A car carries a far bigger battery, and it is usually somewhere else

The size comparison is genuinely striking

Domestic storage units are typically sized to carry a household through an evening. Electric vehicle packs are sized to move a tonne and a half of metal for hundreds of kilometres, and the result is a battery several times larger than the one a household would ever buy for a wall. On paper it is an enormous resource sitting on the driveway doing nothing for most of the day.

That observation is correct and it is where most of the enthusiasm comes from. A vehicle pack could, in principle, carry an ordinary house for days rather than hours, and it has already been paid for as part of buying the car.

The difficulty is that a battery bolted to a wall and a battery attached to a car differ in one crucial respect. The wall battery is always there.

Availability is the constraint that decides it

A home battery is available every hour of every day, which is what allows it to catch a solar surplus at noon and release it at seven in the evening, every single day, for years. A car is available when it is parked and plugged in, and for many households that is overnight and not much else.

Overnight availability isn’t useless. It suits a tariff with a cheap window, since the car can absorb cheap energy and release some of it during an expensive one. But it lines up poorly with solar, because the vehicle is most likely to be absent during precisely the hours the roof is producing a surplus.

The households where the arithmetic works are the ones where the car is home during the day: people who work from home, retired households, second vehicles. For a commuter who leaves at eight and returns at seven, a vehicle pack can’t do the job a wall battery does, however large it is.

Bidirectional hardware, and which direction it goes

Ordinary charging is one-way. Sending energy back out of the pack requires a bidirectional charger, which is a more complex and more expensive device than a standard wall unit, and it requires a vehicle whose manufacturer permits discharge in the first place. Plenty do not, and support has arrived unevenly across models and markets.

Two distinct arrangements get discussed under similar names. Powering the house from the vehicle keeps everything behind the meter and is the simpler proposition. Exporting from the vehicle to the network is the more ambitious one, and it needs an agreement with the network operator and a supplier willing to buy on those terms, which exists in some markets and not in others.

Backup adds a further layer, because using a car to power a house during an outage requires exactly the same islanding hardware that a battery backup requires. The vehicle doesn’t remove that need. It only changes where the stored energy comes from.

Warranty and wear, which nobody enjoys discussing

A traction battery has a warranty stated in years and distance, usually with a retained-capacity threshold attached. Cycling it for household purposes adds throughput that the manufacturer did not anticipate when pricing that warranty, and manufacturers have responded in different ways: some permit it explicitly and count it, some cap it, and some decline to support it at all.

Whether the wear is significant is genuinely contested. Household cycles are shallow and gentle compared with fast charging and hard driving, which are the conditions that age a pack most, so the incremental damage may be small. It is not zero, and anyone relying on the warranty should read what it says about discharge to an external load before installing the hardware.

The value at stake is not symmetrical either. A degraded home battery is a nuisance. A degraded traction battery reduces the range and the resale value of a considerably more expensive asset.

The version that works today

There is a much simpler measure that captures a large share of the benefit and needs none of this. Charge the car from solar surplus. A charger that modulates its current to follow the excess production of the array turns a household’s largest flexible load into a way of consuming its own generation, and it requires no bidirectional anything.

That approach also sidesteps the availability problem partially, because a car that is home at weekends can absorb a great deal of surplus in two days. It is unglamorous, the hardware is mature, and in most households it is worth considerably more than a discharge capability that will be used occasionally.

None of which means bidirectional charging is a bad idea. It is a good idea that is early, and being early is a real cost: fragmented standards, limited vehicle support, uncertain warranty treatment and regulatory arrangements that differ from one country to the next. Anyone buying it today should do so knowing that, rather than on the strength of the pack size alone.