Solar compatible EV car charging
A standard domestic EV charging unit has two modes of operation,
a slow charge, and a semi-rapid charge. A solar compatible
charger has a third mode which uses a current sensor on t
he incoming mains supply cable. If there is sufficient excess
power being exported from solar PV generation then the
charger switches on to allow charging from the electricity that
would otherwise have been exported.
Electric vehicle charging from domestic supply
The UK national electrical supply network has the purpose of taking the output from the power stations and delivering it to industrial and domestic users. It is a complicated network of sub stations and interconnecting cables but in simplistic terms consists of the 400kV pylons that carry the high voltage power from the power stations to the regional distribution hubs and falls under the control of The National Grid, which is a publicly quoted company.
The regional hubs drop the voltage down to 11KV for local distribution, which falls under the control of District Network Operators (DNO), which are private companies that bid for control over the 11 regional distribution areas. The DNOs are responsible for supplying individual businesses and households and use local sub stations to drop the voltage from 11kV down to the 415V three-phase and 230V single-phase supply for connection to individual properties. The whole distribution system is monitored by Ofgem which is an independent body appointed by the government to keep the lights on. This whole private sector operation is funded from consumers’ electricity bills to make sure the shareholders get good dividend returns!
Most domestic properties in the UK have a 100 Amp incoming supply which, in theory would allow a maximum power usage of 23kW (or 8 three pin plugs simultaneously!) however, the District Network Operators rely on not everyone using the full capacity of their individual supplies simultaneously, so the local substations are effectively under sized, which then has consequences for anyone who wants to use high power applications.
Electric vehicles have large batteries, typically 5 to 10 times the capacity of the batteries used for domestic PV storage, which gives rise for potential EV to grid applications. However, a consequence of these larger EV batteries is that charging EV’s from a household supply would be like adding several new houses to the existing substation, and that threatens the grid with serious supply issues.
With this in mind, the DNO’s have placed a 32 Amp limit on domestic EV chargers which gives a maximum power of 7kW (21kW on a three-phase supply) so a standard EV charger will charge at 7kW (semi rapid) or 3kW on slow charge, which means you will need 10 to 24 hours to fully charge your vehicle from a standard domestic EV charger.
The third charging mode available with solar compatible chargers uses a current sensor on the incoming mains supply cable. A PV system will convert any available incident solar radiation into electricity and feed it on to the household consumer unit. Any circuit that is pulling power at the time the solar is feeding in will of course use the solar power rather than mains electricity. If the solar is feeding in more power than the household is using, then the surplus would normally be exported to the grid, however a solar-compatible electric vehicle charger will recognise this exported power and if it reaches the minimum required to activate the vehicle charger (normally about 1.4 KW) the charger turns on and starts charging the vehicle.
