Creating a sport-utility vehicle equipped with a plug-in petrol-electric drivetrain is an unenviable task because SUVs are usually anything but efficient.

With large bodies and all-wheel-drive systems, they readily consume more fuel than most other vehicles. Having to add more weight in the form of an electric drivetrain seems like a bad idea.

Porsche, however, must love challenges, because that is exactly what it has built. The Cayenne S E-Hybrid weighs 2,350kg (265kg more than the regular Cayenne S), but it has a claimed fuel economy of just 3.4 litres for every 100km.

That is a third of what the Cayenne S supposedly consumes. During this test drive, it posted 6.7 litres for every 100km, which is still pretty impressive for a car its size.

The SUV also delivers a better performance and efficiency than its predecessor, the Cayenne S Hybrid.

The key improvement is its more powerful electric drivetrain, courtesy of a 70kW motor and a lithium-ion battery pack, versus a 34kW motor powered by a nickel-metal hydride pack previously.

Most importantly, the Cayenne S EHybrid can be recharged via an electrical outlet. Charging time from a regular wall socket is three hours and with a high-voltage connection, the time is halved.

Running on electric power alone, the car can cover 18 to 36km, at a top speed of 125kmh. The total range is, of course, affected by one's driving style, the terrain and prevailing traffic conditions.

In line with its more eco-friendly stance, this SUV is driven by its electric motor (as long as the batteries are adequately charged) by default.

Like an electric car, it is switched on instead of being cranked up.

Cruising around town in full-electric mode, which Porsche calls E-Power, is an eerie experience - the car operates in near silence and the only movement on the instrument meters is the swinging of the needle on the battery gauge.

The 95bhp produced by the electric motor seems inadequate, given the car's 2.3-tonne mass, but the 310Nm of instantaneous torque it delivers is actually what is needed for driving at low to moderate speeds in built-up areas.

Switching off E-Power puts the car in hybrid mode. In this setting, its internal combustion engine is activated earlier in order to deliver more power and maintain the battery's state of charge.

To minimise fuel consumption, the vehicle recuperates energy from braking and employs a coasting function, which decouples the gearbox from the engine when the driver lifts his foot from the throttle while cruising at highway speeds.

Apart from its "green" talents, the Cayenne S E-Hybrid also boasts athletic abilities.

Select Sport mode and the SUV turns into a veritable mile muncher, with the electric drivetrain providing additional boost to its supercharged 3-litre V6. The combined output of both systems is 416bhp and 590Nm, which powers the Porsche to 100kmh in 5.9 seconds.

The Sport setting is ideal for "bahn-storming", as the electric boost enhances the Cayenne's mid-range and makes overtaking a breeze - even when you are already clocking three-digit speeds.

The cabin could be quieter, though, as there is more road and tyre noise than expected.

True to the brand's sporting roots, the Cayenne S E-Hybrid is pretty handy around corners. Despite its portliness, it can still be hustled up and down twisty mountain roads, with its precise steering and additional grip from its all-wheel-drive system helping the driver keep the sizeable car in line.

Indeed, Porsche has created an SUV that is as adept at thundering down highways as it is gliding about town quietly.

No prizes for guessing which mode is more fun.