The average British car is parked for more than 95% of its life. For most of that time, an EV's battery just sits there. The 55 kWh pack in our reference sub-£30k EV holds enough energy to run a typical UK home for about a week.

A stationary home battery exists to do one job: store cheap or solar-generated electricity and release it when grid power is expensive. An EV parked on the driveway can do the same job, provided its power electronics can push energy both ways with minimal loss.

Until recently they could not do that efficiently. This analysis sets out the hardware that makes it possible: 800V Silicon Carbide inverters, galvanically isolated bi-directional conversion and sodium-ion cells built for heavy cycling.

V2G reference system at a glance

  • Vehicle battery: 55 kWh sodium-ion, 800V nominal architecture.
  • Power semiconductors: 1200V SiC MOSFETs, wide-bandgap.
  • Conversion losses: up to 70% lower than silicon IGBT equivalents.
  • Power electronics round-trip efficiency: above 95%, grid to pack and back.
  • Isolation: high-frequency transformer in a dual active bridge (DAB) DC-DC stage.
  • Grid interface: 230V single-phase or 400V three-phase, 7–11 kW bi-directional.
  • Compliance: G98/G99 connection, G5/5 harmonics, BS 7671 Section 722.

The economics of scale: a battery you already own

The home battery market sells capacity at a premium. A typical installed stationary battery in the UK offers 10–14 kWh and costs upwards of £7,000 fitted. That works out at roughly £500–£700 per kWh of storage, before any inverter upgrade.

Our reference EV carries 55 kWh. As the battery packaging analysis established, that pack costs about £2,258 at cell-to-pack level, or about £41 per kWh. The buyer pays for it because they need a car.

The capacity comparison

  • Stationary battery: 10–14 kWh, £7,000+, a single-purpose asset bolted to the garage wall.
  • Reference EV pack: 55 kWh, already paid for as transport, a multi-purpose asset.
  • Ratio: about four times the storage of a 13.5 kWh wall unit.

The car brings four times the storage at zero extra battery hardware cost to the home. The one extra item is a bi-directional wall box or a V2G-capable onboard charger. That replaces the hybrid inverter a stationary battery would need anyway.

The mobility constraint

A car battery drives away. That is the obvious objection, and the capacity gap answers it. A household that needs 40 km of daily range uses roughly 6–7 kWh. Reserve 20 kWh for driving and unplanned trips, and more than 30 kWh is still available for home storage. That is still more than twice the largest mainstream stationary battery.

The car does not need to be home all day to be useful. It needs to be plugged in during two windows: the midday solar surplus or overnight low-price period, and the evening peak. For commuters, weekends and working-from-home days cover the midday window. Overnight pricing covers the rest.

The Silicon Carbide conversion dividend

Early V2G systems had one main weakness: conversion loss.

Every kilowatt-hour that moves between grid and battery crosses the power electronics twice. It goes AC to DC on the way in and DC to AC on the way out. Older silicon IGBT systems lost 5–8% in each direction once transformer, filter and standby losses were counted. The round trip lost 10–15% before the cells took their share.

That loss hurt the business case. A 15% round-trip loss cancels much of the price spread a tariff offers. On a modest spread, it cancelled all of it.

Why silicon IGBTs lose energy

  • Tail current. An IGBT is a bipolar device. At turn-off, stored minority carriers keep conducting as a tail current, and each switching event wastes energy as heat.
  • Frequency ceiling. Switching loss scales with frequency, so silicon converters run at 10–20 kHz. At those frequencies, magnetics and filters must be large.
  • Fixed voltage drop. The IGBT's knee voltage wastes power even at low load. V2G spends most of its time at low load, for example exporting 2 kW to cover a home's evening demand.

What SiC changes

Silicon Carbide is a wide-bandgap semiconductor. Its bandgap is about 3.3 eV, against 1.1 eV for silicon. Its critical breakdown field is about ten times higher. In practice:

  • Unipolar switching. A SiC MOSFET has no minority-carrier tail. It turns off in tens of nanoseconds, and switching energy falls sharply.
  • Ultra-high switching frequency. Converters can run at 50–200 kHz. The transformer, inductors and EMI filters shrink in proportion.
  • Resistive conduction. A MOSFET behaves like a resistor with no knee voltage, so light-load efficiency is far better. That suits the partial-power operation V2G spends most of its time in.
  • Thermal headroom. SiC keeps working at higher junction temperatures and conducts heat about three times better than silicon. Cooling can be simpler.

Together, these cut conversion thermal losses by up to 70%. Each conversion direction now reaches 97.5–98.5% efficiency. The power electronics round trip exceeds 95%, and conversion loss falls below 5%.

The same die technology drives the wheels. The inverter instalment showed how 800V SiC adds 4–6% to system efficiency on the road. V2G uses the same semiconductor advantage while the car is parked.

Counting honestly: conversion versus cell losses

The 95% figure covers the power electronics. The cells add their own loss through internal resistance. Charge and discharge at 3–7 kW are gentle for a 55 kWh pack, a C-rate below 0.15C, so cell loss stays low. A realistic wall-to-wall round trip for a SiC system is 90–92%. That compares well with stationary home batteries, which typically quote 85–90% AC round-trip efficiency.

800V bi-directional engineering: stepping down to the domestic supply

The pack runs at about 800V DC. The house runs at 230V AC single-phase, or 400V AC three-phase on larger properties. Connecting the two safely takes a carefully designed conversion chain.

The conversion chain

  • Stage 1, active front end. A SiC totem-pole or three-phase bridge rectifies grid AC to an intermediate DC bus when charging, and synthesises a clean sine wave when exporting. It controls power factor and reactive power in both directions.
  • Stage 2, isolated DC-DC. A dual active bridge (DAB) or CLLC resonant converter links the intermediate bus to the 800V pack through a high-frequency transformer. It manages the large voltage ratio and gives the galvanic isolation.
  • Stage 3, battery interface. The pack's contactors, pre-charge circuit and insulation monitoring stay under the vehicle's battery management system.

Where this chain sits is an architectural choice. DC V2G puts it in the wall box and moves DC over the CCS connector. AC V2G puts it in a bi-directional onboard charger (OBC), so the wall box stays simple. The OBC can share SiC power modules, cooling and even inverter legs with the traction drive. The AC route costs less on the wall, but the car itself must then meet grid-code compliance.

Why galvanic isolation is non-negotiable

Galvanic isolation means there is no direct conductive path between the 800V pack and the domestic supply. Energy crosses the transformer's magnetic field. Current cannot flow through a wire. It does three jobs:

  • Safety under UK earthing. Most UK homes use PME (TN-C-S) earthing. An open-PEN fault can raise the car's chassis to a dangerous voltage. Isolation, combined with BS 7671 Section 722 protection, keeps the high-voltage system independent of the house earth.
  • Common-mode containment. SiC's fast edges create high dV/dt. Without isolation, common-mode currents flow through the pack's stray capacitance to chassis and into the home's earth. That causes nuisance RCD trips and EMI.
  • DC injection control. Grid codes strictly limit DC injection into the AC network. The transformer blocks it by design.

Harmonic feedback and microgrid stability

At peak discharge, several V2G cars on one low-voltage feeder can export at the same time. That is when harmonics matter most.

  • The risk. Switching-frequency ripple and low-order harmonics from many converters add up on a street's distribution cable. They can distort voltage, overheat neutrals and trigger resonance with the capacitance of nearby solar inverters. On a weak rural feeder, the result is flicker, tripped inverters and protection faults.
  • The SiC advantage. Switching at 100 kHz or more pushes ripple far above the frequencies where the grid's impedance and resonances sit. A compact LCL filter removes it. Current total harmonic distortion (THD) falls to below 3%, comfortably inside Engineering Recommendation G5/5.
  • Isolation as a buffer. The isolated DC-DC stage decouples the pack from grid disturbances. Grid transients do not pass through to the cells, and battery-side ripple does not reach the feeder.
  • Grid-forming control. Advanced converters use virtual synchronous machine control. They behave like a small spinning generator and resist frequency and voltage swings instead of following them. A street of V2G cars then helps stabilise the network.

Connection rules

Export capacity governs the paperwork. A single-phase unit exporting up to 16A per phase, about 3.68 kW, can connect under G98 on a notify-after basis. A 7 kW or 11 kW V2G unit falls under G99 and needs DNO approval before commissioning, or an export limit set in firmware. Our G99 connection analysis covers the queue in detail.

Sodium-ion: the chemistry built for daily cycling

V2G changes how a car battery is used. A typical driver cycles the pack perhaps 150–250 full equivalent cycles a year. Add daily solar shifting and evening export, and that can rise to 400–500. Grid services add thousands of shallow micro-cycles on top.

That is where chemistry decides the case. It is why our reference platform uses the Sovereign Cell sodium-ion framework set out in the EV silo.

Why NMC struggles with V2G duty

  • Cathode lattice strain. Nickel-rich layered oxides expand and contract anisotropically as lithium moves in and out. Repeated cycling causes micro-cracking in cathode particles, which exposes fresh surface to electrolyte attack.
  • Phase transitions. At high state of charge, high-nickel NMC undergoes a damaging H2–H3 phase transition. V2G keeps cars at high charge for long periods, waiting to export, which speeds up that damage.
  • SEI growth. Every cycle thickens the graphite anode's solid-electrolyte interphase. It consumes lithium that can never be recovered.
  • The result. Automotive NMC typically reaches 80% capacity after about 1,500–2,000 full cycles. Daily V2G duty uses that budget much faster. That is why many manufacturers restrict V2G or its warranty terms.

Why sodium-ion thrives

  • Structurally stable cathodes. Prussian-white and polyanion cathodes, such as NVPF, have open, rigid frameworks. Sodium ions move in and out with minimal volume change, so there is little lattice strain to crack the particles.
  • Hard carbon anodes. Hard carbon's disordered structure takes sodium without the staging expansion of graphite. The risk of metal plating is also lower.
  • Cycle longevity. Commercial sodium-ion cells are rated for 3,000–6,000+ cycles to 80% capacity. That supports thousands of aggressive charge-discharge loops without structural cathode degradation.
  • Zero-volt tolerance. Aluminium current collectors on both electrodes allow safe storage down to 0V. That gives the BMS a wider usable window.
  • Cold-weather export. Sodium-ion keeps most of its power at low temperature. That matters because the UK's highest-value export evenings are in winter.

The degradation cost of V2G is the hidden line in every arbitrage calculation. With NMC, a £4 export can cost £2 in battery wear. With sodium-ion, wear per kilowatt-hour is a fraction of that, so most of the spread stays with the household.

Grid synchronisation and the arbitrage opportunity

Hardware makes V2G possible. Dynamic tariffs make it pay.

How dynamic pricing works

UK time-of-use tariffs such as Octopus Agile set a new import price every half-hour, following the wholesale market. On windy nights and sunny weekend afternoons, prices fall. When renewable output exceeds demand, they can go negative, and the supplier pays you to consume. Our analysis of negative prices and solar cannibalisation explains why these events are becoming more common.

The 16:00–19:00 evening peak reverses this. Solar output collapses as demand surges, and gas plants set the price. Import rates often exceed 30p/kWh, and export tariffs pay a premium.

The automated arbitrage cycle

  • Absorb. The car charges from surplus rooftop solar, or from the grid at low or negative prices during local wind and solar surges.
  • Hold. The pack stores energy through the afternoon, keeping the driver's reserved range intact.
  • Power the home. In the evening peak, the car supplies household demand directly, displacing 30p+/kWh grid imports.
  • Export. Remaining surplus goes back to the grid at peak export rates, or is dispatched as part of a virtual power plant for flexibility payments.
  • Recharge. The car refills overnight on cheap off-peak energy, ready for the morning commute.

The arithmetic

Take an illustrative winter weekday. The car cycles 20 kWh through the home and grid, with a 91% wall-to-wall round trip:

  • Import: about 22 kWh at an overnight 7p/kWh, costing about £1.54.
  • Displace or export: 20 kWh at an evening value of 25p/kWh, worth about £5.00.
  • Net daily value: about £3.46. Over 250 plugged-in days, that is about £865 a year.

Summer changes the source of energy, not the pattern. Free solar surplus that would otherwise be exported at a low rate goes into the car instead. A 13.5 kWh stationary battery fills by lunchtime. A 55 kWh car keeps absorbing all afternoon.

These figures assume a responsive tariff and a plugged-in car. Real returns depend on driving patterns and tariff spreads. The home battery without solar guide covers the tariff side in more depth.

Automation is the enabler

No household will trade half-hourly prices by hand. The value comes from software. The vehicle, the charger and the supplier's platform communicate over ISO 15118-20 and OCPP 2.0.1. They know the driver's departure time, the required range and the next day's price curve, and they schedule every flow automatically. The driver sets two numbers, when they leave and how far they need to go, and the system handles the rest.

Grid-to-gravel: the verdict

The home battery was a workaround for a world where cars could not export power. That world is ending.

  • Capacity: a 55 kWh car pack offers about four times the storage of a £7,000+ stationary battery, at no extra battery hardware cost.
  • Efficiency: 800V SiC conversion cuts thermal losses by up to 70%. Round-trip power electronics efficiency exceeds 95%.
  • Safety: galvanic isolation and high-frequency filtering keep the 800V pack and the 230V/400V home separate, and keep the street's feeder clean.
  • Durability: sodium-ion cells handle thousands of daily cycles without the structural wear that limits NMC.
  • Economics: dynamic tariffs turn the parked car into an automated arbitrage engine worth hundreds of pounds a year.

The same inverter technology that makes a sub-£30k EV efficient on the road makes it a mobile energy store on the driveway. The car and the home battery become one asset. For the UK grid, millions of parked cars become a distributed storage network that did not have to be built separately.