Cost & payback
V2L affordability calculator
The saving is not the cheap rate minus the day rate — it is the cheap rate after round-trip losses, battery wear and equipment cost, and only if your car and charger can follow the schedule. Model the economics before you spend; a competent person still designs and tests any fixed-wiring V2L work.
Quick check
Could V2L load-shifting pay back here?
Answer five quick questions for a first read. Nothing you choose leaves your browser.
Marginal — could go either way
- Tariff type unknown — the peak-to-cheap spread is the single biggest driver of the answer.
- Whether your charger can limit/stretch current is unknown — it decides if the 'whole window' plan works.
The one thing most likely to change this answer: your tariff's peak-to-cheap spread.
What this quick check is — and isn't
This triages whether the detailed economics are worth modelling for your setup — it is the first step, not the saving figure itself. Losses, battery wear and equipment cost still decide the real payback, and a competent person designs and tests any fixed-wiring arrangement to BS 7671. The full calculator, with your tariff, car, charger and losses, follows.
Detailed calculator
Model the economics for your setup
Every figure below is editable and starts from a worked example — replace them with your own. Results recompute live as you type, and nothing you enter leaves your browser.
6. Results
Best net result over the horizon: Smart cheap-rate charging only (no discharge). Confidence: Medium
| Strategy | Status | Annual saving | Payback | Net over horizon |
|---|---|---|---|---|
| Do nothing (pay standard rate) | Computed | £0 | — | £0 |
| V2L load-shifting (EV → essential board) | Computed | £102 | ≈ 5 months | £812 |
| Standalone home battery | Computed | £197 | 20.3 years | -£2,360 |
| Smart cheap-rate charging only (no discharge) Best net | Computed | £584 | No upfront cost | £4,857 |
| Second-hand / budget battery bank | Computed | £175 | 8.6 years | -£42 |
What would change the answer: below a peak-to-cheap spread of about 12.5p per kWh, V2L load-shifting cannot pay back here — after round-trip losses, wear and kit cost.
Net is over a 10-year horizon, after capex, discounted to today. The winner is chosen on net £, not on payback — a cheap kit that pays back fast can still net less than a battery that pays back slowly.
What the model is telling you
V2L load-shifting (EV → essential board)
- Charger minimum charge rate unknown — the optimal slow-charge rate and finish time depend on it.
- Whether this charger can spread the charge across the whole window is unknown — the slow-charge benefit and the optimal rate depend on it.
- Peak load or V2L power ceiling limits how much of the bank can be used during peak hours.
Standalone home battery
- Inverter power is an assumed ~0.33C default (no sourced figure) — edit it if your battery's inverter kW differs.
Second-hand / budget battery bank
- Inverter power is an assumed ~0.33C default (no sourced figure) — edit it if your battery's inverter kW differs.
Charger current-control matrix
Whether your charger can spread the charge across the whole cheap window — and how slowly it can go — decides if the “six full hours” plan works. “Unknown” is a valid, first-class answer; confirm against the manufacturer’s own help page, never a forum.
Sourced charger profiles are still being researched and will appear here with manufacturer references. Until then, enter your charger’s maximum output and lowest controllable rate in the calculator above — and if you are not sure of the minimum, leave it as unknown rather than guessing. The model flags what it cannot assume.
Research checklist — what to gather
Bring these figures to the calculator above, and to any competent person you ask to design, install and test the work. Sourced figures with dates beat remembered ones.
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.
References & sources (5)
- Octopus Energy — 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) — Public, unauthenticated tariff and half-hourly (Agile) unit rates — the source for live Octopus rates (an optional, post-v1 integration).
- Ofgem — energy price cap — Regional unit rates and standing charges that bound a standard-rate baseline.
- GOV.UK — Electric vehicle smart charge point regulations — Default-off charging schedules and the randomised start delay that can disrupt a fixed-window plan.
- myenergi — Zappi ECO/ECO+ charge rates — An example of a charger's minimum controllable charge current — confirm your own model against its manufacturer help page, never a forum.
- Last reviewed
- 16 June 2026
- Written against
- BS 7671:2018 + A4:2026
General information, not project-specific design advice. Standards are cited by reference only and never reproduced. How we source this.