# How V2L load-shifting saves money — the method & worked example

> This is the method behind the calculator: how running an essential-loads board off your EV's V2L output through the expensive hours can save money — and exactly where it cannot. Read it once, then model your own setup on the tool. It is education, not an instruction to carry out the work — a competent person designs, installs and tests any fixed-wiring arrangement to BS 7671.

## How this works

The principle is load-shifting through the vehicle battery: charge the EV at the off-peak rate overnight, then — when the car is at home — supply your essential circuits (refrigeration, router, lighting) from its V2L output during the peak-rate hours. You import energy at the low rate and use it in place of grid energy at the high rate. This tool determines whether that yields a net saving for your tariff, vehicle and charger, once round-trip conversion losses and battery degradation are accounted for.

1. **Charge at the off-peak rate.** Overnight, within the cheap window (for example 23:30–05:30), charge the vehicle — preferably at a low, steady current, so the whole window is used and the battery is charged at a lower C-rate, which reduces wear.
2. **Transfer essential loads to the vehicle at peak.** When the high rate begins, a small essential-loads board is supplied from the vehicle's V2L outlet rather than from the grid — refrigeration, router and lighting only, never the whole consumer unit.
3. **Account for the difference.** You displace peak-rate grid import on those circuits. The saving is the margin between your peak rate and the effective cost of the V2L energy — the off-peak rate plus round-trip losses and an allowance for battery wear.

> Illustration only — not a live price or a recommendation. Charge approximately 10 kWh at around 7p overnight, then supply a ~300 W essential board from the vehicle through peak hours at a grid rate of roughly 27p. After about 20% round-trip loss and a wear allowance of approximately 9p per delivered kWh, the calculator reports the net result — and where the figures do not pay back, it states so directly.

Each value the calculator requests changes the result materially: your off-peak and peak rates (the saving is the margin between them); your vehicle's V2L output and usable capacity (the energy available to shift); your charger's controllable range (whether it can hold a low current across the window); the vehicle's availability during peak hours (V2L pays only when the car is present); and your loss and battery-wear assumptions (delivered energy is always less than imported energy). Where a value is unknown, leave it — each field carries a sourced default you can override.

## How the saving is calculated

All rates are p/kWh (VAT-inclusive) and energy is in kWh; the day is modelled as 48 half-hour slots. The saving is never the cheap rate minus the peak rate — it is the cheap rate after the conversion losses, the battery wear and the equipment cost, and only for the energy the car can actually deliver while it is home.

- **Recharge cost (round-trip loss):** `gridKwhToRecharge = houseKwhDelivered / (ηcharge × ηdischarge)` — you buy more cheap-rate energy than you deliver back to the house.
- **Effective delivered cost:** `cheapRate / ηroundtrip + evWearPerKwhDelivered + allocatedStandby` — the true p/kWh of V2L energy — the figure that must beat your peak rate for the method to work.
- **Battery wear:** `replacementCost / lifetimeThroughputKwh × 100` — e.g. £8,000 / 90,000 kWh ≈ 8.9p per delivered kWh — often a third or more of an attractive spread, and the figure optimistic claims omit.
- **Break-even spread:** `cheap × (1/ηroundtrip − 1) + wearPerDelivered + installPerDelivered` — below this peak-to-cheap spread, V2L cannot pay back — and the tool says so plainly.
- **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, not a failure of the model.
- **Winner:** `highest net £ over the horizon (optionally discounted ~3.5%)` — chosen on net £, 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 never overlap; V2L serves a small essential-loads board only — selecting the whole house returns a safety stop, never a saving; a 2.2–3.6 kW V2L output supplies base load, not a shower, oven or hob; on a leased car the wear is shown but not deducted from the saving (it falls on the lessor, and V2L cycling may breach lease terms); and the cheap whole-home tariff window is fixed — charging more slowly does not lengthen it.

## A worked example — the author's own setup

This is the calculator, pre-filled with the author's own setup, so you can see the method applied to a real installation rather than a tidy illustration: a Hyundai Ioniq 5 Standard Range (58 kWh), a ~1.5 kW essential board, a 3.2 kWp solar array, and an Intelligent Octopus Go tariff. Three inputs are still labelled assumptions pending the author's bill and the live Octopus feed — whole-home consumption, the essential board's evening hours, and the export rate — so treat the headline figure as indicative, not final. Every field below is editable: change any value, or model your own setup on the main calculator.

The setup: Hyundai Ioniq 5 Standard Range (58 kWh, 3.6 kW V2L), a ~1.5 kW essential board through the evening peak, a 3.2 kWp solar array, on Intelligent Octopus Go (7p off-peak / 28p day — illustrative, verify current). On these inputs the calculator makes V2L the cheapest option on total annual cost — roughly £280/yr below doing nothing, and below a £4,000 home battery — while the freed-PV export earns nothing here, because a 15p export rate does not beat the all-in V2L cost (charge loss + EV wear). Whole-home consumption, the board's hours and the export rate are assumptions pending the author's bill and the live Octopus feed.

## 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 reduce the unit rate — off-peak is off-peak — and delivers less energy over a fixed window. Its value is matching the charge to the energy actually required and reducing battery wear by lowering the C-rate. The calculator flags whether your charger and vehicle can reach the low 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.

Model your own setup: https://v2lworkshop.co.uk/affordability. Compare against a home battery: https://v2lworkshop.co.uk/affordability/ev-vs-home-battery.

## Sources

- [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 behind this page's optional “Connect Octopus for live rates” button. Octopus is the only supplier currently verified and integrated here; every other supplier stays manual or preset.
- [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: 2026-06-18. Written against: BS 7671:2018 + A4:2026. Education, not project-specific design advice; standards cited by reference only._