# V2L affordability calculator — methodology, formulas & research checklist

> An honest model of whether running an essential-loads board off an EV's V2L output pays back, compared against a standalone home battery, a second-hand battery, smart cheap-rate charging, and doing nothing. The saving is the cheap rate AFTER round-trip losses, battery wear and equipment cost — and only if the car and charger can follow the schedule.

**Safety frame.** This is education, not an instruction to carry out work. Work connecting V2L equipment to fixed wiring is safety-critical and may be notifiable under Part P. It must be designed, installed, inspected and tested by a competent person to the current edition of BS 7671. Vehicle manufacturers generally do not sanction back-feeding fixed wiring from V2L outlets; follow manufacturer instructions.

## How the saving is calculated

All rates are p/kWh (VAT-inclusive); energy is kWh. The day is modelled as 48 half-hour slots.

- **Recharge cost (round-trip loss):** `gridKwhToRecharge = houseKwhDelivered / (ηcharge × ηdischarge)` — you buy more cheap-rate energy than you deliver back.
- **Effective delivered cost (p/kWh):** `cheapRate / ηroundtrip + evWearPerKwhDelivered + allocatedStandby`.
- **Battery wear (p/kWh):** `replacementCost / lifetimeThroughputKwh × 100` — e.g. £8,000 / 90,000 kWh ≈ 8.9p/kWh, often a third or more of a typical peak-to-cheap spread. This is the figure rosy claims omit.
- **Break-even spread (p/kWh):** `cheap × (1/ηroundtrip − 1) + wearPerDelivered + installPerDelivered`. Below this peak-to-cheap spread, V2L cannot pay back — and the tool says so.
- **Annualisation:** `annualSaving = dailySaving × 365 × availabilityFactor` — availability (the fraction of days the car is home and dispatchable) is applied exactly once.
- **Payback:** `annualSaving > 0 ? capex / annualSaving : never pays back`. "Never pays back" is a successful, honest result.
- **Winner:** chosen on net £ over the horizon (optionally discounted at ~3.5%), NOT on payback — a £40 kit that pays back in months can still net less than a battery that pays back in years.

Hard rules in the model: V2L and charging cannot overlap; V2L serves a small essential-loads board only (whole-house selection returns a safety stop, never a saving); a 2.2–3.6 kW V2L output serves base load, not a shower, oven or hob; on a leased car the wear is shown but not deducted from the saving; and the cheap whole-home tariff window is fixed — charging more slowly does not lengthen it.

## Research checklist — what to gather

### Your tariff

- Cheap-window and peak unit rates (p/kWh, VAT-inclusive).
- When the cheap window starts and ends — and whether the cheap rate is whole-home or car-only.
- Who controls the schedule and charge rate (you, or the supplier's smart system).
- Standing charge (p/day), and any export / SEG rate if you have or plan solar.

### Your car

- Usable battery capacity (kWh) and the V2L output power (kW) and socket type.
- On-board AC charger maximum (kW) and the car's own minimum accepted charge rate (kW).
- Whether the car can discharge via V2L while charging (usually it cannot).
- How you hold the car — owned, PCP or leased (this decides whether battery wear counts against you).

### Your charger

- Maximum output (kW) and whether it can limit or stretch its current across the cheap window.
- The lowest controllable charge rate (kW) — or record it as "unknown" rather than guessing.
- Scheduling capability, and any regulated randomised start delay.

### Your essential loads

- The appliances you would actually back up (fridge, freezer, router, lights) and their wattage.
- The fraction of that essential energy that falls in the expensive peak hours.
- Confirm what is NOT on the board: heating, immersion, kettle, oven, tumble dryer and heat pumps do not belong on a small V2L board.

### Equipment & cost

- Indicative raw-product cost of the V2L changeover kit (interlocked changeover, protection, essential board, adapter, cable).
- Indicative cost and usable capacity of any standalone or second-hand battery you are comparing against.
- Remember: labour, inspection, testing, certification and any DNO/export work are extra and need a competent-person quote.

### Losses & assumptions

- Round-trip efficiency (what you put in is not what you get out — typically ~80% for V2L).
- Battery wear: replacement cost divided by lifetime throughput (often about a third of an attractive spread).
- Driving reserve to keep in the car, and the fraction of days the car is home and dispatchable.

## Frequently asked questions

**Can I run the whole house from V2L?**

No. A V2L output (typically 2.2–3.6 kW) cannot run a whole consumer unit — that is a gross overload and outside V2L scope. V2L suits a small essential-loads board only, and any fixed-wiring arrangement must be designed, installed and tested by a competent person to the current edition of BS 7671.

**Why does lowering the EV charge rate matter?**

Charging more slowly does not make energy cheaper (off-peak is off-peak) and banks less over a fixed window. Its real value is right-sizing the fill and lowering battery wear by reducing the C-rate. The calculator flags whether your charger and car can actually reach the slow rate the plan assumes.

**Does a smart EV tariff make the whole house cheap while the car charges?**

Not by charging slower. On tariffs such as Intelligent Octopus Go the cheap whole-home window is fixed (for example 23:30–05:30) and does not extend with the charge session; extra smart slots are usually car-only. The genuine whole-home saving is shifting flexible loads into the fixed cheap window. Always check your supplier's published terms.

**How much is lost charging and discharging through V2L?**

Round-trip losses are typically around 15–20% (about 80% efficiency), depending on the equipment — so you buy more cheap-rate energy than you deliver back to the house. The calculator lets you set your own figure and shows the effect on the saving.

**Is a standalone battery cheaper long-term?**

It depends on your numbers. A fixed battery costs far more upfront but is automatic and available every day (the car may be away), so over a long horizon it can net more than a cheap V2L kit even though V2L pays back faster. The calculator compares them on net cost over your horizon, not on payback alone.

**Should I include battery degradation — and what if the car is leased?**

Yes. EV battery wear is often about a third of an attractive peak-to-cheap spread, so leaving it out overstates the saving. If the car is leased the calculator still shows the wear figure but does not deduct it from your saving (it falls on the lessor) — and note that V2L cycling may breach lease wear or mileage terms.

**Can I use V2L while charging?**

Generally no — most EVs cannot discharge through V2L while AC charging, and the calculator assumes the two never overlap. Check your vehicle's manual for the specific behaviour.

**What data do I need before asking an electrician for a quote?**

Work through the research checklist on this page: your tariff's rates and cheap window, your car's usable capacity and V2L output, your charger's controllable range, your essential-load list, and indicative equipment costs. Bring those numbers and a competent person can design, install, test and price the work.

## Standards & sources

Standards (notably BS 7671) are cited by reference only and never reproduced. See the standards clause-index at https://v2lworkshop.co.uk/standards and the safety guidance at https://v2lworkshop.co.uk/safety. Notifiable electrical work must be designed, installed and tested by a competent person. Tariff and product figures are volatile — verify the current values from the supplier or manufacturer before relying on them.

- [Octopus Energy — Intelligent Octopus Go](https://octopus.energy/smart/intelligent-octopus-go/) — The cheap whole-home window structure (e.g. 23:30–05:30, whole-home; smart slots car-only). Rates are volatile — verify the current p/kWh.
- [Octopus Energy — REST API (products & unit rates)](https://docs.octopus.energy/rest/guides/endpoints/) — Public, unauthenticated tariff and half-hourly (Agile) unit rates — the source for live Octopus rates (an optional, post-v1 integration).
- [Ofgem — energy price cap](https://www.ofgem.gov.uk/energy-price-cap) — Regional unit rates and standing charges that bound a standard-rate baseline.
- [GOV.UK — Electric vehicle smart charge point regulations](https://www.gov.uk/guidance/regulations-electric-vehicle-smart-charge-points) — Default-off charging schedules and the randomised start delay that can disrupt a fixed-window plan.
- [myenergi — Zappi ECO/ECO+ charge rates](https://support.myenergi.com/hc/en-gb/articles/5780558509201-ECO-ECO-charge-rates-in-a-three-phase-zappi) — An example of a charger's minimum controllable charge current — confirm your own model against its manufacturer help page, never a forum.

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_Last reviewed: 2026-06-16. Written against: BS 7671:2018 + A4:2026. Education, not project-specific design advice; standards cited by reference only._