Undeniably doable, but might be less worthwhile than it first seems.

Consider as an example, a battery that provides 25 kwh on discharge. There will be small but real losses in the charger, in the inverter, and in the battery itself. So to provide 25 kwh of electricity output might require about 30 KWH of input.

Then consider the cost of the battery and the depreciation thereof. If the battery lasts for 2,000 cycles, and costs £10,000 then each cycle costs about £5, IN ADDITION to the energy cost of charging.

So if the cheap electricity to charge the battery costs 10 pence a unit, it will cost £3 per charge, plus another £5 for battery depreciation, or £8 in total.

The 25kwh produced on discharge will therefore have cost in total about £8. That is about 32 pence a unit. Slightly more expensive than the capped domestic price of about 30 pence a unit.

You can`t tell folk that. They make their own facts and conspire to make all results fit into those facts

for those wondering the capped unit pricing by region info

is here most expensive regain is SW with a capping

£1018.19 per 3100KWh = 30.44 pence unit.

and the sums behind it in this OFGEM spreadsheet
Mike

Wot 'e said.

At some point, on some TOU tariffs, it might be worthwhile, but the maths needs to be done properly or as broadgage says there will be unhappy people when time comes to buy a new battery earlier than expected. To be fair, it is worth noting that battery life is defined arbitrarily and it doesn't stop working at all beyond the nominal cycles, but you will be beyond warranty and capacity will have fallen, which might well be a big deal for the primary use of the car. Or not if you only drive round the block anyway.

For commercial / utility scales this arbitrage with batteries it is already being done as part of the "revenue stack", because the price differential between demand peak and trough is somewhat greater and they're not paying to wrap their battery in a car.

Also don't forget that it turns your site into a generator so G98 / G99 required, and not all car chargers can do it.

broadgage said:

Undeniably doable, but might be less worthwhile than it first seems.

Not just doable, but done: https://octopusev.com/powerloop

When energy companies offer you preferential rates for electricity, in return for the use of your vehicle batteries (and perhaps when it gets going fully, solar "self-consumption" batteries), it is likely to be the best option (perhaps you will pay through the nose for energy unless you have this flexibility for your energy vendor).

The sums look more favourable if you're less pessimistic on the figures used.  Octopus Go is still available at 7.5p/kWh, for 4 hours of overnight charging, or about 28kWh.

On a modern car, 25kWh may only be about half a charge.  So it's not using a full charge cycle every day.  There is no way the battery would need replacing after only 2000 cycles if the charging/discharging is managed properly.  The solar battery in my loft has a 10 year warranty*, and is cycled every day, so that's 3600 cycles.

*Or would have if the installer had bothered to register it.

I think you are about to hit a "manufacturers warranty" issue here. If you add extra battery cycles, you are basically doing a lot more miles. Clearly this is serious for the vehicle warranty and I would expect you pay a lot for that. BTW I have a number of high capacity Li batteries, and get nowhere near 3000 cycles from them, despite there being a lot of "load management" electronics inside. At about 1500 cycles they are probably at half capacity, why you think vehicle batteries will be better is beyond me, as they get far more high rate charge / discharge cycles. Second hand electric vehicles often have trouble with batteries, probably why they are sold, and warranties on batteries are probably unobtainable. Battery storage is very unlikely to ever provide cheap power, it certainly doesn't today.

I don't think the average bear is going to understand what is best for them. There are going to be some very angry car owners when the grid has knackered their car battery and the car dealer says, "Nothing to do with us guv".

David Stone said:

I think you are about to hit a "manufacturers warranty" issue here. If you add extra battery cycles, you are basically doing a lot more miles. Clearly this is serious for the vehicle warranty and I would expect you pay a lot for that.

There are only a select few models (I thin you can count them on one hand) out there in the UK that are capable of V2G.

You might be right in terms of the warranties, but I'm not conversant with the battery warranty small-print for those models - I guess it would be easy to check. I understand the most prolific model is the Nissan Leaf.

David Stone said:

BTW I have a number of high capacity Li batteries, and get nowhere near 3000 cycles from them, despite there being a lot of "load management" electronics inside.

This definitely depends on the battery chemistry and physical battery architecture ... and what profiles the batteries were designed for in the first place. What's good for one particular battery is not always true for all.

hmm the same things that make a good battery for high peak currents, such thicker, better supported electrodes (less foil-like) better control over impurities is electrolytes  and less capacity in amp hours per unit volume, also work for cooler running and longer life. Attempts to cram as many AH as possible into the volume tend to result in batteries that run hot and die young. One of the reasons that Tesla have got themselves involved in the battery making business, rather than just phoning up Panasonic or whoever, is to have better control over that side of things. To be focusing on lifetime is an interesting change of direction for an industry that previously had been chasing capacity at the expense of reliability for some years.

Even so,  I too would be wary of matching the wilder claims in real life use, it is not yet an exact science.( but intelligent charging controls charging counting electrons out and back in again, low voltage lock off and watching temperatures and cell balancing is still necessary, but not  on its own enough.)

If not already, then soon there will be huge volumes of ex car batteries that are not exactly dead, but not really good enough to drive round on either, so things that use them, perhaps as local power, will probably become more popular.

Mike