I wanted to know exactly how much I save - or don’t - by running my 2020 Mercedes-Benz E300 de on battery instead of diesel. So I took the last seven months of my driving, December 2025 to June 2026, and roughly 1,200 electric miles of it, and did the sums.

Here’s what I’m working with:

  • 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

How diesel prices moved

Diesel pump prices bounced around a lot over these seven months.

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

All in, that averages out to roughly 163p (£1.63) per litre, which is the figure I’ve used throughout.

Realistically, I only went to the pumps three times, and I avoided a full tank each time so I wasn’t just carting 65kg of diesel around constantly.

Working out the grid cost

First, the electric side. To cost up those 1,198.9 electric miles, I needed to know how much energy I actually pulled from the grid.

  • 1,198.9 electric miles ÷ my real-world 21-mile range = 57.09 full charge cycles.
  • × the 13.5 kWh battery = 770.72 kWh drawn from the grid.
  • × 25p per kWh = an electric running cost of £192.68.

Then the alternative: what those same miles would have cost me burning diesel in the 2.0-litre engine.

  • 1,198.9 miles ÷ my real-world 54.3 MPG = 22.08 UK gallons.
  • × 4.546 litres per UK gallon = 100.37 litres.
  • × the 7-month average of £1.63 per litre = an equivalent diesel cost of £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 over the winter, so my electricity usage was probably a bit higher than the raw numbers suggest, and the recent heatwaves have meant a lot of pre-cooling before journeys too.

Making it actually pay off

So on standard 25p daytime electricity, charging the car actually costs me more than just burning diesel would have. That makes sense when I think about it: I’m hauling a whole diesel engine and a tank of fuel around as dead weight, which drags down the electric efficiency, and the diesel engine on its own is genuinely frugal.

To turn that loss into a saving, I need to stop charging at the standard daytime rate.

1. Charging overnight on a cheap tariff: If I move all my charging to an overnight off-peak tariff (around 7p per kWh), that same 770.72 kWh only costs £53.95. That flips the whole thing around, giving me a saving of £109.65 against diesel at the 7-month average price.

2. Charging from my own solar: Charging from a home solar array is a bit different - it’s really about opportunity cost. If I put spare generation into the car instead of exporting it to the grid at the standard 15p Smart Export Guarantee (SEG) rate, that 770.72 kWh effectively “costs” me £115.61 in lost export payments. Even then, I’m still £47.99 better off than diesel, and I’ve sidestepped peak pricing entirely.

Couple of other thoughts

1. Maintenance and wear: If I ran everything on the diesel engine, my main trips - a 4km round trip to the nursery twice a day - would be about the worst kind of driving for it: short hops from cold, never really getting up to temperature. That’s exactly where the electric motor shines. The trips are short, so the 21-mile range covers them easily, and I’m not cold-starting a diesel over and over.

2. The Rule of Cool: It’s just 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’ve got the windows of the house open. And coming back home down the cul-de-sac without the noise of the diesel engine in the afternoon or evening feels a bit more neighbourly.