Every business electricity bill in Slovakia has a part that follows the energy you use, in kWh, and a part that follows the capacity you reserve on the grid, in kW. You pay for that capacity every month, whether you use it or not, and going over it costs several times more. A battery that covers your short peaks lets you reserve less and stop paying for overruns. This is called peak shaving.
Guide · Energy costs
Peak shaving: how a battery cuts the cost of reserved capacity
A few short peaks can set what you pay for your grid connection all year. Here is how reserved capacity is charged in Slovakia, and when a battery that trims those peaks is worth it.
1. Two numbers in your connection contract
Your contract with the distribution system operator sets a maximum reserved capacity. It is the most your connection may draw, and raising it means a new connection request and a connection fee. Below it sits the reserved capacity, the part you actually pay for each month.
At medium voltage, you book reserved capacity for 12 months, 3 months or a single month. The longer the booking, the lower the price per kW, but its value cannot simply be lowered while the booking runs, and except at seasonal sites it can never be less than half of the maximum. In 2026, a 12-month booking costs €8.02 per kW per month at Východoslovenská distribučná and €4.94 at Západoslovenská distribučná.
2. What going over costs
Your demand is measured as the average power in each 15-minute interval. The highest quarter-hour of the month is compared with your reserved capacity, and every kW above it is charged as a penalty for that month:
- Východoslovenská distribučná: five times the capacity price for each kW over the reserved capacity, and fifteen times for each kW over the maximum.
- Západoslovenská distribučná: €33.19 for each kW over the reserved capacity, and €99.58 for each kW over the maximum.
One quarter-hour is enough. At the 12-month price of Východoslovenská distribučná, a single 15-minute overrun of 30 kW costs around €1,200, as much as reserving those 30 kW for five months. Stredoslovenská distribučná and local distribution systems publish their own prices, so the figures for your site depend on where it is connected.
3. How a battery shaves the peak
The battery’s controller watches the power your site draws from the grid. When it approaches a limit you set, the battery discharges and covers the difference. When the load drops, it recharges from the grid or from your photovoltaic system. Your equipment runs exactly as before. Only the shape of what you take from the grid changes.
Two numbers decide the size. The battery’s power must cover the gap between your peak and the new limit. Its capacity must last as long as the peak does, with a margin for peaks that come close together. For most sites, one to two hours of storage at full power is enough.
A worked example
- Maximum reserved capacity: 600 kW
- Peaks of up to 400 kW, a few times a month
- The rest of the time: below 300 kW
- Battery: 100 kW / 200 kWh
- Reserved capacity lowered from 400 to 300 kW
- Saving at 2026 prices of Východoslovenská distribučná: about €9,600 a year, before any overrun charges avoided
4. Low voltage works differently
At low voltage, the fixed charge follows the rating of your main circuit breaker and is billed per ampere every month. A larger breaker means a higher charge, whether or not you ever use the capacity.
If your site has 15-minute metering read monthly, you can agree a reserved capacity below the breaker rating, down to half of the maximum, for 12 months at a time. Overruns are then charged in the same way as at medium voltage. You can also fit a smaller breaker to lower the maximum itself. If a generator is connected at the same point, check the distributor’s conditions first.
When it pays, and when it does not
Peak shaving pays best when the peaks are short and infrequent, when you regularly pay for overruns, or when you are about to add load, such as EV chargers or a heat pump, that your connection cannot take. In that last case a battery can be cheaper and faster than raising the maximum reserved capacity.
On its own, the capacity saving rarely pays for a battery. It works best as one of several jobs for the same system: storing your solar surplus, backup power and peak shaving. We set a reserve for the peaks in the controller, so the jobs do not get in each other’s way.
What we need to assess it
- 15-minute load data for the last 12 months
- Your connection contract: maximum and reserved capacity, voltage level
- Invoices that show overrun charges
- New loads you are planning, such as chargers or production lines
- Your photovoltaic system, if you have one
Questions we hear
Not always straight away. The value of a running 12-month or 3-month booking cannot simply be lowered, although a 12-month booking can be switched to a shorter one after three months. At low voltage you can lower it only 12 months after the last change. We check your contract and plan the change so the saving starts as early as the rules allow.
The controller keeps a reserve for peak shaving and recharges as soon as the load allows. We size the battery on a full year of your data, including the worst days, so the hardest peaks are planned for rather than left to chance.
Yes. Most of our systems do both. Part of the capacity is held back for peaks, and the rest stores your midday surplus for the evening.
No. It changes when you draw energy from the grid, not how much. Charging and discharging cost a few per cent in losses, which our calculation includes.
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