A plug-in hybrid carries both a battery large enough for daily journeys and a combustion engine for longer ones. Which of those does the work depends on the owner rather than the vehicle.
The design assumes regular charging
The battery is sized to cover typical daily distance on electricity, with the engine reserved for journeys beyond that.
Official consumption figures are calculated on the assumption that this pattern holds, which is why they appear extraordinarily low.
An owner who charges consistently can genuinely achieve them for most journeys, since the engine simply does not run during a normal week.
Unplugged, the vehicle is a heavy conventional hybrid
With a depleted battery, the vehicle relies on its engine while still carrying the battery, motor, power electronics and associated structure.
That mass is substantial, and it must be accelerated and carried on every journey regardless of whether the electric capability is used.
Consumption in this state is therefore worse than a comparable conventional hybrid and often worse than a well-chosen petrol car.
Charging opportunity determines who benefits
An owner with off-street parking can plug in nightly and rarely use fuel, making the arrangement highly effective for their circumstances.
An owner dependent on public charging faces a poor proposition, since these vehicles charge slowly compared with fully electric ones and occupy a charger for modest gain.
This is why the same vehicle produces wildly different real-world figures between households, more than almost any other powertrain type.
Company fleets exposed the gap
Where tax and benefit schemes reward low official emissions, plug-in hybrids became attractive on paper regardless of how they would be used.
Fleet data has generally shown that vehicles provided with fuel cards and without home charging spend much of their life running on the engine.
Regulators responded by revising how the figures are calculated to reflect a more realistic share of electric running, which changed the arithmetic considerably.
The revision worked by weighting the official figure according to how far a vehicle can genuinely travel on electricity and how often a comparable driver would recharge, rather than assuming a full battery at the start of every journey. Vehicles with the smallest batteries were affected most, and several left their ranges as a result.
The engineering compromise is real either way
A plug-in hybrid carries two complete drivetrains, which costs space, weight, complexity and eventually maintenance across two systems.
Engines that run infrequently have their own issues, including fuel ageing in the tank and condensation in the exhaust, which some vehicles manage by running the engine deliberately.
The format suits a specific pattern of use extremely well and suits everything else poorly, which is an unusual characteristic for a mainstream vehicle.