EV Battery, Tariff & Mileage Details
📊 EV Charging Cost Breakdown
Driving an electric vehicle (EV) in the UK delivers dramatic fuel savings compared to traditional petrol or diesel cars, particularly when charging at home on dedicated overnight off-peak EV electricity tariffs.
While a petrol car averaging 40 MPG costs 16.5p per mile in fuel, an EV charged overnight at 7.5p per kWh costs just 2.14p per mile—saving average drivers over £1,200 per year in fuel costs.
⚙️ UK EV Efficiency & Electricity Tariff Rules for 2026/27
1. The EV Cost per Mile Formula
- Total Annual Electricity Consumed (kWh):
Annual kWh = Annual Mileage ÷ Real-World Efficiency (mi/kWh). - Cost per Mile (p/mi):
Cost per Mile = Electricity Tariff Rate (p/kWh) ÷ Real-World Efficiency (mi/kWh). - Annual Charging Cost (£):
Annual Cost (£) = [ Annual kWh × Electricity Tariff Rate (p) ] ÷ 100. - Example: Driving 10,000 miles per year in an EV averaging 3.5 miles/kWh on a 7.5p/kWh off-peak tariff:
Annual kWh = 10,000 ÷ 3.5 = 2,857 kWh.Cost per Mile = 7.5p ÷ 3.5 = 2.14 pence per mile.Annual Electricity Cost = (2,857 × 7.5p) ÷ 100 = £214.29 per year.
2. Electricity Tariff & Charging Location Rates
Where you charge your EV has the biggest impact on your driving costs:
- Home Off-Peak Smart EV Tariff (e.g. Octopus Go / OVO Charge): 7.0p to 9.0p per kWh (2.0p to 2.5p per mile).
- Home Standard Peak Tariff (Ofgem Cap Rate): 24.5p per kWh (7.0p per mile).
- Public Rapid DC Charger (50 kW): 55.0p to 65.0p per kWh (15.7p per mile).
- Ultra-Rapid Highway Charger (150 kW - 350 kW): 79.0p per kWh (22.5p per mile).
3. Real-World EV Efficiency Benchmarks (Miles per kWh)
- Compact Electric Cars (e.g. Fiat 50e, Peugeot e-208): 4.0 to 4.8 miles / kWh.
- Medium Family EVs (e.g. Tesla Model 3, Hyundai Ioniq 5, VW ID.4): 3.5 to 4.0 miles / kWh.
- Large Luxury EVs / SUVs (e.g. Audi Q8 e-tron, BMW iX): 2.2 to 2.8 miles / kWh.
📊 Practical EV Charging Cost Worked Examples
Below are two worked calculation examples illustrating EV driving costs:
- Annual Mileage: **10,000 Miles**
- EV Efficiency: **3.5 Miles / kWh** (2,857 kWh total)
- Electricity Rate: **7.5 p / kWh** (Home Off-Peak Tariff)
Calculation: Cost per mile = 7.5p ÷ 3.5 = 2.14p/mi. Annual EV cost = £214.29 (Saved £1,285.71 vs 16.5p/mi petrol car).
- Annual Mileage: **10,000 Miles**
- EV Efficiency: **3.5 Miles / kWh** (2,857 kWh total)
- Electricity Rate: **79.0 p / kWh** (Public Ultra-Rapid Charger)
Calculation: Cost per mile = 79.0p ÷ 3.5 = 22.57p/mi. Annual EV cost = £2,257.14 (More expensive than petrol!).
📑 Common Pitfalls & EV Charging Warnings
- Relying Exclusively on Public Rapid Chargers: Public chargers attract 20% VAT (compared to 5% VAT on domestic home electricity), plus commercial profit margins. Charging exclusively at public rapid chargers negates the fuel savings of owning an EV.
- Ignoring Winter Range & Efficiency Drops: Cold winter temperatures reduce lithium battery efficiency by 20% to 30% due to battery cabin heating demands. An EV averaging 4.0 mi/kWh in summer drops to ~3.0 mi/kWh in winter.
- HMRC Advisory Electricity Rate (AER) Reimbursement: If you drive a company electric car for business travel, HMRC’s Advisory Electricity Rate (AER) reimburses business mileage at 8.0p per mile.
❓ Frequently Asked Questions (FAQ)
An Intelligent EV Tariff (e.g. Octopus Intelligent Go) connects directly to your EV or smart charger via an app. You set your required morning charge level, and the supplier automatically schedules cheap 7p/kWh charging during low grid demand hours.
Charging a 60 kWh battery from 10% to 80% requires **42 kWh of energy** (60 × 0.70). At a 7.5p/kWh off-peak rate, this 70% charge costs just **£3.15**.
To protect battery cell chemistry from overheating and degradation, the EV's Battery Management System (BMS) automatically throttles (slows down) charging speed after reaching 80% capacity.
AC (Alternating Current) charging is used for home wallboxes (7.4 kW), relying on the car's onboard converter. DC (Direct Current) charging bypasses the onboard converter to supply high-power electricity (50 kW to 350 kW) directly to the battery.