Setup guides → Cheapest hours

Charge in the cheapest hours

What the grid is asked to do, and when. Set a level per car and a price you will not buy above; Sunplug finds the hours, at whatever time of day they happen to fall.

What it does

Each car has two handles on the same slider: the lower one is the level the grid should reach, the upper one the target you are happy for the sun to chase. While the car is plugged in, Sunplug fills it to its grid level in the cheapest hours it can see, at full rate, in blocks of at least an hour, and never in an hour priced above the ceiling you set. Whatever the sun adds goes on top, up to the solar target.

Cheap is not a time of day. Off-peak contracts put the cheap hours at night, so that is where the habit comes from — but on a market tariff a breezy midday, with every solar roof in the country exporting at once, is regularly cheaper than three in the morning. A car on the drive at lunchtime is charged at lunchtime if that is what the prices say.

Full rate is not a contradiction of everything Sunplug does in daylight. On sun the rate follows what the roof is making; on price the cost of an hour is fixed, so the cheapest way through it is at full current and out the other side.

Blocks of an hour

A cheap quarter of an hour on its own is not taken. Every start and stop cycles a mechanical contactor in the charger or the car, and those have a finite life — so a block is an hour or more, and the few cents a shorter one might have saved are not worth the wear.

The price you will not buy above

One figure for the household: the price above which grid charging simply does not happen. Through a cold, still week, when every hour is dear, this is what stops a routine top-up from costing several times its usual amount. The car waits for a cheaper hour, or for the sun.

What overrides it

A trip wins. Set a departure time and a target and the car is ready at that hour. If the only way to get there is to buy above your ceiling, Sunplug buys above your ceiling and sends you a notification saying so — a price you set is a preference, and a time you have to leave is not.

Boost charges now, as it always has.

How each car is told

A Tesla is given the plan as charge schedules, written into the car at plug-in while it is already awake. No wake-ups are spent on it after that: the car starts and stops on its own clock.

Any other car charges at full rate through a supported smart charger, which is what starts and stops it at the right hours.

On the home screen

Home shows the next 24 hours of prices with the cheapest hours marked and the planned charge underlined, so the plan is visible before it happens rather than explained afterwards. A switch changes the view between 15 min and 1 h.

Market prices: ENTSO-E Transparency Platform and ESIOS (Red Eléctrica). Solar forecast: Open-Meteo.

What it will tell you

Notifications, each of which can be switched off:

• Negative prices ahead.
• Prices are missing and a charge is at risk.
• A trip needed more than your ceiling allowed, so it bought above it.

What it adds to the statistics

Beside what the sun saved, which is unchanged: the average price you paid per kilowatt-hour taken from the grid, and where that charging sat among the day's prices — the cheapest tenth of the hours, or the cheapest third. The second is the honest test of whether any of this is working.

In France: Tempo

On a Tempo contract the colour of the day is part of the price, so the cheapest hours follow it: a blue day is worth filling on, a red one is mostly worth waiting out. Sunplug can send you the colour the evening before, so a red day is something you know about rather than something you discover on the bill.

Not set up yet? Your tariff comes first. Stuck? Email me.