Revero’s successor, is still listed on Karma’s website.

Plug-In Hybrids (PHEVs)

A plug-in hybrid can be a hard concept to explain. In the simplest terms, it’s a regular hybrid with a much larger battery that can be plugged in to recharge from the grid. That lets it operate as an electric vehicle for some distance, from 10 to 60 miles.

The best-known plug-in hybrid may be the 2011–2018 Chevrolet Volt. Many drivers found the second-generation Volt, with an EPA-rated range of 53 miles, could provide daily driving solely on electricity, with the engine switching on only for longer trips or extreme hills. Reports of more than 1000 miles per tank of gasoline weren’t uncommon—if the Volt was plugged in every night.

The Volt, unique among PHEVs, offered nearly identical performance regardless of whether it was running on battery alone or acting as a regular hybrid after the battery charge had been depleted. Other plug-in hybrids have more powerful engines than electric motors. When more power is needed, the engine has to switch on. The frequency of engine use falls as battery size (and hence electric range) rises; higher battery capacity is needed for 40 miles of electric range versus, say, 10 miles.

All plug-in hybrids today are adapted from power-split hybrids, including Toyota’s Prius Prime and RAV4 Prime and a variety of PHEV versions of Hyundai and Kia hybrids. Stellantis, however, offers no conventional hybrids (without a plug); its Chrysler Pacifica Hybrid and a growing lineup of Jeep 4xe hybrids all have plugs.

Carmakers see plug-in hybrids as a way to meet rising zero-emission-vehicle requirements without the expense of engineering entirely new battery-electric cars. Those EVs require battery packs of 60.0 to 100.0 kWh, versus 10.0 to 25.0 kWh in a PHEV. The challenge is that the manufacturers get credit for producing emission-free vehicles even if the PHEV is never plugged in, meaning it operates only as a heavier, pricier conventional hybrid.

Mild Hybrids

Finally, mild hybrids have electric motors that aren’t powerful enough to propel the vehicle alone. They may sit between the engine and transmission, or be part of an integrated starter-generator system in which a beefier electric motor replaces both those components. Fuel-economy benefits here are far less dramatic, and they may not even be marketed under the name “hybrid.”

In this case too, a small battery pack is required to store energy recaptured from excess engine power and/or regenerative braking. The motor effectively adds some extra boost to the engine when needed, meaning potential fuel savings by avoiding a downshift.

Recent mild-hybrid systems often run at 48 volts. They deliver more power than the vehicle’s 12-volt system, but are far less expensive than full hybrid systems that operate at 280 to 400 volts.