How we calculate running costs

One model for every car

A car's running cost is just the energy it uses times what that energy costs. We express consumption per 100 km — kWh for electricity, litres for petrol or diesel — and multiply:

• Cost per 100 km = energy per 100 km × price per unit, added up across each energy type the car uses.
• Cost per year = cost per 100 km × (your annual km ÷ 100) + any fixed fees.

Nothing about the car changes with distance, so the per-year cost rises in a straight line as you drive more. That's why the cost-vs-distance chart is made of straight lines — and why, when two lines cross, the cheaper car flips.

Electricity loses a little in charging

An electric car draws more from the wall than actually reaches the battery. We account for that with a 90% charging efficiency: the electricity you pay for is the car's battery figure divided by 0.9. Petrol and diesel have no equivalent loss.

Plug-in hybrids are a range, not a point

A plug-in hybrid runs on electricity until the battery is low, then on fuel — so its real cost depends on how much of your driving is electric. We estimate that electric share (the "utility factor") from three things: the car's electric range, your mean daily distance, and how often you can plug in. A car whose range comfortably covers most days runs mostly on electricity; long days or rare charging shift it toward fuel.

Our formula follows the shape of the SAE J2841 standard for electric share, which we check with a unit test (a 40-mile-range plug-in at the US mean daily distance lands near 62% electric, as the standard says). Because the answer is a range, plug-in hybrid costs are shown as a band — cheapest if you always charge, dearest if you never do, with the expected value in between. Charging defaults to 100% when you can charge at home and 40% when you can't; both are overridable.

Subscription charging plans

Some charging plans add a flat monthly fee for a lower per-kWh rate. We amortise that fee across the year, so a subscription only comes out ahead once you drive far enough for the cheaper energy to outweigh the standing cost — exactly the trade-off the comparison should surface.

What these figures are — and aren't

These are energy running costs only. On purpose, they do notinclude:

• purchase price or depreciation,
• insurance, registration or road tax,
• servicing, tyres or repairs.

So this answers "which is cheaper to fuel/charge", not full cost of ownership. Adding purchase price — which is what makes a cost-vs-distance chart actually cross at a break-even point — is on the roadmap, not in this version. We'd rather show one honest number than a total-cost figure built on guesses.

Where the numbers come from

Cars & economy. The Danish catalogue — 270models — is built automatically from the EEA CO2 monitoring (Regulation (EU) 2019/631) dataset (the official EEA record of cars registered in Denmark), table co2cars_2025Pv31, generated 2. jul. 2026. Economy is the WLTP figure from those records, converted to kWh or litres per 100 km. It refreshes weekly, and every value passes a plausibility gate before it can be published — a mis-converted figure fails the build rather than shipping a wrong page. You can also edit any car's economy in the tool.

Electricity. 1,72 kr./kWh, from Energi Data Service (Energinet) — 30-day avg day-ahead spot (DK1+DK2) + Danish grid/system tariffs + electricity tax, incl. 25% VAT (as of 3. jul. 2026).

Fuel. Petrol 17,45 kr./L and diesel 14,97 kr./L, from EU Weekly Oil Bulletin — Weekly DK pump price, Eurosuper 95, incl. all taxes (as of 29. jun. 2026).

Illustrative, for now. These default prices are real and sourced, but they're a snapshot — illustrative, not a live per-user quote. Tax and grid-tariff components are typical published values that vary by supplier, region and time of day, and the branded per-provider prices in the charging matrix are examples. Live, continuously-refreshed pricing (a Cloudflare Worker in front of the same sources) is planned for v2. Every price is editable in the tool, so you can drop in your own tariff and get your own number.

Defaults & assumptions

• Annual distance defaults to 15.000 km; charging defaults to at home.
• Charging efficiency: 90%.
• All prices include taxes and VAT, in Danish kroner.
• Every input — cars, economy, distance, charging, prices — is overridable.

Neutrality

The ranking is decided only by computed running cost. It is never influenced by referrals, advertising or any commercial arrangement — the neutral comparison is the product.