The default evening
Home at half six, out at half six the next morning, 45 kWh to put back through an 11.5 kW charger. That is 3 h 55 min of charging and twelve hours of standing there, so the timing is entirely free — the only question is which twelve hours the four sit in.
Start immediately and the session runs 18:30 to 22:26, every minute of it on-peak. Fifty kWh off the meter at 28 cents is $14.00. Delay to the window opening at 23:00 and it runs 23:00 to 02:55, every minute off-peak, and the same fifty kWh costs $6.00. Eight dollars for a setting nobody has to think about again.
The case that is actually interesting
Change the departure to half four in the morning and the picture shifts. There are now ten hours of plug time, the window still opens at 23:00, and a delayed start finishes at 02:55 — still fine. Shorten the charger to 7 kW instead and the session takes 6 h 26 min: a full delay finishes at 05:26, after you have gone, and the car is short 6.5 kWh — about 21 miles.
That is what the latest-start row is for. It gives the last moment a session can begin and still complete, and in cases like that one it is earlier than the window opens. The honest answer becomes a compromise start that catches part of the window rather than all of it. On that case it is 22:04, 56 minutes before the window opens: 5 h 30 min of the session lands off-peak and 56 minutes does not, for $7.15 against $11.60 for plugging in and going. The page finds that by sweeping the start time rather than by asserting a rule.
The rate gap decides whether any of this matters
Every saving on this page is the rate gap multiplied by the kWh that moved. The default gap is 16 cents and the default session moves fifty kWh, which is why it is worth eight dollars. A tariff whose off-peak discount applies only to the supply portion might have a real gap of four cents, and the same session then saves two dollars — still real, but no longer worth rearranging a household around.
This is why both rate fields ask for all-in delivered numbers. Rate cards advertise the supply gap, bills contain the real one, and the difference between them is regularly a factor of three.
Where constant power stops being true
The arithmetic assumes the charger holds its output flat for the whole session. AC charging is close to that until the pack is fairly full, at which point it eases off, so a session that ends near 100 percent runs a little longer than the page says and slides a little further into the next rate period. Where a session already ends within twenty minutes of the window closing, that drift is enough to eat the saving. Where it ends hours inside, it changes nothing.
Questions people ask
What time should I set my EV to start charging?
It depends on whether the session fits inside the window. With the defaults, delaying to the 23:00 opening puts the whole session in the cheap rate and saves $8.00 against starting at plug-in. Where the session is longer than the window has room for before you leave, the cheapest start is earlier than the window opens, and the page sweeps for it rather than guessing.
How much does off-peak EV charging save?
The rate gap multiplied by the kWh that move into the window, and nothing else. The defaults move fifty metered kWh across a 16 cent gap, which is $8.00 a session. Halve the gap and you halve the saving. If your tariff only discounts the supply portion of the rate, the real gap can be a quarter of the advertised one, so take both numbers off a bill.
What if the charge does not finish before I leave?
The page says so and reports the shortfall in kWh and in miles rather than reporting a tidy cost for a session that never happened. If it does not fit even starting at plug-in, no scheduling helps and the levers are a faster charger, a longer plug-in period or arriving with more charge. If it fits but a full delay does not, there is a latest-start time that still finishes and the page prints it.
Does the calculator handle a window that crosses midnight?
Yes. Most off-peak windows do — the default is 23:00 to 07:00 — and the overlap is worked out on an unrolled timeline rather than by clock arithmetic, so a session starting at 22:00 and ending at 02:00 is split correctly. A departure time earlier than the plug-in time is likewise read as the next morning.
Is a delayed start always cheaper?
No, and the page computes it rather than assuming. If the window opens too late for the session to finish, delaying costs you range instead of money. If the two rates are equal, delaying is worth nothing. And on any tariff that also bills demand, everything in the neighbourhood starting at the same window opening creates a peak of its own, which is a different charge with different arithmetic.