Working out what an electric car charge really costs, including the losses
Most EV charging calculators take the battery capacity, multiply by the electricity price, and stop there. That misses two things that matter: the energy lost between the meter and the battery, and the taper that slows down rapid charging past 80 per cent but does not apply to your home charger at all. This article walks through every control on the EV Charging Cost and Range calculator, works through a real charging session with numbers, and explains the gap between the headline range and the range you actually get.
What the calculator works out
The tool takes your battery size, your driving efficiency, the charge window you want, and the power of your charger, then produces three answers: the cost of the charge, the range it buys, and how long it takes. It does this while accounting for charging losses and for the taper that affects DC rapid chargers above 80 per cent but not AC home chargers.
It also offers a petrol comparison. Enter an electricity price, a fuel price, and a fuel economy figure, and the output shows the cost of the same journey in both an electric car and a petrol car. That comparison is where most of the real-world decision-making happens, because the question people actually ask is not what does charging cost, but is it cheaper than fuel.
Battery capacity and driving efficiency
The first field, Battery capacity, takes the usable kilowatt-hours from the car's spec sheet. This is not the same as the gross capacity; manufacturers hold back a buffer at the top and bottom to protect battery health. A car advertised as having a 64 kWh battery might have 60 kWh usable. Use the usable figure.
The Type of car dropdown offers six presets: Small hatchback at 14.5 kWh per 100 km, Compact or family at 16.5, Saloon at 17.5, SUV or crossover at 20.0, Large SUV or truck at 25.0, and Van at 26.0. These are typical real-world figures, not laboratory ones. If your trip computer shows a different number, enter it in the Your real efficiency field and the preset is overridden. The Units dropdown switches between kilometres and miles.
Weather and its effect on range
The Weather dropdown adjusts efficiency with a multiplier. Mild conditions at 15 to 25 degrees Celsius apply no penalty. Cool weather at 5 to 15 degrees adds 12 per cent. Cold weather at minus 5 to 5 degrees adds 30 per cent. Freezing conditions below minus 5 degrees add 45 per cent. Hot weather above 30 degrees adds 15 per cent.
Cold weather is the single biggest range killer for electric cars. A petrol engine heats the cabin with waste heat for free. An electric motor produces very little waste heat, so the cabin heater draws directly from the battery. On a freezing day, the heater alone can consume nearly as much energy as moving the car, which is why winter range can drop by almost a third compared with summer. Knowing this before a long trip prevents unpleasant surprises at the charger.
The charge window and charger power
Two percentage fields, Charge from and Charge to, set the range of the charging session. The default is 20 to 80 per cent, which is the everyday habit most manufacturers recommend for daily use. Charging to 100 per cent is fine before a long trip, but sitting at full charge for extended periods is harder on lithium cells. Cars with LFP batteries are an exception and actually benefit from a periodic full charge to keep the state-of-charge meter accurate.
Charger power, in kilowatts, is the critical number for charging time. A typical home wall box is 7.4 kW. A workplace or destination charger might be 22 kW. Public rapid chargers range from 50 kW to 350 kW. The tool uses this number to decide which charging model applies. At or below 22 kW the charge is AC, handled by the car's onboard charger, and the rate is essentially constant from start to finish. Above 22 kW the charge is DC, and the car tapers the current as the battery fills.
Charging losses: what the meter counts
The Charging loss field, set to 10 per cent by default, represents the energy that the charger draws from the wall but that never reaches the battery. It is lost as heat in the charging cable, the onboard charger, and the battery management system. You are billed for all of it.
A 60 kWh battery going from 20 to 80 per cent needs 36 kWh delivered to the cells. With a 10 per cent loss, the meter reads 40 kWh. Ignoring this underestimates the cost of every charge by about a tenth. Over a year of daily charging, that tenth adds up to a meaningful amount of money. The default of 10 per cent is a good average for home AC charging. DC rapid charging losses can be higher, sometimes 12 to 15 per cent, because the higher power generates more heat.
A worked example: a family car on a home charger
Take a compact car with a 60 kWh battery, a real efficiency of 16.5 kWh per 100 km, mild weather, charging from 20 to 80 per cent on a 7.4 kW home wall box with 10 per cent loss. The battery needs 36 kWh delivered. With the loss, the meter reads 40 kWh. At 7.4 kW, the charge takes about 4 hours and 52 minutes, which fits neatly into an overnight slot. The clock runs on the 36 kWh the cells take, not on the 40 kWh the meter counts.
The range gained is roughly 218 km, because 36 kWh divided by 16.5 kWh per 100 km gives about 218 km. If the electricity price is 0.30 per kWh, the charge costs 12.00 in total. Compare that with a petrol car doing 7 litres per 100 km at 1.80 per litre: the same 218 km costs about 27.49 in fuel. The electric car costs less than half as much for the same distance.
Now change the charger to a 150 kW rapid unit and set the price to 0.65 per kWh. The same 36 kWh costs 26.00 at the higher rate, and the saving over petrol nearly vanishes. That is the real lesson: charging at home is almost always cheaper than fuel, but public rapid charging can be comparable or more expensive.
The 80 per cent taper and why it only applies to DC
On a DC rapid charger, the car reduces the charging current as the battery fills past about 80 per cent. This protects the cells from damage. The tool models the tail at half rate, so the last 20 per cent costs the time that 40 per cent would have taken at full speed. That is why rapid charging sessions are always quoted as 10 to 80 per cent.
On an AC home or workplace charger, this taper does not happen. The onboard charger already limits the current to well below what the cells can absorb, so there is no need for the battery management system to intervene. An overnight charge runs at a constant rate all the way to 100 per cent. Many online calculators apply the taper to every charge, which makes home charging look slower than it really is. This tool uses the charger power to decide which model applies and only tapers when the physics calls for it.
For long journeys with DC stops, two shorter charging sessions often take less total time than one long one. Stopping twice at 10 to 60 per cent avoids the slow tail above 80 per cent and gets you back on the road faster.
Common mistakes when estimating EV costs
The most frequent error is ignoring charging losses. The battery says 36 kWh but the meter says 40 kWh, and the bill is based on the meter.
The second most frequent is using the manufacturer's range number. That comes from a laboratory test at mild temperatures with no motorway cruising and no heating or cooling. Real driving is typically 10 to 20 per cent worse in summer and 30 to 40 per cent worse in winter. Use the figure your trip computer actually shows.
A third mistake is comparing home electricity prices with public charger prices as if they are the same thing. Home overnight rates can be a third of the public rapid rate, so the cost per kilometre swings dramatically depending on where you charge.
When this calculator is not enough
The tool covers a single charging session on a single tariff. It does not plan a multi-stop road trip, account for time-of-use tariffs that change through the day, or model battery degradation over years of ownership. For trip planning with charger locations and stop durations, a route planner designed for EVs is the right tool.
It also does not give financial advice about whether buying an electric car saves money overall. That calculation includes the purchase price, insurance, servicing, tyre costs, and resale value, none of which belong in a per-charge calculator.
Privacy and where the numbers stay
The entire calculation runs in your browser on your own device. Nothing you enter -- your battery size, your electricity price, your driving habits -- is sent to any server. You can disconnect from the internet after loading the page and the calculator still works.
Charging data might seem harmless, but it reveals driving patterns, home energy costs, and daily routines. Running the calculation locally means that information stays with you and nobody else.