As part of my electric car charger control project I thought I’d try to estimate the benefit of such a system. I’d already decided on resource grounds to do a simple on/off system. The car offers the choice of charging at 6 or 10 Amps. 10 Amps would of course charge more quickly, but would be available for fewer hours each day, so I thought I’d look at what might be possible with 6 Amp charging. My chosen date for analysis was March 16th a day on which we generated 19 kWh. The Immersun provided a hourly profile of consumption and generation through the day which included diversion of 5.4 kWh to generate hot water via the Immersun.
For my charging analysis I decided to prioritise EV charging over making hot water (an option within the Immersun) which would result in hot water being generated until 1.5 kW was available, once 1.5 kW was available then the car charger would be turned on, and then as the power increases beyond 1.5 kW then the water heater turns back on to absorb the excess.
The result of this strategy was that 7.5 kWh was diverted to charge the car, while water heating received 4.8 kWh (a slight reduction on the 5.4 kWh delivered for water heating without EV charging). Exported electricity dropped from from 7.5 kWh to 0.2 kWh – so effectively all the available power was used. 7.5 kWh is more than 50% of a full charge for the car and probably exceeds my average daily charge.
For the alternative 10 Amp charging strategy only a single hour could provide 2.3 kW for charging (a total of 2.3 kWh) so the 6 Amp charging strategy provided around three times the energy transfer to the vehicle.
If the project saved me the same 7.5 kWh from my night time electricity (my current pattern of car charging) that would save me 60 pence per day. Of course I wouldn’t save that every day – some days (particularly in winter) we don’t generate enough power and other days I’m at work (although it should work quite well at the weekend when I spend more of the day at home). Normally I start each day with the car fully charged, but on most days I return with a significant amount of unused charged (typically 50%), but I could choose a charging strategy where I don’t attempt to fully charge each day allowing some charging to be displaced from weekdays to the weekend. It thus gets quite hard to estimate an annual benefit.