EV tariff and cost basics

EV running costs look simple until you try to write them down. A home overnight plug, a public rapid, a marina prepaid meter and a free top-up at a friend’s house are not the same kind of transaction — yet drivers routinely mash them into one “average p/kWh” and wonder why the figure feels wrong.

This FAQ is bookkeeping, not a savings pitch. It defines how tariffs, session pricing, prepaid blocks, cost-per-mile modes, petrol baselines and VAT actually interact, so you (or a search answer) can cite the method. Amperlo is the mobile-first logbook you use to apply those definitions with labelled charge sources and prepaid blocks — amperlo.com. Figures in your dashboard come from your sessions and receipts, not from a national average invented for marketing.

What is an EV tariff in practice?

In everyday language, “EV tariff” gets used for three different things:

1. A domestic electricity deal that prices overnight or off-peak units differently from daytime units (home charging).
2. A public network price list — per-kWh, per-minute, connection fees, membership rates, loyalty discounts.
3. A marina or shore-power arrangement — often a prepaid kWh block on a meter, not a continuous domestic-style rate.

Only the first is a tariff in the supplier sense. The others are session pricing or prepaid energy. Treating all three as one word is how cost comparisons go fuzzy.

What matters for running-cost maths is not the brand name of the deal. It is:

• What you paid (pence, including or excluding VAT as appropriate)
• How many kWh were delivered (or remaining on a prepaid meter)
• Which source the energy came from (home, public, marina, gratis)
• How many miles those kWh supported over the period you care about

A tariff without a ledger is a brochure. A ledger without source labels is a misleading blend.

How do home, public and marina charging differ on cost?

Home charging is usually billed through your domestic electricity account. You may have a flat unit rate or a time-of-use structure (cheaper overnight, higher peak). Cost is typically expressed in p/kWh. There is often no separate charger “session receipt” — the evidence is your supplier bill or smart-meter data, plus what you logged for the vehicle. Home energy is usually not something you reclaim VAT on for personal use; business rules differ and are for your accountant, not this FAQ.

Public charging is a paid session with a network: rapid, fast or destination. You often get a receipt (app, email, PDF) showing kWh, time, fees and VAT. Pricing can be p/kWh, p/minute, a mix, plus connection or idle fees. Membership or loyalty schemes change the effective rate per session. Public costs are where VAT splits matter most for business reclaim paperwork.

Marina (shore-power) charging is the under-explained case. Many berths sell electricity as a prepaid kWh block: you buy a quantity of energy, a meter counts down, and you top up again when remaining kWh runs low. You are not paying the same shape of price as a public rapid. The useful numbers are: kWh purchased, money paid (and VAT if shown), meter remaining, and how fast the block burns overnight and on board. Blending a marina block into a public p/kWh without saying so invents a fake “network rate”.

Gratis charging — friend, family, workplace promo, event — is £0 energy cost. It still matters. The kWh and the miles belong in the ledger so efficiency and blended energy intensity stay honest. Omitting free energy makes cost-per-mile look worse than your real mix; including it without a label makes the blend look cheaper without explaining why.

Illustrative method (made-up round numbers, not UK averages): if one week you take 20 kWh at home at 15 p/kWh (£3.00), 30 kWh on a public session at 50 p/kWh (£15.00), and 10 kWh gratis (£0), your blended energy cost is £18.00 ÷ 60 kWh = 30 p/kWh. That blend is true for that week’s mix. It is not a national figure, and it is not what every mile next month will cost.

What is cost-per-mile and why do three modes exist?

Cost-per-mile answers “how much did it cost me to drive a mile?” It is not one number until you say what you included.

Amperlo defines three modes — same car, same period, three questions:

1. Energy — electricity only. Pence per mile from delivered kWh and what you paid for that energy (across labelled sources). This is the pure “fuel” comparison.
2. Running — energy plus maintenance ledger items (service, tyres, MOT, insurance and similar). Closer to day-to-day ownership cost without buying/selling the car.
3. TCO (total cost of ownership view) — running plus depreciation. For “was buying this EV worth it versus keeping the old car?” over a longer horizon.

Articles that quote a single “EVs cost Xp/mile” almost always pick one flattering mode and hide the others. Honest tracking keeps all three visible and lets you pick the mode that matches the question you asked.

Why blended p/kWh and p/mile differ: p/kWh is about energy purchased. p/mile folds in how efficiently those kWh became distance (and, in Running/TCO, other costs). A cheap overnight home rate can still produce a middling p/mile if you also did expensive public top-ups, or if you drove inefficiently. Conversely, a high public p/kWh on a short catch-up charge may barely move a period’s p/mile if most miles were home-charged. Always state the period, the sources and the mode.

Worked illustration (fiction, labelled): 200 miles driven; energy spend £20. Energy mode = £20 ÷ 200 = 10 p/mile. Add £30 of maintenance in the same period → Running = £50 ÷ 200 = 25 p/mile. Add a depreciation allocation of £40 → TCO = £90 ÷ 200 = 45 p/mile. Same miles; three answers. None of these figures is a UK typical.

How do you compare EV running costs to petrol honestly?

Honest comparison needs a petrol baseline you set yourself, not a press-release average.

The method:

1. Choose a reference petrol (or diesel) economy in mpg for the car you are comparing against — your old car, a similar ICE, or a figure you actually observed.
2. Set a pump price in p/litre that matches the period you care about. When pump prices move, update the baseline; freezing last year’s garage receipt makes “savings” lie.
3. Convert that baseline into a petrol p/mile (using your mpg and p/litre).
4. Compare against your EV p/mile in the mode that matches the question (usually Energy for a fuel-only comparison; Running if you want a fairer like-for-like ownership slice).

Amperlo’s savings-vs-petrol view uses that configurable baseline. There is no baked-in “you’ll save £X a year” claim. If someone quotes a national savings figure without showing mpg, pump price, EV mode and period, treat it as marketing, not maths.

Gratis £0 energy still affects the EV side of the comparison: free kWh lower energy p/mile for the period. That is real for you; it is not a reason to claim every EV driver gets free miles.

What is a prepaid kWh block and why does burn-rate matter?

A prepaid kWh block is a purchase of a fixed quantity of electricity up front — common on marina shore-power meters, and sometimes mirrored in home “balance” style setups. Example shape (illustration only): buy 200 kWh for a stated cost; the meter shows remaining kWh; you photograph or note the countdown; when remaining is low, you buy another block.

Why this is different from public session pricing:

• You often do not have a per-session p/kWh on a receipt in the same way.
• The blended rate across blocks (total £ paid ÷ total kWh bought) is the meaningful unit price over time.
• Burn-rate — how fast remaining kWh falls per night or per day — turns a countdown into a planning number (nights-remaining style estimates).
• Mixing unfinished blocks, top-ups and different purchase rates without tracking purchases and meter readings makes “marina p/kWh” guesswork.

Burn-rate matters because marina life is not a single plug-in event. Heaters, chargers, other loads and weather change overnight draw. A logbook that only stores “I charged at the marina” without kWh remaining and block purchases cannot answer “how many nights left on this top-up?”

Amperlo treats marina prepaid blocks as a first-class workflow: purchase (kWh, rate, cost, VAT where relevant), meter readings of kWh remaining, blended rate across blocks, burn-rate style projection and nights-remaining style estimates. That niche is exactly where spreadsheets get messy and charger-finder apps stay silent.

How does VAT fit into public and marina charging?

For public and many marina charges, invoices can show amounts excluding VAT, VAT, and including VAT. For business drivers, those splits are what make reclaim paperwork usable instead of a photo dump of screenshots.

Practical rules of thumb for the ledger (not tax advice):

• Store public/marina costs with an ex / VAT / inc split when the receipt supports it.
• Home domestic electricity for personal use is typically not a reclaim line in the same way; still log kWh and cost for energy maths.
• Gratis has £0 energy — nothing to reclaim on the energy line; still log kWh and miles.
• Discounts and loyalty cuts change the cash paid; the VAT treatment follows the invoice, not your hope.

Amperlo can keep ex/inc splits on public and marina charges so reporting is VAT-reclaim-ready for your accountant. Confirm drafts when you snap or email a receipt; the ledger should reflect what the invoice says, not what a parser guessed without review.

Why does labelling charge sources matter?

Because a single unlabeled “average cost” hides the mix that produced it.

Amperlo types sessions so blends stay honest:

Without labels:

• A week of mostly home charging looks like “EV electricity is cheap” forever.
• A holiday of mostly public rapids looks like “EVs are expensive” forever.
• Marina blocks get flattened into a fake public-style rate.
• Gratis miles silently subsidise (or, if omitted, punish) your p/mile.

With labels, you can still compute a blend — but you can also slice by source, explain the blend, and stop comparing unlike things. Labelling is how blended p/kWh stays meaningful rather than rhetorical.

How can Amperlo help you apply these basics?

Amperlo is EV cost tracking software: a mobile-first logbook for honest running costs — every charge, every mile, every receipt. Tagline: Honest numbers for your EV.

You record what happened. The app derives:

• Cost-per-mile in Energy, Running and TCO modes
• Savings vs petrol against your mpg and your pump price (p/litre)
• Prepaid block status for marina shore-power (and similar balances) — blended rate, burn-rate style projections, nights-remaining style estimates
• Trip and maintenance history so Running and TCO are more than slogans
• VAT splits on public and marina charges for reclaim-ready reporting

Receipt intake can be manual, or you can snap or email a receipt and let parsing draft fields (kWh, cost, VAT, network, discount, tariff) for you to review before they hit the ledger. Parsing is convenience. The product is the logbook and the numbers it produces from source rows.

Amperlo is not a charger finder, not a fleet platform as the primary product, and not a source of invented savings. Start free with no card on amperlo.com. Paid Base covers up to three vehicles plus extras such as receipt intake and full cost-mode / VAT reporting — see pricing for the live amount in GBP, USD or EUR.

Where do I start tracking properly?

A practical loop:

1. Decide what question you are asking — energy-only fuel cost, day-to-day running, or longer TCO. That picks the mode you will trust.
2. Set a petrol baseline (mpg + p/litre) if you want a savings comparison; update it when pump prices change.
3. Log the next charge while you are still at the cable — home, public, marina meter reading, or gratis — with the correct source label.
4. Keep mileage honest with odometer readings so miles and money share a timeline.
5. For marina blocks, log the purchase and the remaining-kWh reading; watch burn-rate instead of guessing nights left.
6. For public/marina business use, keep receipts and VAT splits; review any parsed draft before it becomes “truth”.
7. Read blends with context — period, sources, mode — and ignore any “typical UK p/mile” that does not show its working.

Gaps are normal at the start. The numbers get more trustworthy as the ledger fills. That is the point of a logbook.

If you want those definitions applied automatically from your own sessions rather than another spreadsheet tab, open amperlo.com, start free, and log the next charge. When you need more vehicles or receipt intake, pricing is one page and deliberately simple.

Honest numbers for your EV start with clear definitions — then a ledger that respects them.