Wattrix

Why it exists

Most EV planners want an account, a route upload and a live connection, and pay for themselves with data or ads. That is a lot of machinery for arithmetic you could do on the back of an envelope if you knew which numbers mattered.

Wattrix does the arithmetic properly and keeps it on your device. There is no backend to send anything to. The page you load is the entire product, which is also why it still works in a basement car park with no signal.

How the maths works

Four things drive every result.

1. Energy for a drive

Distance multiplied by consumption. The only interesting part is that consumption is not a constant, so Wattrix adjusts your rated figure by a multiplier built from temperature, speed, load and tyres:

• Temperature - below 20 °C, roughly 1.1% added per degree. At -10 °C that is about a third more energy.
• Speed - city 0.90×, mixed 1.00×, 100 km/h 1.16×, 120 km/h 1.28×, 140 km/h 1.45×.
• Load - full car 1.08×, roof box 1.16×, trailer or caravan 1.60×.
• Winter tyres - 1.05×.

They multiply together, which is why a cold motorway run with a roof box is so much worse than any single factor suggests.

2. Charging time

Charging power falls as the battery fills, so dividing kWh by kW gives an answer that is wrong in the direction that hurts. Wattrix walks the session in half-percent steps and, at each step, takes the lower of what the charge point can supply and what the car will accept at that state of charge. On DC that acceptance follows a taper curve: flat to about 35%, easing off through the middle, and down to a fraction of peak past 85%. Below 10 °C the DC curve is derated further, because a cold pack genuinely will not take the power.

AC is simpler. Your onboard charger is the ceiling until the BMS starts balancing near the top.

3. Charging stops

The trip planner works out what the drive needs, subtracts what is in the pack above your reserve, and fills the gap in realistic windows - arrive near your reserve, leave at the cap you set, default 80%. Each of those windows is timed with the same taper model, then added up.

4. Cost

Energy at the meter, not energy in the battery. Charging losses of roughly 5% on DC and 11% on AC sit between the two, and you pay for them, so Wattrix bills them.

Accuracy, and where it ends

These are planning estimates. They are built on defensible models rather than round numbers, but a calculator cannot know about the headwind, the roadworks, the 600 m climb, the charger that is sharing a cabinet with the car next to you, or eight years of battery degradation.

• Consumption is the biggest source of error. Use your own long-term average and the rest gets much better.
• Elevation is not modelled. A mountain crossing will cost more than this says, though you get some of it back coming down.
• Charging curves are generic. Your car has its own, and it changes with temperature and state of health.
• Battery degradation is not modelled. Reduce the usable capacity yourself on an older car.

Always keep the reserve you set. The tool is designed around that reserve for a reason.

Privacy and cookies

There is no account, no analytics of your inputs, and no server that could receive them. The calculator does not make a single network request while you use it.

• Strictly necessary - local storage holding your inputs, saved scenarios and your cookie choice. No third party sees any of it.
• Analytics - optional Google Analytics 4 with IP anonymisation, purely page views. Off unless you accept it, and the script is never loaded if you decline.
• Advertising - off, and there are no ad units on the site. The category exists so the control is honest if that ever changes.

Your choice is stored for 180 days and then Wattrix asks again. Withdrawing analytics consent deletes the analytics cookies immediately.

Open cookie preferences

Frequently asked

Do you track me?

Only if you say yes. Analytics is off until you accept it in the cookie banner, and rejecting means the Google Analytics script is never loaded at all - not loaded-but-muted. You can change your mind from the footer on any page, and withdrawing consent clears the analytics cookies straight away.

What is stored on my device?

Three things, all in local storage: your calculator inputs so the page comes back the way you left it, any scenarios you save by name, and your cookie choice. Clearing your browser data for this site removes all of it. There is no server copy because there is no server.

Does it work offline?

Yes. A service worker caches the pages and scripts on your first visit, so Wattrix opens and calculates with no connection. That is deliberate: underground car parks and rural charging stops are exactly where you want it.

Why does my car charge slower than this says?

Cold cells, a shared charging cabinet, a high starting state of charge, or a car that is throttling to protect the pack. The taper curve here is a good generic model, not your specific car on that specific day. Treat it as a planning figure.

Where do the car presets come from?

Published usable-capacity and peak-power figures, with consumption set to a realistic mixed-driving average rather than the optimistic official number. They are a starting point - your own long-term average is always more accurate, so override it.

Can I use miles?

Yes. Distance switches between km and mi, and consumption can be entered as Wh/km, Wh/mi, kWh/100km or mi/kWh. Currency has a few common symbols and you can type any price.

Is it really free?

Yes, with no account and no paywall. It is built and maintained by FHATAL as a genuinely useful tool.

How do I report a wrong number?

Email support@fhatal.com with the car, the inputs and what you saw in reality. Corrections to the preset data are welcome and easy to apply.

Who builds it

Wattrix is made by FHATAL, a small Finnish software company. We build web software, SaaS and digital products, and Wattrix is the tool we wanted for our own cars.