Consumption, not range: the number you actually work with

Range is not something a manufacturer installs. It is a fraction: usable energy on top, consumption per distance underneath. The two quantities are independent of one another, and only one of them appears reliably on a spec sheet.

The top of the fraction is not the pack's gross capacity but the share the car releases. Some of it is held back at the bottom as protection against deep discharge, some at the top as a buffer against ageing. How wide that gap is varies a great deal between cars — the usable figure is the one you calculate with, not the larger number printed in the brochure.

Underneath sits consumption, and that is the figure worth knowing, because it moves: with speed, with temperature, with load, with the terrain. The range readout in the car is nothing more than this same fraction, recalculated continuously from the consumption of the last few kilometres. It does not forecast the future; it extends the past.

The useful unit is kilowatt-hours per hundred kilometres. It maps directly onto the litres-per-hundred figure a combustion driver already thinks in, it is stable, and it works in both directions: usable capacity divided by consumption, times a hundred, gives range; consumption times distance gives the energy a planned trip will need.

Some displays invert the fraction and show kilometres per kilowatt-hour. That is the same information, but it distorts comparisons: a gap of two kilowatt-hours per hundred kilometres looks tiny in the reciprocal and is not. When comparing two cars, convert both to the same direction first.

One more distinction belongs here: the car's display almost always counts energy leaving the battery. The bill at a charger and the meter at home count energy going in. Charging losses sit between the two. If you want your cost per hundred kilometres, use the energy you paid for; if you want to plan a leg, use the on-board figure.

Aerodynamic drag rises with the square of speed. The power it demands is force times speed, so it rises with the cube. Raise your pace by a fifth and the drivetrain has to find roughly three quarters more power for air alone. This is not a matter of how a car is tuned, and not a quirk of one model; it is fluid mechanics.

Rolling resistance behaves differently: across a wide range it is roughly proportional to mass and barely to speed. At urban pace it dominates; at motorway pace it is a side item. That is why an electric car is frugal in town and thirsty on the motorway — the exact inverse of a combustion car, which throws its heat away in traffic and runs near its best operating point on a long haul.

Practically, this yields the one measure that works immediately and reliably on a long trip: ease off. It costs driving time and saves charging time, and whether the net result is faster depends on how steep the charging curve still is in the range you are using. The only way to settle it is with your own consumption figure — which brings us back to the number this piece is about.

After speed comes frontal area. A roof box or a set of bars changes precisely the item that consumes most of the power at speed — and it does so whether or not anything is inside. An empty carrier left on all year is the most expensive accessory on the car.

Then tyres. Width, diameter, compound and tread pattern set the rolling resistance, and pressure sets it too. A tyre run soft through a winter costs a measurable amount of energy and, incidentally, tread at the shoulders. Tyres are the one component you can change without a workshop visit and thereby shift consumption permanently — in either direction.

Finally the auxiliaries. Heating and air conditioning draw a roughly constant load regardless of speed. So they weigh heavily in percentage terms on a slow town trip and lightly on a motorway run. Comparing a winter figure with a summer figure is really comparing two different ratios of traction power to auxiliary power.

The type-approval figure comes from a rolling road running a prescribed profile, at a prescribed temperature and a prescribed load. That is not a deception; it is the point. A procedure that runs identically for every car makes cars comparable with one another. It does not turn them into a forecast for one particular person on one particular route.

One detail matters for electric cars and is little known: the declared energy consumption is derived from the energy drawn from the mains to recharge — so charging losses are inside it. The on-board display does not count them. Anyone who compares the declared figure with the dashboard and feels pleased about the gap is comparing two different measuring points.

The declared figure is still useful, but only as a ranking. Two cars measured by the same procedure will stand in roughly the same relation to one another on the road as on the rig. The absolute number is not yours; the ratio is.

No app and no instrument are needed, only patience over a distance long enough to mean something. A single commute says nothing; a fortnight of ordinary mixed use says everything. Reset a long-term memory, drive the way you drive, and read it off.

Two numbers will serve for the rest of the car's life: one from summer, one from winter. Everything else lies between them, and the two extremes are enough for planning. If you want the cost as well, keep a record over a few months of how much energy actually went in — that is the number on the bill, and it includes the losses.

Planning follows from those numbers with no further effort. Usable capacity, times the share of it you are actually willing to use, divided by consumption — that is the leg you can drive without thinking about it. Anything beyond that is a decision, not an uncertainty.

Two cars with the same range can be built on opposite principles: one with a small pack and low consumption, the other with a large pack and high consumption. The listing shows the same figure; in use they behave in opposite ways.

The efficient car costs less per kilometre, replenishes a given distance sooner — because less energy has to be moved — and is content with a smaller charging arrangement at home. The big pack buys something else: fewer stops, more reserve in winter, a longer leg without interruption. Both are legitimate, but they are two different purchases.

The choice is not a matter of taste but of the longest trip made regularly and of where the car spends the night. Charge every night and you want efficiency. Charge rarely and travel far and you want capacity. Anyone who claims to need both has not yet written the routes down.

The same car shows two different consumption figures on two European motorways, and the reason is not the country but the pace. Where the limit is lower, or where traffic effectively imposes one, consumption drops sharply; where speeds are higher it climbs with the cube. Which limit applies is published by each country's traffic authority — and it changes, which is why no number appears here.

Terrain adds to it. An Alpine crossing, a run across a high plateau or a coastal leg with a permanent crosswind move consumption more than most drivers expect. So when planning a trip across several countries, do not use one consumption figure for the whole distance — use one per leg.

Temperature does the same again. A drive from north to south ends in a different climate from the one it began in; the legs then differ not only in pace but in the load the heating or air conditioning draws. Plan with the winter figure and arrive in the south and you will have carried too much reserve — which is the pleasanter of the two mistakes.