The cars
The same household ran two premium diesel SUVs — a Mercedes GLE diesel handling long-distance and family duties, and a Mini diesel for town and shorter trips. Both have been replaced with electric equivalents, keeping the same premium positioning and broadly the same roles.
* EQE 53 list price exceeds £50,000 threshold, attracting £195 standard VED + £495 luxury supplement = £690/year for years 2–6. Mini Cooper Electric under £40,000 threshold, standard £195 rate.
The fuel cost calculation
Context — May 2026: The conflict in the Middle East — ongoing since 28 February 2026 — has driven diesel up by more than 34p/litre within weeks, with the RAC reporting an average UK pump price of 175.73p/litre by late March 2026. The Strait of Hormuz has seen a massive decline in shipping traffic, with around 20 million barrels of oil per day affected and production cut by at least 10 million barrels in the Gulf countries. The figures in this case study use £1.85/litre — the current price at time of writing. If diesel continues to rise, every penny adds directly to the diesel owner's annual cost. The EV owner's cost does not change.
All home charging is done overnight on the Octopus Intelligent Go tariff at 7p/kWh — the same overnight rate used for the Sunamp hot water heat battery and Sonnen battery storage at Rose Cottage. This is the key variable. At the standard day rate of 27.75p/kWh the charging economics change significantly; the cheap overnight rate is what makes the numbers genuinely compelling.
The 7p/kWh overnight rate is not a special EV deal — it's the same tariff the household already uses for battery storage and hot water. The EV charging is simply another overnight load that charges at 6am at the same rate. The solar array means some charging also happens during the day from surplus generation, potentially pushing the effective per-mile cost even lower.
Mercedes EQE 53 AMG — annual charging
Mini Cooper Electric — annual charging
The full cost comparison
| Cost category | Diesel (both cars) | Electric (both cars) | Annual saving |
|---|---|---|---|
| Fuel / charging — Mercedes | £2,803 | £302 | £2,046 |
| Fuel / charging — Mini | £1,008 | £121 | £724 |
| Fuel subtotal | £3,811 | £423 | £2,770 |
| Road tax (VED) — Mercedes | £620 | £690* | −£70 |
| Road tax (VED) — Mini | £195 | £195 | £0 |
| Annual service — Mercedes | ~£750 | ~£375 | ~£375 |
| Annual service — Mini | ~£375 | ~£200 | ~£175 |
| AdBlue (diesel only) | ~£80 | £0 | £80 |
| Diesel fuel duty escalation risk | Ongoing exposure | None | — |
| Total annual running costs | ~£5,831 | ~£1,883 | ~£3,330 |
* EQE 53 AMG attracts the £495 EV luxury supplement (list price over £50,000) under April 2025 VED changes, slightly higher than the diesel GLE. Service cost estimates based on typical Mercedes and MINI EV service plans; EV servicing is lower due to absence of oil changes, filters, exhaust components, and less brake wear via regenerative braking. Fuel price based on current UK pump price of £1.85/litre diesel (May 2026). At this price the per-mile diesel fuel cost is approximately 17.3p/mile for the GLE.
The solar advantage
The charging economics above assume 100% overnight grid electricity at 7p/kWh. Rose Cottage has a 15.8 kWp solar array generating an estimated 14,000–16,000 kWh annually. The Zappi charger runs in Eco mode, prioritising surplus solar for EV charging before drawing from the grid.
In practice this means a meaningful proportion of charging — particularly for the Mini doing shorter local trips — happens from daytime solar surplus at an effective cost of 0p/kWh (displaced export at 15p/kWh rather than grid import at 27.75p/kWh). The Zappi's April 2026 charge history showed 394 kWh consumed — much of it during daylight hours from solar surplus.
Conservative estimate: if 30% of annual EV charging (approximately 1,816 kWh) comes from solar surplus rather than grid electricity, the effective annual charging cost falls from £424 to approximately £300 — and the saving versus diesel widens to nearly £5,000/year on fuel alone.
Carbon: the numbers that matter
Annual CO2 from the two diesel cars at 18,000 miles combined: approximately 5.4 tonnes. At the UK grid carbon intensity of 136 gCO2/kWh, the 6,054 kWh used by both EVs annually produces approximately 0.82 tonnes on a full-grid basis — an 85% carbon reduction. With solar generation covering 30%+ of charging, the actual carbon footprint falls below 0.6 tonnes — a 90%+ reduction against the diesel baseline.
The costs nobody talks about
The smart buying strategy: let someone else take the depreciation hit
Both cars in this comparison were bought at 18 months old. This is the deliberate strategy — and it changes the economics of premium EV ownership fundamentally.
The Mercedes EQE 53 AMG has a new list price of approximately £115,000. Early EQE models depreciate heavily in the first 18–24 months — the combination of rapid technology improvement in newer models, EV market softness, and luxury-tier buyer caution has driven used prices down sharply. The owner purchased an 18-month-old EQE 53 AMG for £60,000 — a 48% discount on new list price, for a car with a full remaining warranty, under 15,000 miles, and all the technology intact.
Someone paid £115,000 for this car new. Eighteen months later it was available for £60,000. The first owner absorbed a £55,000 depreciation hit. The second owner — this household — acquired a performance electric SUV at less than the price of a new mid-range diesel equivalent, with running costs a fraction of what the diesel would cost.
This flips the usual EV purchase price argument entirely. The common objection — "EVs cost more to buy" — is true at new list prices. It is emphatically not true when you buy an 18-month-old EQE 53 AMG at £60,000 versus a new GLE AMG diesel at £75,000+. The EV was £15,000 cheaper at point of purchase, and then saves £3,330/year in running costs on top.
The depreciation arbitrage opportunity
EV depreciation in the £80,000–£130,000 bracket has been particularly steep because early adopters at that price point tend to upgrade frequently, and the second-hand market for six-figure EVs is thinner than for mainstream models. If you purchased an EQE new, you have likely lost more than a third of the car's value on paper within three years — making slightly-used examples an exceptional value proposition for the second buyer.
The same logic applies to the Mini, though less dramatically. The Mini Cooper Electric bought at 18 months old saves a meaningful amount versus new, and at a list price under £40,000 the residual values are stronger than the EQE. The key principle is the same: the first owner subsidises the second owner's acquisition cost — and in the EV market at the premium end, that subsidy has been exceptionally generous.
Total cost of ownership — the full picture
| 5-year ownership scenario | Diesel (both cars, bought used) | Electric (both cars, bought used) | 5-year difference |
|---|---|---|---|
| Purchase price — Mercedes | ~£45,000 | £60,000 | EV £15,000 more |
| Purchase price — Mini | ~£18,000 | ~£18,000 | Roughly equal |
| 5-year running costs (×£3,330/yr) | ~£31,155 | ~£9,415 | EV saves £16,650 |
| Estimated residual after 5 yrs | ~£25,000 combined | ~£30,000 combined | EV holds better |
| Net 5-year cost | ~£66,155 | ~£57,415 | EV ~£6,650 cheaper |
Purchase prices are estimates based on 18-month-old used market values. Running costs use the £3,330/year combined saving from the full running cost comparison above. Residual values are indicative. The diesel GLE bought used at 18 months assumed at ~£45,000 (new GLE diesel ~£78,000 with similar depreciation curve). All figures approximate — the purpose is directional, not precise.
The 5-year total cost of ownership favours the electric pair by approximately £9,390 — and that's before accounting for any solar self-consumption reducing charging costs to near zero on sunny days, or any further grid carbon intensity reduction improving the EV's environmental case. The gap widens every year electricity stays cheaper than diesel per unit of energy delivered.
The smart money move: finance it and let the law protect you
Buying an EV outright exposes you to the full force of depreciation — as the first owner of an EQE 53 AMG discovered when their £115,000 car was worth £60,000 eighteen months later. Financing it changes the risk profile entirely, particularly when you understand two things most car buyers don't: what happens at the end of a PCP contract, and what you can do halfway through one.
End of contract: just hand it back
A PCP (Personal Contract Purchase) agreement is structured so that at the end of the term — typically 2–4 years — you have three options: pay the final balloon payment and own the car, part-exchange it, or simply hand it back and walk away. If the car's market value has fallen below the guaranteed future value (GFV) set by the finance company at the start, that's entirely the finance company's problem, not yours. You owe nothing beyond the balloon payment you've chosen not to pay.
For EVs — where depreciation has been steep and unpredictable — this is a genuinely powerful protection. If the technology moves on, a new model with significantly better range arrives, or the market softens further, the end-of-PCP handback means you have effectively transferred all residual value risk to the lender. They set the GFV at the start; they absorb the shortfall at the end.
The finance company has to guess what your EV will be worth in three years when they write the deal. If they guessed wrong — and with EVs, many have — you hand the car back, shake hands, and walk away. The lender absorbs the depreciation. You absorbed none of it beyond your monthly payments.
Halfway through: Section 99 of the Consumer Credit Act 1974
This is the provision almost nobody knows about, and it is one of the most powerful consumer rights in UK finance law. Under Section 99 of the Consumer Credit Act 1974 — known as the Voluntary Termination right or the "Half Rule" — once you have paid 50% of the total amount payable on a regulated PCP or HP agreement, you can hand the car back and walk away with no further liability. No penalty. No shortfall. Nothing.
The key numbers: The 50% threshold is calculated on the total amount payable — your deposit, all monthly payments, interest, fees, and the balloon payment combined. It is not 50% of the car's purchase price. Check your agreement document carefully for the "Total Amount Payable" figure and halve it. That is your threshold. Once crossed, write to the finance company citing "Section 99 of the Consumer Credit Act 1974" explicitly — not just "I want to hand the car back." The statutory language is what triggers the legal protection.
For EV owners worried about a sudden technology shift or market collapse midway through ownership, this is a genuine safety net. Voluntary termination is a legal right, not a favour your lender can grant or refuse — no finance company can contractually remove this right, regardless of what their terms and conditions say. The car goes back, the agreement closes, and you have the option to move on to whatever comes next — a newer EV, a different model, or simply no car finance at all.
How these two protections work together
The strategy in practice
For an EV like the EQE 53 AMG — a car with a high list price and demonstrated steep early depreciation — the smart financing approach is:
Finance it on PCP at a reasonable deposit and monthly payment. If after 18–24 months you decide the technology has moved on and a newer model suits you better, calculate your 50% threshold. If you're there, invoke Section 99 and hand it back. If you're not, assess whether topping up to the 50% figure makes sense versus continuing. At end of contract, if you've run it for the full term and the residual is below the GFV, simply hand it back rather than paying the balloon.
Either way, you never own a depreciating asset outright — you pay for the use of it, and the law gives you structured exit points that cap your downside. In a market where EV values have been volatile and technology has been advancing rapidly, that flexibility is worth a great deal.