The Objective: Determine the exact financial delta of running a 2020 Mercedes-Benz E300 de on battery power versus its diesel engine over a 1,200-mile sample size, calculated against the last 7-month window of your ownership (December 2025 to June 2026).

The Baseline Data:

  • Total Distance: 2,294.7 miles
  • Electric Distance: 1,198.9 miles
  • Diesel Price: £1.63 per litre (averaged over the past 7 months)
  • Grid Electricity Price: 25p per kWh
  • E300 de Battery Capacity: 13.5 kWh
  • Real-World EV Range: 21 miles
  • Real-World Diesel Economy: 54.3 MPG

The 7-Month Fuel Data Context

Over the past 7 months diesel pump prices have been highly volatile.

  • Prices started relatively stable, averaging around 145p per litre in December and dropping to roughly 141p by February.
  • Spring 2026 saw a sharp spike, peaking mid-April at over 192p per litre.
  • Prices then tumbled rapidly in June, dropping nearly 17p to close the month at 167.14p.

Considering this, the average pump price across the window is roughly 163p (£1.63) per litre.

Realistically, I only actually went to the pumps three times, avoiding a full tank to prevent just carting 65kg of diesel around constantly.

The Grid Calculation

To calculate the cost of the 1,198.9 electric miles, the total energy drawn from the grid must be known.

  • Dividing the total electric miles by the 21-mile real-world range yields 57.09 full charge cycles.
  • Multiplying by the 13.5 kWh battery capacity gives a total energy consumption of 770.72 kWh.
  • At a standard domestic rate of 25p per kWh, the electric running cost is £192.68.

To establish the alternative, the cost of driving those same miles using the 2.0-litre diesel engine is calculated.

  • Dividing 1,198.9 miles by the real-world average of 54.3 MPG requires 22.08 UK gallons.
  • Converting to metric (4.546 litres per UK gallon) gives 100.37 litres.
  • At the 7-month average of £1.63 per litre, the equivalent diesel cost is £163.60.

The Verdict

Power Source Total Energy/Fuel Total Cost
Grid Electric (25p/kWh) 770.72 kWh £192.68
Diesel Engine 100.37 Litres £163.60
Net Margin   -£29.08 (Loss)

Observations

I used pre-heating in the winter, so electricity usage was probably higher overall, and similarly, the heatwaves recently have meant a lot of pre-cooling before journeys too.

The Optimization Layer

Even with the volatile spike in spring diesel prices, running the E300 de on standard 25p domestic electricity with a 21-mile range is an economic loss. The dead weight of the internal combustion engine and fuel degrades the electrical efficiency, while the diesel engine remains highly efficient on its own.

To invert this deficit and make the plug-in hybrid mathematically viable, the charging architecture must bypass standard daytime utility pricing.

1. Time-of-Use Arbitrage (Smart EV Tariffs) Routing the charging schedule exclusively to an overnight off-peak tariff (averaging 7p per kWh) drops the cost of the 770.72 kWh draw to £53.95. This flips the script, creating a positive savings margin of £109.65 compared to burning diesel at the 7-month average price.

2. Solar Export Arbitration Charging from a local home solar array shifts the math to opportunity cost. If excess generation is dumped into the vehicle instead of exported to the grid at a standard Smart Export Guarantee (SEG) rate of 15p, the 770.72 kWh effectively “costs” £115.61 in lost revenue. This yields a net positive savings of £47.99, avoiding peak utility pricing completely.

Couple of other thoughts

1. Cost of Maintenance and Wear If was running constanly on the diesel engine the main trips that I undertake, a 4km round trip to the nursery twice a day, would be the worst kind of driving for it. That is where the electric motor shines, the trips are short, and generally

2. The Rule of Cool It’s fun to drive under electric power, pulling away in the morning without having to start the diesel from cold on the driveway is more pleasant, especially when we have the windows open. Likewise returning home down the cul-de-sac without the noise of the diesel engine in the afternoon or evening feels a bit more neighbourly.