Though the feel good factor of charging with solar can't be discounted I have seen it pointed out that if you charge at night on a 5p KWh tariff (Octopus) it is cheaper than solar direct .. because you would be paid slightly more than that for exporting your solar to the grid. (This is second hand information but the solar users here will no doubt be able to confirm or deny!)
Depends on when the system was installed and whether or not it has an export meter. We're on 50% deemed export, so currently get paid 5.57p/kWh for 50% of our total generation, irrespective of how much we self-consume. We generate around 6 MWh/year usually, and consume about 3 MWh/year from the grid, so in our case the 50% is pretty accurate, I think. Our income from export payments is about £167/year, plus we get a FiT payment of about £850/year (14.16p/kWh of total generation). We pay about £600 for our imported electricity (including the standing charge), so on balance our total household energy cost (we only use electricity) is about -£417/year.
Because this figure is always negative, I don't tend to bother much about fine tuning something to gain another few pence of income. I have kept track of the car energy use both when using a smart charging system, rather like the way the Zappi works, to try and maximise solar charging, and also since I binned that as being to complex and reverted to a fixed, slower, charge rate for summer daytime charging. So far, using a fixed, slower charge rate, has been marginally "cheaper" than using the smart charging system, although not by enough to be able to say with confidence that it will always be slightly cheaper.
The difference between the two methods is so small though, that I'm not at all convinced that solar smart charging is worth the hassle, and the relatively high cost of a charge point like the Zappi. I didn't need to buy a Zappi, as the instrumentation and telemetry system I built in to the house measures instantaneous true power at the incoming supply and broadcasts this via a 433 MHz radio link to any device that needs it. It was intended to just send data to two devices, the main data logging system (that records things like total and individual appliance energy use, temperatures from a dozen or so locations, CO2 concentration, humidity etc), plus a simple import/export power display we have plugged in to an outlet in the kitchen as an indicator as to when to do things like turn the dishwasher or washing machine on during the summer.
The old version of one of my DIY charge points just used that instantaneous power broadcast to determine when to start and stop car charging during the day, and vary the advertised maximum current to the charger(s), based on a simple algorithm that tried to use a combination of grid and solar power to charge the car if that was cheaper than charging at the overnight cheap rate. It doesn't need much solar to achieve this, just enough to reduce the cost of charging by about 6p to 7p/kWh. For a 10 A charge current, that means that we need about 1 kW of excess solar generation to make it worthwhile. More than 1 kW, up to 2.3 kW, reduces the charging cost below that of the off-peak rate. Over 2.3 kW of excess PV generation means we're "wasting" electricity by exporting to the grid.
For anyone on E7, that has a reasonable size PV system, then having a cheap and simple charge point that is turned on only during the off-peak period for normal charging, and just using the UMC for charging during the summer to utilise excess solar generation, is probably the most cost-effective solution. It will take a heck of a long time to recover the cost of something like a Zappi installation, just from the saving in charging costs.
