How big should commercial battery storage be: capacity, power and how to size them
Storage is not designed to be “as big as possible”. Capacity and power follow from the purpose and the load profile – a company storing solar surplus needs a different size from one shaving its peaks.
Two numbers: kWh and kW
Capacity in kilowatt-hours says how much energy the storage holds. Power in kilowatts says how fast it can take that energy in and give it out. The ratio of the two is expressed as discharge time: a system whose capacity is twice its power discharges at full power in two hours. The right ratio is set by the purpose and the data, not by a catalogue.
The design also has to allow for usable capacity being lower than nominal capacity, for part of the energy being lost in charging and discharging, and for batteries gradually losing capacity over the years. We therefore size storage with a margin for the end of its life, not for the first day of operation.
Size by purpose
| Purpose | What sets the power | What sets the capacity |
|---|---|---|
| Storing surplus from solar PV | midday export to the grid that is not worth selling | how much surplus an ordinary sunny day produces and how much of it the site uses in the evening and morning |
| Peak shaving | how much the 15-minute maxima need to come down | how long the peaks last – the energy above the chosen threshold |
| Trading and services through an aggregator | the power that can be offered on the market | the required length of the market offer |
| Backup power | the load of the backed-up equipment | how long it has to run during a grid outage |
Storage usually serves several purposes. It is sized for the one that brings the most, and the others are checked to fit.
What the calculation is based on
The basis is a year of 15-minute consumption data. Companies connected at medium voltage have it in the grid operator's portal; at low voltage, consumption points with interval metering have it, and otherwise it can be measured. We add the solar PV output – actual or calculated – and plans: new machines, EV charging, heat pumps. The storage is then simulated over that year quarter-hour by quarter-hour, showing how much energy actually passes through it and how much it saves.
No design without data
We do not size storage by eye, and we do not quote indicative figures “for a typical factory”. Every site has a different consumption and generation profile, and the same storage can earn money on one site and sit idle on another. Our professional approach does not allow us to guess: without 15-minute consumption and generation data and a clear operating goal, storage cannot be designed responsibly. That is why we always start with the numbers – power, capacity and economics follow from them.
Grid connection, location and reliefs
- Grid connection – storage needs reserved power for export to the grid and reserved capacity for charging, so its size feeds into the connection application. See the article on reserved power.
- Location – container storage needs foundations, clearances and access for firefighters; smaller cabinet systems fit next to the switchroom. The fire safety solution is part of the design.
- Reliefs – since 1 August 2026, electricity that storage draws from the grid and returns to it benefits from reliefs on part of the regulated charges. The conditions include registration with the Czech Electricity Data Centre (EDC) and suitable metering; for storage behind a shared meter with solar PV, separate metering of generation as well. We therefore deal with metering in the design.
What makes up the price and the payback is covered in the article on commercial battery storage, and the whole service on the battery storage page.
FAQ
Isn't bigger storage better, just in case?
No. Capacity that goes unused on most days lengthens the payback. It is better to size the storage for normal operation and leave space and connection capacity for a later extension.
Can storage be designed without solar PV?
Yes. Stand-alone storage for peak shaving or market trading is common; it connects to the grid as both a generating plant and a consumer.
What data do you need from us?
A year of 15-minute consumption data (or at least the bills), solar PV data, the single-line diagram of the switchroom and an idea of the purpose. From this we prepare a study with a proposed size and options.
Planning a similar project?
Send us the site and your intent. We reply within 48 hours.
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