Edit: I recently revised my opinion of V2G implemented with Panasonic 18650 batteries, as used in Tesla cars. It may be possible to achieve good enough lifetime even in solar daily cycling applications. For details, see: http://blog.evandmore.com/lets-talk-about-the-panasonic-ncr18650b/
Of particular importance is this figure: 28 000 cycles from 10% to 90%
If you use only 80% of the battery capacity by using a slightly larger than needed battery, you can theoretically achieve over 75 years of lifetime. Theoretically, as the calendar life may be more limiting than the cycle life.
New cost calculation: 100 USD / kWh, of which you can use 80%, giving 125 USD / usable kWh. If 50% of the days are sunny, you achieve about 180 cycles per year, or 1800 cycles in 10 years (best to limit the considered period to 10 years, as we don't know the calendar life of these batteries, and money has a time value). It's 0.07 USD / kWh then. You need to add 1 USD / watt of solar, with 16% capacity factor, meaning 1 watt produces about 1.4 kWh per year, or 14 kWh per 10 years. So, about 0.07 USD / kWh for electricity production and the same for storage. 0.14 USD / kWh is more than 2x as expensive as the electricity where I live, but the cost is coming down all the time. Perhaps, some day, we will see V2G programs.
Here's the old answer:
Don't consider such stupid ideas! Hype is hype.
Tesla uses a different battery chemistry (NMC, nickel manganese cobalt) for PowerWall products that are intended for grid energy storage. For the vehicle use, Tesla uses NCA (nickel cobalt alunimum), which is cheaper but not durable enough for daily cycling. I guess they have a valid reason for the different chemistries.
And before somebody claims driving is daily cycling, it isn't, because Tesla batteries are so large that driving the car uses an infinitesimal amount of its capacity.
Now, if a NCA battery is already at its end of useful lifetime (like holding only 50% of the capacity), you could use it for grid energy storage for a while until it is finally scrapped and the materials are reused. So, as reuse grid energy storage is a good application, but these NCA batteries degrade very quickly when used for grid energy storage. So anybody planning to reuse batteries needs to have a plan of how to safely scrap them, and needs to be aware of their limited lifetime.
I guess the reason why such programs in general don't exist is that electric vehicle users are well aware of the properties of their expensive toys. Specifically, they are aware of the limited lifetime of batteries when rapidly cycled.
There is one form of cycling that is beneficial and helps stabilize the grid: charge when electricity is cheap, drive on charged electricity when electricity is expensive. Of course, this works only if the battery is large like on the most expensive Tesla cars.
In no circumstance, would I discharge the battery while supplying power to the grid. I could consider supplying power to my own house in an emergency, but not to the grid (unless I get paid really large amounts of $$$).
It is almost always cheaper to use electricity from the grid than to discharge your EV battery, reducing the useful lifetime of the very expensive battery.
- Cycle durability of Li-Ion is 400-1200: https://en.wikipedia.org/wiki/Lithium-ion_battery ...and this means full discharge, then full charge, then repeat
- It is well known in the automotive circles that shallow cycling allows a battery to last for a very long amount of time. For example, Toyota hybrid batteries that use only ~20% of the battery capacity can do so for 100 000 cycles. Example source: http://www.bitsonchips.com/wp-content/uploads/2017/07/ref33.pdf quoted: "in shallow state of charge (SOC) swing applications (such as hybrid electric vehicle and certain power
quality applications) the cycle life can be many hundreds of thousands of cycles"
- Tesla batteries when used for driving last the lifetime of the car: https://electrek.co/2016/11/01/tesla-battery-degradation/ ...and note this means for driving, not for 100% cycling every day. It is well-known that Teslas have a range of 500 km or so, and typically people drive 50 km or so per day, then charge, using only 10% of the capacity.
- Tesla PowerWall uses NMC, not NCA: https://teslamotorsclub.com/tmc/threads/something-fishy-about-the-batteries-in-powerwall.75391/ -- "NMC has much better durability (5000+ full cycles vs 500-1000 full cycles), whereas NCA has much better energy density (280 Wh/kg vs something like 150 Wh/kg)."
- Also: https://medium.com/the-unfortunate-tetrahedron/the-cathodes-of-the-powerwall-6aa52df8cd66 -- "It is speculated that the NCA chemistry is the cell in the Model S. To the best of my knowledge Tesla has not used the NMC chemistry in any car to date."
- Cost analysis of NCA: $100 / kWh probably will be achieved on a pack level after few years, 500-1000 full cycles, $0.1 - $0.2 / kWh. I guess in some areas where the authorities are insane (preventing the construction of thermal power, necessitating massive building of intermittent renewables), the electricity price can be over $0.2 / kWh or even higher with taxes. I am fortunate to live in an area with 0.06 EUR / kWh electricity.