Car & Vehicle

EV Battery Size Calculator

How much battery a journey actually needs, and whether the one you have covers it without a stop.

7 inputs Free, no sign-up

Your figures

What your car's own trip computer reports, usually as km/kWh or Wh/km. It swings a long way with speed, temperature and the climate control, so use the figure from conditions like the ones you are planning for.

How much more than the bare figure you want in hand. Leave blank for none.

From the handbook or the manufacturer's specification. Usable capacity is a little below the gross figure, because a buffer is held back at each end to protect the cells - and it differs by variant, so it is not filled in here.

From your own bill for home charging, or the price shown on the charger for a public one. Public rates are commonly several times the domestic tariff.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A journey needs a quantity of energy, and a battery holds one. Comparing the two answers the question people really have, which is whether a particular drive is possible without stopping.

Enter the distance and the efficiency you expect on that kind of road. If you name a battery, the result says what share of it the trip would take.

How to use this calculator

  1. Enter the distance you need to cover.
  2. Enter the efficiency you expect — highway figures are worse than city ones for an electric vehicle, which is the opposite of a petrol car.
  3. Set a margin. Twenty per cent is a reasonable starting point on an unfamiliar route.
  4. Optionally name the usable capacity you have, to see whether it fits.

The formula

Energy needed = Distance / Efficiency in km/kWh
With margin   = Energy needed x (1 + Margin / 100)
Share used    = With margin / Battery capacity

A share above 100% means the trip does not fit the battery entered and needs a charging stop. The page states that; it does not suggest a different vehicle.

SymbolMeaningUnit
Distance Distance to cover km or mi
Efficiency Expected on this route km/kWh or Wh/km
Margin Spare capacity you want %

Worked example

250 km at 6 km/kWh with a 20% margin:

Energy needed = 250 / 6      = 41.67 kWh
With margin   = 41.67 x 1.20 = 50 kWh

A 30 kWh battery would be at 167% of capacity — this trip needs a stop. A 60 kWh one would be at 83%.

Notes and limits

  • Usable capacity, not gross. A manufacturer's headline kWh figure usually includes a buffer the car will not let you use.
  • An older battery has lost some capacity. If the car is a few years old, the usable figure is below the original one.
  • Highway speed is the enemy of EV efficiency. Air resistance rises with the square of speed and there is no gearbox to hide behind.
  • Charging on the way is normally the answer for a long trip rather than a bigger battery. Two twenty-minute stops to 80% are quicker than one charge to 100%.
  • To turn the energy into a duration, use the EV charging time calculator.

What this calculation assumes

  • Efficiency holds for the whole journey.
  • Battery capacity, if entered, is the usable figure rather than the gross one.
  • This is the energy the trip takes. It is not a recommendation about which vehicle to buy.

Frequently asked questions

What size battery do I need?

Enough energy for your longest regular journey plus a margin, which is what this page works out. For most people the answer is smaller than they assume, because the longest journey is rare and charging on the way is routine.

How much energy does 250 km take?

Divide by your efficiency. At 6 km/kWh it is 41.7 kWh; at 4.5 km/kWh, which is realistic for fast highway driving in winter, it is 55.6 kWh.

What margin should I allow?

Enough that a detour or a cold morning does not strand you. Twenty per cent is a common starting point; on an unfamiliar route with few chargers, more. The page leaves the number to you.

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