Dynamic energy prices
The Dutch electricity market operates with hourly prices that are set one day in advance on the EPEX Spot exchange (the so-called day-ahead market). Every day around 13:00, the exchange publishes the prices for the 24 hours of the next day. These prices vary significantly: at night and on weekends they are often low, while during peak moments (morning and evening rush hours) they can be considerably higher.
Stekker automatically retrieves these hourly prices as soon as they become available. The platform supports multiple European market areas via ENTSO-E, the European transparency platform for energy data. For each location, Stekker maps the correct market region, ensuring the planner always works with current, local prices.
Direct charging vs smart charging
When a vehicle is plugged in, Stekker splits the charging session into two phases:
Direct charging — By default, Stekker first charges 20 percent of the battery capacity immediately (configurable per location). This guarantees the vehicle always has a minimum driving range, regardless of energy prices. This phase deliberately ignores the market price: availability comes first.
Smart charging — The remaining volume up to the battery target (default 90 percent) is scheduled by Stekker at the cheapest moments before the configured departure time. The planner optimizes a charging schedule by minimizing energy costs across all available time intervals, taking into account the connection capacity and the vehicle’s charging power.
The departure time and battery target are configured through the location’s charging preference (default 08:00, 90 percent). Users can adjust these values per location or per session. The target is configurable as a percentage of the battery capacity (the smart fraction), allowing you to determine per location how much should be charged.
Calculating savings
After each charging session, Stekker calculates the savings by comparing two scenarios:
- Smart charging (actual) — The actual energy costs based on the hourly prices at the moments when charging actually took place.
- Direct charging (reference) — What it would have cost if the same volume had been charged immediately after plugging in, at the average kWh rate for that country and period.
The difference between these two amounts is the savings. Stekker stores this per session and aggregates it in the dashboard, where savings are visible per day, week, month and year.
When imbalance prices are available (see below), the costs and revenues from imbalance responses are also included in the calculation.
Tariff types
Stekker supports three tariff types, configurable per location:
Dynamic — Hourly prices from the day-ahead market (EPEX Spot). The planner uses the actual market prices per hour to determine the optimal charging moment. This is the most common tariff type and typically yields the greatest savings.
Fixed tariff — A single fixed price per kWh, regardless of the time of day. With a fixed tariff, Stekker optimizes on other factors such as solar yield and grid congestion, but not on price (which is constant).
Time-of-use tariff — Multiple tariff zones, each with their own price and time window. Stekker schedules charging as much as possible in the cheapest tariff zone. This type is often used with commercial contracts that have peak and off-peak rates.
Imbalance market
Beyond the day-ahead market, Stekker can also respond to signals from the imbalance market. This is an optional feature that is enabled per location.
TenneT, the Dutch transmission system operator, continuously publishes data about the balance of the electricity grid. When there is a surplus of electricity, the imbalance price drops — sometimes to negative values. During a shortage, the price rises sharply.
Stekker uses this PTU data (Programme Time Unit, 15-minute periods) to adjust charging behavior:
- During grid surplus: increase charging (utilize cheap or negatively priced energy)
- During grid shortage: reduce or pause charging (avoid expensive energy and relieve grid stress)
This response to imbalance signals yields additional savings on top of the day-ahead optimization, and contributes to the stability of the electricity grid. Imbalance costs and revenues are tracked separately in the savings calculation.
The imbalance market feature is not active by default. Stekker enables it for locations that can benefit, depending on the connection and contract type.
Solar energy
For locations with solar panels, the planner takes expected solar yield into account. Charging with your own solar energy avoids grid costs (estimated at EUR 0.09/kWh), making it more cost-effective than charging from the grid — even when the market price is low at that moment. The planner factors this into the optimization: when sufficient solar energy is available, it takes priority over cheap grid energy.
