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 S 500 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 S 500 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's it like?
Whether gliding around city streets or cruising out on the open road, the S 500 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 S 500 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 S 500 Plug-in Hybrid you need to become familiar with its four main driving modes – a process which takes time and persistence.