Eight worked examples

What Wattrix is for

Every example below opens the calculator already filled in with a realistic car, route and price. Change anything you like - it is your copy.

The long weekend drive
The winter range reality check
Home tariff versus the motorway
Should I switch from petrol?
Towing, roof boxes and full cars
Mileage claims and small fleets
Is the fast charger worth the detour?
Will it be full by morning?

The long weekend drive

“How many stops does 620 km actually take, and what does the charging cost?”

Long drives are where range anxiety comes from, and it is almost always the charging stops rather than the distance that decide whether the day works. A 620 km run in a 77 kWh car is not one charge, and pretending it is turns into an unplanned hour at a service station.

The trip planner works backwards from your reserve. It fills the gap between what is in the pack and what the drive needs, then charges in realistic windows - arrive near your reserve, leave at 80%, because the last fifth of the pack is where the charging curve falls off a cliff.

Worth knowing

  • Set the reserve to what you would genuinely accept arriving on, usually 10 to 15%.
  • Charging to 80% twice is normally faster than charging to 100% once.
  • Motorway speed is the single biggest lever - 120 km/h costs about 28% more than mixed driving.
Open this trip: 620 km, VW ID.4, 150 kW chargers

The winter range reality check

“It says 380 km on the dash. It is -15 °C. What have I actually got?”

Cold hits an EV twice. The chemistry itself delivers less, and the cabin heat has to come out of the same battery rather than out of waste engine heat. Add winter tyres and a motorway cruise and the gap between the rated figure and reality gets big enough to strand people.

The range tool applies each of those penalties separately so you can see which one is costing you. The chart plots range against outside temperature across the whole year, which is the fastest way to understand your own car.

Worth knowing

  • Preconditioning while still plugged in spends grid energy instead of battery.
  • A cold pack also charges slower on DC - Wattrix derates for that too.
  • Winter tyres add roughly 5%, worth knowing before you blame the battery.
Open this: full charge at -15 °C on winter tyres

Home tariff versus the motorway

“How much is public charging really costing me over a year?”

The difference between a cheap overnight home rate and a motorway rapid charger is often three to four times per kWh. Spread over a year of normal driving that gap is usually worth more than most people expect, and it is the strongest argument for a home wallbox.

Set the split between home and public charging in the comparison tool and watch the annual figure move. It also folds in charging losses, which you pay for at the meter but never see in the battery.

Worth knowing

  • Charging losses are real money: about 11% on AC, 5% on DC.
  • Even 25% public charging can double your effective price per kWh.
  • Try the slider at 0% and 100% to see the range you are working within.
Open this: 20 000 km a year, 30% public charging

Should I switch from petrol?

“Forget the brochure. What is the running cost difference for my mileage?”

Fuel cost per 100 km is the number that decides this, and it is easy to work out honestly: your real consumption, your real electricity price, your real annual distance. No residual values, no incentives, no assumptions about what you should want.

The comparison tool puts the two side by side per 100 km, per year and over five years, and shows the tailpipe CO2 the combustion car emits. It deliberately does not invent a well-to-wheel figure for the EV, because that depends entirely on your grid.

Worth knowing

  • Use the consumption you actually see, not the WLTP number.
  • Include a realistic share of public charging or the result flatters the EV.
  • The five-year line is usually the one that changes minds.
Open this: 15 000 km a year against a 6.5 L/100 km petrol

Towing, roof boxes and full cars

“We have got the caravan on the back. Does the plan still work?”

Aerodynamics dominate at speed, so anything that changes the shape of the car changes the trip. A caravan can add 60% to consumption, a roof box around 16%, and a fully loaded car with four people and luggage about 8%. Planning a holiday drive on the empty-car figure is how people end up stopping three times instead of two.

Pick the load in the conditions panel and the whole plan updates - energy, stops, time and cost all move together.

Worth knowing

  • Towing at 100 km/h instead of 120 recovers a surprising amount.
  • Remember the roof box on the way home too, even when it is empty.
  • Check whether your charging stops can physically take a trailer before you rely on them.
Open this: 400 km towing a caravan

Mileage claims and small fleets

“What did that client visit cost in energy, and can I file it?”

If you claim mileage or run a handful of vehicles, you need a defensible number rather than a guess. Wattrix gives you the energy used, the price you paid and the cost per 100 km for a specific journey, and exports the inputs and the result together as a CSV so the working is visible.

Trips you repeat can be saved by name and reloaded in one click, which is quicker than retyping the same route every month.

Worth knowing

  • Export the CSV: it contains every input as well as the result, so the number is auditable.
  • Save each regular route by name, then just update the starting charge.
  • Print to PDF gives you a clean one-page record with the branding stripped out.
Open this: a 180 km round trip on a 50 kW charger

Is the fast charger worth the detour?

“150 kW site ten minutes away, or the 50 kW one right here?”

Charging power is never just the number on the pillar. Your car has its own ceiling, and the taper means average power over a session is always well below peak. A car that peaks at 100 kW gains nothing from a 350 kW post, and above 80% almost every car is slow regardless of what it is plugged into.

The session tool shows the power curve across the window you actually charge in, plus the average you will really see and the time per 100 km of range added - which is the number that decides whether the detour pays.

Worth knowing

  • Compare 20-80% on both chargers rather than peak against peak.
  • Look at the time per 100 km added, not the headline kW.
  • A cold battery can halve DC power. Precondition if the car lets you.
Open this: Ioniq 5 from 15 to 80% on a 150 kW post

Will it be full by morning?

“Home from work at 30%, leaving at seven. Is the wallbox enough?”

Everyday charging is an AC question, and AC is refreshingly simple: your onboard charger is the ceiling, and a 22 kW post does nothing for a car that accepts 11 kW. The only real surprise is how long a big pack takes on a household socket - a 77 kWh car on 2.3 kW is a two-day job.

Set the session from where you plug in to where you want to be and the tool gives you the hours, including the AC losses you pay for but never see in the battery.

Worth knowing

  • Check your onboard AC limit - some cars are 7.4 kW, a few are 22 kW.
  • AC losses run around 11%, which shows up on your bill.
  • For daily use, 80% is kinder to the pack than 100%.
Open this: 30 to 90% overnight on an 11 kW wallbox

Your situation is not on the list?

Every one of these is the same four tools with different numbers. Open the calculator and put yours in.

Open the calculator Take the tour first