What is it?
The Mercedes-Benz S500 Plug-in Hybrid is the first of up to ten petrol- and/or diesel-electric powered models confirmed for launch by the German car maker before the end of 2017.

It succeeds the S300 BlueTEC Hybrid and S400 Hybrid but adds one vital new element, namely the ability of its battery to be charged via either standard household mains electricity or a high voltage electrical system – rather than simply by recuperated means or on the go via the petrol engine.

This allows the S500 Plug-in Hybrid to use a much larger lithium ion battery than its predecessors, which in turn provides its electric motor with far greater energy reserves and, crucially, a more than ten-fold increase in electric range.

On a feathered throttle in optimal weather conditions, it is claimed to be able to travel up to 20 miles before its battery is depleted and the petrol engine is fired up.

At the heart of the hi-tech luxury saloon is a newly developed plug-in hybrid system described as boasting modular properties that will allow it to be tailored for use with varying outputs in a wide range of upcoming alternative drive Mercedes-Benz models.

In the S500 Plug-in Hybrid the new system employs a turbocharged 3.0-litre V6 direct injection petrol engine developing 242bhp and 354lb ft. The electric motor, meanwhile, is set within the forward section of the gearbox, and generates 114bhp and 251lb ft.

Together, the petrol engine and electric motor provide a combined system output of 436bhp and 479lb ft. This gives the S500 Plug-in Hybrid greater reserves than the Porsche Panamera S E-Hybrid, whose supercharged 3.0-litre petrol engine and electric motor deliver a combined 415bhp and 435lb ft.

The combined output is channeled through a seven-speed automatic gearbox to the rear wheels in any one of the four different driving modes: Hybrid, E-mode, E-save and Charge. Additionally, the gearbox offers three separate modes: E (Efficiency), S (Sport) and E (Economy).

As an incentive to prospective buyers, Mercedes-Benz is offering a six year/60,000 mile performance guarantee for the S500 Plug-in Hybrid’s 8.7kWh battery.

More than anything else, though, it is the battery which adds to the weight of the new car. At 2140kg, it tips the scales 145kg above the more traditional turbocharged 4.7-litre V8-powered S500.

What is it like?
Whether gliding around city streets or cruising out on the open road, the S500 Plug-in Hybrid is a superbly serene and relaxing way to travel.

Its hi-tech driveline operates with Swiss watch-like precision and whisper quiet properties in all modes. The transition between pure electric running and the engagement of the petrol engine as it switches into hybrid state is virtually imperceptible.

The combination of petrol power and electric propulsion also provide the plush leather lined saloon with a broad and entertaining range of performance qualities. In electric mode it provides pleasingly urgent step off and flexible properties at speeds up to a limited 87mph.

There are also sufficient reserves on offer in petrol-electric hybrid mode to haul it along with an impressive turn of pace and compelling high speed vigor when the conditions permit.

Despite the weight burden brought on the large battery and all its various ancillaries, Mercedes-Benz says the S500 Plug-in Hybrid is capable of accelerating from 0 to 62mph in 5.2sec and reaching a top speed limited to 155mph. By comparison, the 45kg lighter Porsche Panamera S E-Hybrid offers respective figures of 5.5sec and 167mph.

Official combined cycle economy is put at 101mpg – a figure that corresponds to average CO2 emissions of just 65g/km.

Mercedes-Benz is realistic enough to acknowledge few motorists, if anyone, will be able to replicate this in everyday driving conditions. Although with a fully charged battery and resolutely light throttle loads you can manage to extract some amazing economy figures out of it, even over extended distances.

I got close to the prescribed consumption figure over an average of 88mpg over a 48-mile route, although 14 miles of it was achieved on electric power alone and it was all at a fairly moderate speed over relatively flat terrain. For the record, the Porsche Panamera S E-Hybrid is claimed to deliver 91.1mpg and 71g/km.

It may offer a bewildering choice of driving modes but it is not a difficult car to operate. You can jump in and drive off as in any of Mercedes-Benz’s more traditional petrol or diesel flagship saloon models without giving it so much as a second thought. But to really get the best out of the S500 Plug-in Hybrid you need to become familiar with its four main driving modes – a process which takes time and persistence.

Hybrid combines the petrol engine and electric. E-mode offers pure electric running until the battery charge is depleted. E-save allows you to preserve the energy within the battery by favouring the petrol engine until another mode is chosen and allowing the stored energy to be used for pure electric running later on, when entering the London congestion charge zone, for example. Charge employs the petrol engine to top up the battery on the run.

At one point during our first drive of the S500 Plug-in Hybrid its digital instrument display indicated the battery charge had dipped to just 25 per cent, suggesting its electric range was limited to around five miles. But when I placed it into Charge mode, the energy store was replenished to 85 per cent within 25 minutes, giving it almost 18 miles of electric range.

Happily, the added weight hasn’t affected the ride. Riding on standard 18-inch wheels shod with low rolling resistance tyres – 245/50 at the front and the 275/45 at the rear – the new Mercedes-Benz provides the same cosseting qualities as other S-class models.

A neat feature is the so-called Haptic Accelerator. It operates in combination with the stereo camera mounted within the windscreen, providing an impulse within the throttle to signal you are too close to the car ahead and can recuperate energy by lifting off – at which the S-class automatically switches into a freewheeling state with the petrol engine and electric motor disengaged from the gearbox.

As we noted on the earlier S300 BlueTEC Hybrid and S400 Hybrid, the action of the brake pedal is compromised by the need to recuperate energy during braking, lacking for uniform firmness throughout its travel.

Unlike the battery of S300 BlueTEC and S400 Hybrid which resided underneath the bonnet, the battery used by the S500 Plug-in Hybrid is mounted within the forward section of the boot. This robs 95 litres of luggage capacity, reducing it to a total of 395 litres.

Should I buy one?
The S500 Plug-in Hybrid is a very convincing proposition, offering a truly impressive electric range and stout combined petrol-electric performance together with outstanding refinement and the sort of economy you might expect from a compact hatchback rather than a fully loaded luxury saloon.

It is a limousine you can drive with a largely clear conscience – albeit one that is burdened with weight, limited on luggage space, requires a charging infrastructure and, with a price tag of £87,965 in the UK, only likely to sell in limited numbers.

If you have the financial means and commute into central London, it is likely to appeal. Otherwise, the S350 BlueTEC is an attractive alternative at a price of £62,905 and without the hassle of having to find a socket to plug in to.

Mercedes-Benz S500 Plug-in Hybrid

Price £87,965; 0-62mph 5.2 secs; Top speed 155mph; Economy 100.9mpg; C02 65g/km; Kerb weight 2140kg; Engine V6, 2996cc turbocharged petrol; Electric motor brushless synchronous; Combined power 436bhp; Combined torque 479lb ft; Gearbox seven-speed automatic

In the S500 Plug-in Hybrid the new system employs a turbocharged 3.0-litre V6 direct injection petrol engine developing 242bhp and 354lb ft. The electric motor, meanwhile, is set within the forward section of the gearbox, and generates 114bhp and 251lb ft.