From November to February, days in Estonia are short and there is little sunshine. Solar panels that kept the battery charged for the morning and evening in summer now produce only a fraction of that. At the same time, electricity is more expensive and price peaks are more frequent, especially in the morning and evening, when all of Estonia is heating, cooking and switching on the lights at once.
This changes what we expect from a battery. In summer, the challenge is where to put the surplus solar energy. In winter, it is the opposite: where to get cheap energy so that the battery is full when electricity is expensive?
What is felx down?
The power system must be in balance at every moment: the amount of electricity produced must equal the amount consumed. When more electricity unexpectedly enters the grid than the market needs, the transmission system operator (TSO) Elering asks someone to reduce production or increase consumption. This is called balancing down, or flex down.
For a home battery, this means something simple: the battery starts charging from the grid, helps balance the power system and benefits in two ways:
- a fee for availability (balancing capacity) or for the service itself (balancing energy) – this is the "earned money" people usually talk about;
- the energy itself – the kilowatt-hours charged into the battery, which at the moment of balancing down are usually much cheaper than the exchange price, sometimes even free or negatively priced.
In summer, often only the first point matters. In winter, the second comes to the fore.
Free refuelling or a big discount
Imagine pulling into a petrol station and being told: "Today we'll fill your tank for free. Or at a very large discount." On top of that, you get a small bonus for showing up at exactly this moment.
Which is worth more, the bonus or the full tank? If fuel is cheap and the tank is already half full, the difference is small. But if fuel is expensive and the tank is empty, a full tank is worth many times more than the bonus.
Balancing down works the same way in winter:
- The tank is your home battery.
- The fuel is the electricity you would otherwise buy and use during expensive morning or evening hours.
- Free or discounted refuelling is the energy charged into the battery during balancing down.
- The bonus is the balancing fee earned.
In summer, the sun fills the tank on its own. In winter, it does not, and that is when every cheaply charged kilowatt-hour is especially valuable.
A real example: the night of 4–5 November
During the night of 4–5 November, one of the homes we manage in Tallinn charged 38 kWh of energy into its battery through balancing down, at a net price of about €0.07/kWh. During the evening peak of the same day, electricity in this home costs €0.39/kWh on average, more than five times as much.
The house has an 80 kWh battery and a 10 kW solar plant. By 11 a.m., the sun had produced only 3.2 kWh, so it could not fill the battery. Balancing down took place mainly between midnight and 6 a.m., and briefly around 10 a.m.
The €2.05 earned is the bonus. The greater value is that the battery was filled overnight with energy that cost, net, about one fifth of the evening price. That is discounted refuelling: not quite free, but at a very large discount.
A few clarifications for a fair picture:
- Prices are based on the customer's own tariff, including network fees and taxes; monthly fixed fees are not included.
- Not all of the energy goes to household consumption. Part of it covers the home's evening consumption instead of expensive purchases, and part is sold back to the grid at €0.15–0.28/kWh. In both cases, the difference from the night-time net price is large.
- Battery losses reduce the savings by a few percent.
Evening prices are known from the day-ahead exchange; actual evening use becomes clear by the end of the day.
Why MarkeDroid's strategy changes with the seasons
We do not trade on the flexibility market the same way all year round, because the value a battery creates depends on where the next kilowatt-hour comes from. In summer and in winter, the answer is different.
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That is why at MarkeDroid we adjust our strategy continuously. In winter, it makes sense to leave room in the battery so that the cheap energy from balancing down fits in, and to make sure the battery is full before the evening price peak. In summer, the focus is on making the best use of solar energy.
Summary
In winter, balancing down is best seen as discounted refuelling, not just extra income. The fee earned is a bonus; the main value is a full battery, charged cheaply at a time when the sun does not provide energy and electricity is expensive.
For a MarkeDroid customer, this means:
a lower electricity bill in precisely the most expensive months;
a battery that is full even when there is no sun;
a contribution to the balance of Estonia's power system, which the system itself pays for.
Next time you see the amount earned from balancing down in your report, also think about how many kilowatt-hours went into the battery and what they would have cost in the evening.