Plug-in hybrid: the arithmetic that comes before the purchase

A plug-in's declared consumption arises differently from every other drivetrain's. Two runs are measured: one with a charged battery and one with an empty one. A weighted average is formed from them, and the weighting comes from an assumption about what share of its kilometres such a car covers electrically.

That assumption is made by the procedure, not by the buyer. It is identical for every car in the same category, which makes it useful for comparing two of them. For the question of what your own car will use, it is worthless — it describes an average driver, and no such person exists.

Your own figure is easy to obtain and does not need estimating. Write down the length of your trips for a fortnight. Count the kilometres that fit inside the electric range and divide by all the kilometres. The result is your share, and with it you compute the consumption yourself: share times the electric figure plus the remainder times the combustion figure.

Describing a plug-in honestly takes two numbers: consumption with a charged battery and consumption with an empty one. The first is good; the second is worse than the same model without the hybrid system — because several hundred kilograms of pack, machine and power electronics are being carried without contributing anything.

There is no meaningful average between those two numbers, because nobody drives on the average. Every day is either a day with a charged battery or a day without, and the distribution of those days across the year is the only quantity that counts.

In practice: ask the seller for both figures, or establish them yourself on the test drive. One run with a full battery and one with an empty one over the same route say more about the car than any figure in a listing — and the second run is the more revealing of the two.

In a plug-in the size of the pack matters less than the number of charges. A small pack filled every evening delivers more electric kilometres over a year than a large one that gets its turn once a week. That inverts the rule for a pure electric car, and the reason is simple: a plug-in empties its pack daily; a battery-electric car does not.

From which follows what kind of parking space suffices. A plug-in does not need a powerful charging installation — it needs one that is available every evening. So the question before buying is not how fast it charges but whether the car spends its nights where there is power, reliably.

And it follows who should not buy one. If you can only charge in public, you pay public-charger rates for small amounts of energy and spend time doing it — for a pack that is empty again after a short distance. In that case the arithmetic fails not only financially but on time as well.

A plug-in's on-board charger is small, often single-phase, and the pack holds little. The two fit together: a small pack is content with a low charging rate, because the night is long. Capacity divided by charging power gives the hours, and for a plug-in that figure stays manageable at almost any socket.

Few plug-in models accept DC at all, and it rarely pays. A rapid charger's power electronics are built for packs many times larger; whatever session and time charges a tariff applies get spread across very few kilowatt-hours when the amount of energy is small. Structurally, a motorway charger is the wrong place for a plug-in.

That is not a weakness but a consequence of the design. A plug-in is built for a parking space, and for the motorway it has an engine. Use it the other way round and you are using both drivetrains at their respective worst points.

A plug-in used exactly as intended has a problem that appears in no brochure: its engine runs rarely, briefly and often cold. That is precisely the duty cycle hardest on an engine. Condensation collects in the oil and in the exhaust, fuel reaches the cylinder walls and thins the oil film, and none of it clears until the engine gets properly hot.

Then there are the parts that need operating temperature to clean themselves. A particulate filter only regenerates when the exhaust is hot enough; if regeneration is repeatedly interrupted because the trip ends, it keeps filling. And fuel standing in a tank for months ages — at low annual mileages that is not a theoretical concern.

This is why a plug-in's service schedule is tied to time and not only to distance. A car that covers little ground still needs its oil change, and postponing it by odometer reading is a false economy. When buying used, check the service book for exactly that: dates, not only mileage.

The decisive question about a used plug-in cannot be answered directly, but it leaves traces. The supplied charging cable is the first: a cable with clean contacts and unbent packaging has rarely worked. A cable with scuffed insulation and wear marks at the plug worked every evening.

The second trace is the charging flap itself. A lid opened hundreds of times shows it at the seal and the hinge; one last moved on the day of delivery looks new while the rest of the car does not. The contacts inside the socket tell the same story of use or disuse.

The third trace is the long-term display in the trip computer. Ask to see it without resetting it first — a figure close to the combustion consumption of the same model settles the question for good. And the fourth is the brake discs: a car that regenerated heavily shows a rust lip at the rim of the disc; one driven almost always on the engine shows ordinary wear.

A small pack loses roughly the same percentage as a big one, but it has far less buffer in absolute terms. What means a shorter leg in a battery-electric car means, in a plug-in, that the electric distance shrinks to a fraction in winter — and with it the share on which the whole calculation rested.

Heating makes it worse. Many plug-ins heat the cabin wholly or partly with engine heat; as soon as it turns cold the engine starts, full battery or not. Others heat electrically and drain the small pack quickly doing it. Either way, far fewer electric kilometres are driven in January than in July.

For an honest annual calculation, then, use two shares: one for the warm half of the year and one for the cold. The mean of the two sits noticeably below what a summer test suggests — and that gap is exactly why many plug-in owners report a different figure from the one they expected.

There are cases where the arithmetic works cleanly. A one-car household with short daily journeys and a socket at the parking space, which drives a long distance a few times a year into an area where charging is thin. A car that has to tow a trailer for which the electric range would never suffice anyway. A region with hard winters and long distances.

What is missing from that list is the reason plug-ins are often bought: a tax or traffic-law advantage. How a vehicle is treated in a low-emission zone, who may enter a city centre and on what conditions, is decided by the city or the state concerned. Those rules change more often than any other argument for buying, and they generally change to the disadvantage of the cars already bought.

So the advice is plain: buy a plug-in because of your driving pattern, not because of a regulation. The binding statement of the rule as it stands today comes from the city or the country's competent authority, and it holds today. You will still be driving the car years from now.

Answer these three honestly and no comparison test is needed. The answers yield the calculation, and the calculation yields the decision.