What is Wh/km and how does it relate to mi/kWh?

Wh/km measures energy consumed per km. mi/kWh measures distance per energy unit. To convert: mi/kWh = 1609.34 ÷ (Wh/km × 1000). Lower Wh/km = more efficient.

Why use usable battery instead of total capacity?

EV batteries reserve a buffer (typically 5–15%) to protect battery health. Usable capacity is what you can actually use. Setting this accurately gives a more realistic range estimate.

EV Economy Calculator — Wh/km, mi/kWh, Range & Cost

Efficiency

Battery capacity

kWh

Electricity price

/kWh

Usable battery

Efficiency conversion & range

Wh / km

miles / kWh

kWh / 100 km

Range (km)

Range (miles)

Cost / 100 km

About the EV Economy Tracker

Electric vehicle running costs differ fundamentally from petrol vehicles. Instead of MPG or litres per 100km, EV efficiency is measured in kWh per 100km or miles per kWh. Real-world range depends on driving style, temperature (cold weather reduces range by 15-30%), speed (motorway driving uses more energy than city driving), and payload. Tracking your own consumption gives a far more accurate cost estimate than published WLTP figures.

Key EV running cost metrics

Home charging — at typical UK rates (~28p/kWh), a 60 kWh battery costs about 16.80 for a full charge giving 250-300 real-world miles.
Public rapid charging — typically 70-85p/kWh, making it 2-3x more expensive than home charging but still usually cheaper than petrol per mile.
Cost per mile comparison — a typical EV at 3.5 miles/kWh at home rates costs about 8p/mile; a petrol car at 40 MPG at 1.55/litre costs about 17.6p/mile.

WLTP vs real-world range

WLTP range figures are measured under controlled laboratory conditions. Real-world range is typically 15-25% lower. In winter, range can fall 25-30% below the WLTP figure. Use 80% of the advertised range as a conservative estimate when planning journeys.

Charging speed and battery health

Not all charging is equal. Fast charging (DC rapid, 50-350kW) replenishes 80% in 20-45 minutes but repeated fast charging slightly accelerates long-term battery degradation. Home AC charging (7-22kW) is slower but gentler on the battery. Most manufacturers recommend limiting DC fast charging to when needed for long journeys and relying on home charging day-to-day.

Level 1 (3kW) — standard 3-pin plug; adds 10-15 miles per hour; suitable for plug-in hybrids
Level 2 (7-22kW) — dedicated home charger; adds 25-80 miles per hour; overnight charging recommended
DC Fast (50kW) — adds 100-150 miles in 30 minutes; common at motorway services
Ultra-rapid (150-350kW) — adds 100 miles in 10-15 minutes; requires compatible vehicle

Frequently Asked Questions

How do I calculate EV running cost per mile?

Divide your electricity rate (pence per kWh) by your car's efficiency (miles per kWh). At 28p/kWh and 3.5 miles/kWh: 28 / 3.5 = 8p per mile. At a 80p/kWh rapid charger: 80 / 3.5 = 22.9p per mile. Home charging is typically 2-3x cheaper than public rapid charging.

How does cold weather affect EV range?

Cold reduces battery capacity and increases energy demand for cabin heating. Typical range reduction is 15-30% at 0 degrees C. Pre-conditioning the cabin while plugged in, using seat heaters instead of cabin heating, and driving at moderate speeds all help preserve range in winter.

Is home charging or public charging cheaper?

Home charging is much cheaper. UK home electricity averages around 28p/kWh. Public rapid chargers charge 70-85p/kWh. Off-peak home tariffs (like Octopus Go) can bring charging below 10p/kWh — making EVs extremely cheap to run for drivers with home charging access.

What is the difference between kWh/100km and miles per kWh?

They express the same efficiency in opposite directions. Lower kWh/100km means more efficient. Higher miles/kWh means more efficient. To convert: miles/kWh = 62.14 / (kWh/100km). Example: 20 kWh/100km = 62.14/20 = 3.1 miles/kWh.

How do I calculate annual EV electricity cost?

Divide annual mileage by efficiency (miles/kWh) to get kWh used, then multiply by rate per kWh. Example: 10,000 miles / 3.5 miles/kWh = 2,857 kWh. At 0.28/kWh = 800/year at home. Compare to petrol: 10,000 miles / 40 MPG x 4.546 litres/gallon x 1.55/litre = 1,762/year.