While Zappi or openEVSE are great in this regard, it is only one way: charging. So benefit is also only one way: charging your car. You can't offset your dinner oven cooking with energy you have cheaply stored in your car.
Let's consider this, my home uses 10 kWh normally. Let's say during peak evening I use 3-4 kWh. That means if I were to set V2G (or rather, V2H) to discharge the car during that peak evening period, I would only use about 15 miles of range. If I arrive home with 30% remaining on a 50kWh car, use it down to 22% over evening is still not too low.
Would increased use accelerate battery degradation? From data we currently have, it seems age of battery plays a bigger role in degradation while gentle slow charging and discharging does not put a lot of stress on the battery. We see 100k taxi Leaf with all health bars while older average mileage Leaf loosing health bars. A hosuehold peak rate of 7kW, <1kW typical will be a lot kinder than 20kW constant drain when driving on motorway.
In summary, I'd happily put cycles on my battery, especially our local runabout, in exchange for cheap tariff. Considering age of the battery will reduce its range at a steady rate.
TLDR: battery health is use it or loose it, might as well use it to pay <5p per kWh rather than 30p per kWh.
Home battery are a nice idea, and it's more mature tech than V2G. But I honestly don't see any point in this at early stage where we have a shortage of battery production. When we are home and using electricity, our cars are most likely also home. You'd never run your car down to almost 0% risking getting stranded, so there's pretty much always energy left in the car ready to off-set your peak time usage.
