Features → What a dynamic tariff is

What a dynamic tariff is, and what it changes for an EV

A dynamic tariff gives every hour its own price, published the day before. The cheap hours move from one day to the next, so the question is no longer when off-peak starts, but which hours to take.

The tariff you already have

Same price all day: when the car charges makes no difference to the bill. Nothing below applies.

Cheaper at set hours: off-peak hours fixed in the contract, sometimes with a cheaper band inside them, other hours at weekends, or other prices in summer and winter. Easy to plan around, but not always the cheapest hours of the day.

A price for every hour

A market tariff passes on the wholesale price, set the day before for every hour of tomorrow. A still winter evening is expensive; a sunny, windy midday can cost almost nothing.

Sunplug reads these prices for your bidding zone from the ENTSO-E Transparency Platform, which covers the EU, the EEA and Switzerland. Norway, Sweden, Denmark and Italy have several zones, so you pick yours. In Spain, the regulated PVPC price comes from Red Eléctrica's ESIOS. In Great Britain, Sunplug reads Octopus Agile's published rates.

Prices come in quarter-hour steps. Choose quarter-hours or whole hours, whichever your contract bills.

Market prices: ENTSO-E Transparency Platform and ESIOS (Red Eléctrica).

The market price is not your price

Your price is the market price adjusted for losses, plus your supplier's markup, plus VAT, plus the grid fee, which usually differs by day and night. You enter these once, from your contract. There are no supplier presets: an out-of-date preset is worse than an empty field.

The app shows your price now, so you can compare it with your supplier's app and check your numbers.

Why the cheapest hours move

Prices follow wind, sun and demand, so the cheap block can be at 2 am one day and 1 pm the next. A schedule set by hand is wrong most days, and nothing tells you. Sunplug picks the hours again each day, as soon as the prices are out.

Cheap is not a time of day. Charging at night is a habit from off-peak contracts. On a market tariff the cheapest hours are often at midday, when solar floods the grid, and a car on the drive can take them.

How the blocks are picked

Each car has a grid level. While it is plugged in, Sunplug puts the charge it needs into the cheapest hours, at full rate, so it all fits inside them instead of spreading into dearer ones.

If you have solar panels, it also reads the solar forecast and your own history: when enough sun is coming to reach the grid level, it waits for the sun instead of buying.

Why a block is at least an hour

Every start and stop cycles the contactor in the charger or the car, and contactors wear out. A lone cheap quarter-hour is not worth that, any more than a passing cloud is. Blocks last an hour or more.

The price you will not buy above

Set a ceiling and Sunplug will not buy above it: the car waits for a cheaper hour or for the sun. On a cold, still week, that keeps a routine top-up from costing far more than usual.

Unless you have somewhere to be

A trip overrides the ceiling. Set a departure time and a target, and the car is ready on time, even if that means buying above your ceiling. Sunplug tells you when it does. Boost does the same in one tap.

When tomorrow's prices are not out yet

Prices are sometimes late, or missing. Sunplug does not guess them: if a charge depends on them, it tells you in time to press Boost.

Negative prices are not free electricity

On windy days with little demand, wholesale prices can drop below zero. Your kilowatt-hour is still not free: the grid fee and taxes stay on the bill, and they never go negative.

These are still the cheapest hours there are, so the car should charge then. Sunplug can warn you they are coming; you can turn that off.

In France: Tempo

Tempo is an off-peak contract where the price also depends on the colour RTE gives the day: blue is cheap, white ordinary, red expensive. Each colour has its own peak and off-peak rate, six prices in all. EDF Tempo is the original; Octopus's OctoTempo uses the same red days.

Tomorrow's colour is announced the evening before. Sunplug follows it day by day and can tell you the night before a red day.

On another kind of contract, or in another country? Tell us which — that is how the next one gets added.

Setting it up: your tariff, then charging in the cheapest hours. Stuck? Email me.