There have been a few trials of V2X technology. In the ones I saw, you were on the hook for the installation cost (~£5k) if you didn’t plug in before 6pm on 50% of days. There was a cost to keep the kit at the end of the trial. Worth checking carefully what you’d be signing up to.
The first phase of the trial is for Chademo so worth considering if you want to tie yourself to this technology. Newer cars don’t have it so you could be stuck with a charger that not only doesn’t do V2H but can’t charge a subsequent car at all.
The subject of powering the house in the event of the failure of supply is an interesting one and one that affects not just V2X but static batteries as well.
First consideration is load management. The inverter isn’t going to be able to support all of the load in the house so your going to need to decide which circuits to support during an outage. Those need to be rewired so that they are supplied via the inverter. If you’re thinking about keeping loads like computers going, remember, this isn’t a UPS so there may be a break in supply. You then need to know what happens if the load is too great for the inverter. If the power fails whilst you are drawing a lot of power, will the inverter just shut down, leaving you with no power anyway?
The next challenge is earthing. In most houses in the UK, earthing is delivered by the electricity supplier. You can’t depend on that earth being present in the event of a power cut so you need your own. You then need a method of making sure you don’t connect your earth to the supplier’s earth. This should be in the inverter but, certainly in the case of static batteries, that has not always been properly considered. Without proper earthing, you are in danger of losing your protection against electric shock in the event of a fault.
Last is over current protection. The fuses/circuit breakers in your consumer unit are based on having a very high capacity supply. The reason the electrician needs to measure “source impedance” is to make sure that the supply can deliver enough current to operate the protection. An inverter that can deliver up to say 7kW will happily deliver 32A into a fault in your wiring without ever being able to operate the fuse. For context, a 32A MCB will take 5 minutes to operate carrying a fault current of 50A so effectively the protection you currently have is no use if the supply is from an inverter.
All of this is solvable but (1) it’s pretty unlikely that the trial installer will going to do any of this for you and (2) even if you decide to spend the money on reconfiguring your wiring to get power outage protection, I wouldn’t trust a domestic-installer-who-calls-themselves-an-electrician to do it.
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