Last updated: 2026-07-02

What this comparison measures

This calculator estimates the yearly carbon dioxide emissions of driving an electric vehicle (EV) versus a petrol car over the same distance. It looks at the use phase only: the electricity an EV draws from the grid, and the petrol a combustion car burns at the tailpipe. The headline number is the difference between the two — how much CO₂ switching to electric would add or avoid in a typical year.

How the estimate is built

Two parallel calculations run over the same annual distance.

Electric car. An EV has no tailpipe, so its emissions come from generating the electricity it consumes:

EV CO₂ = (distance ÷ 100) × EV efficiency × grid intensity ÷ 1000

Distance is in kilometres, EV efficiency in kilowatt-hours per 100 km, and grid intensity in grams of CO₂ per kilowatt-hour. Dividing by 1,000 converts grams to kilograms.

Petrol car. Burning petrol releases a well-established amount of CO₂ — about 2.31 kg per litre:

Gas CO₂ = (distance ÷ 100) × fuel economy × 2.31

Fuel economy is in litres per 100 km. (Diesel is denser and emits roughly 2.68 kg/L; this model assumes petrol.)

The difference is simply the petrol figure minus the EV figure. A positive result means the EV emits less over the year.

Worked example

Take a 15,000 km year, an EV using 18 kWh/100km on a grid at 400 g CO₂/kWh, and a petrol car managing 7 L/100km:

• EV: (15,000 ÷ 100) × 18 × 400 ÷ 1000 = 150 × 18 × 0.4 = 1,080 kg CO₂.
• Petrol: (15,000 ÷ 100) × 7 × 2.31 = 150 × 7 × 2.31 = 2,425.5 kg CO₂.
• Difference: 2,425.5 − 1,080 = 1,345.5 kg CO₂ avoided by the EV.

Now change only the grid intensity. On a clean grid at 50 g/kWh the EV drops to 135 kg, widening the saving to about 2,291 kg. On a coal-heavy grid at 750 g/kWh the EV rises to 2,025 kg, shrinking the saving to roughly 400 kg. Same car, same driving — the grid mix does most of the work.

Why grid intensity dominates

Petrol emissions are fixed by chemistry: a litre burned always releases about the same CO₂. EV emissions are not — they scale directly with how the local electricity is generated. Grid intensity ranges from under 50 g CO₂/kWh where hydro, nuclear, or wind dominate, to over 700 g/kWh on coal-reliant grids, and it shifts hour to hour as different plants come online. Because that factor can vary more than tenfold, it is the single most important input here. Charging on a cleaner grid, or at times of day when renewables are plentiful, meaningfully lowers an EV's footprint.

Converting from mpg and miles

If figures are quoted in imperial units, switch the distance field to miles and convert fuel economy: litres per 100 km ≈ 235.2 ÷ mpg (US) or 282.5 ÷ mpg (imperial). For the EV, some sources quote miles per kWh — invert and rescale to get kWh/100km.

What this leaves out

• Manufacturing. Building a car, and especially its battery, carries an upfront carbon cost not counted here. A full lifecycle assessment adds it; the extra is usually repaid within the first years of driving, sooner on cleaner grids.
• Fuel supply chains. Extracting, refining, and transporting petrol (well-to-tank) adds emissions beyond the tailpipe, and generating electricity has its own upstream losses. This model counts only tailpipe combustion and grid generation.
• Real-world variation. Efficiency swings with weather, terrain, driving style, and load. Cold weather in particular reduces both EV range and petrol economy.

Treat the output as an order-of-magnitude comparison for a given grid and driving pattern, not a precise measurement. The clearest takeaway is usually directional: on most grids the EV emits less, and the advantage grows as electricity gets cleaner.