Ummm some of that is some what incorrect and that is the nub of the dilemma.A quick catchup for anyone not wanting to watch the mixed tennis/squash game above.
On Rob's Tarif. Daytime electric is 42 Pence per KWH. Cheap rate is 10 Pence Per KWH (1000w for one hour). He'll also pay a standing charge of about 45p a day just to be connected to the electricity supply.
Rather than paying 42p for his daytime rate, he'd like to capture and store 20kwh's (for example) in batteries on the cheap rate and use that during the day.
To do that he needs to invest in a large pile of (relatively) expensive batteries and some equipment to control the charge/discharge/change the voltages/stop the batteries getting damaged.
That then leads to the economy question.
How long does it take to pay off that investment on the 32p (42p-10p) saving he makes per KWH, also keeping in mind that the batteries and possibly other parts of his system, will need replacing over time.
If his land allows he could also build two ponds and use the cheap electric to pump water to the upper pond and then run that water through a mini hydroelectric generator during the day to create his electricity. Or just use a motor to lift an obscenely heavy weight up a ramp/pole on cheap rate and lower it to create power during the day. Gravity examples. Alternatively, he could put a load of sand in a large well insulated container, heat that to about 700c with his cheap electric and then use that heat to create electricity during the day.
The other issue being discussed is Supply.
He has a 100 amp supply (limited by the thickness of the cable to the house). Therefore 230v * 100 = 23000w he can use at once (peak)
The electric cars can charge on 32amp (7360w) or 16amp (3680w).
He also needs to pull about 3500w per hour to charge his 20000wh batteries (ignoring efficiency losses for simplicity)
Rob has the choice of paying on several tariffs for his electricity. He has chosen a variable tariff of 42P for 20hours and only 10P for the other 4 hours. He could also only pay 28.2P across the entire 24 hours.
Although he has a 100A surge fuse in his incoming electric cable he cannot pull constantly that current, to prevent overheating and excessive power load on the likes of local inter connectors in the street supply, it would be normal not to exceed a constant max of 63A on a local 16mm2 cable or 80A if the street supply to his house is 25mm2 ( only relatively new supplies will have this ).
Just to confuse the issue for the non electrical versed amongst you, no Rob will not be charging at 32A’s, the 32A’s he is talking about is the inline 32A fuse to prevent the charger overloading his house supply board. Now that lower figure of the actual charge rate ( possibly as low as 3.6KW or 7KWs if he has the car and EV charger to match ) will dictate just how much he can download in the 4 hours he has the cheap tariff ( 7KW x 4 = 28KWh, most EV‘s these days are 60KWh or more ).
As with all these tariffs it’s a bit of a maths game to actually see who wins, Rob or his suppliers in the monetary stakes.