Rooftop solar on a factory or warehouse produces most around midday. Operations, though, need power in the morning, in the evening and often overnight. Whatever is not used straight away flows into the grid, and that is where much of the plant's value is lost.
Why midday surplus is worth so little
On sunny days, every solar plant in the region peaks at the same time. On the day-ahead market, which has traded in 15-minute periods since 1 October 2025, midday prices then fall close to zero and sometimes below it. If the price you get for exported power follows the spot market, your midday surplus earns little or nothing.
The evening is the opposite. Solar output fades, demand rises and prices are often several times higher. And when you buy from the grid, you pay distribution and other regulated charges on top of the energy itself, none of which you receive for exported power. The gap between what your surplus earns and what evening power costs is therefore wider than wholesale prices alone suggest.
How storage raises self-consumption
Storage captures the midday surplus before it leaves the site and releases it when solar output falls short: in the afternoon, in the evening or overnight. The share of your own power in site consumption rises and grid purchases fall. With 95% efficiency, most of the stored energy comes back into use.
On our energy storage page we show an illustrative example day on which solar plus storage cut grid purchases by 24%. It is a model day, not a result from a real site. The actual benefit depends on the size of the solar plant, your demand profile, the season and the size of the storage.
Sizing storage for solar
The right size comes from two curves: how much the solar plant generates in each quarter-hour, and how much the site uses at the same time. The difference shows how large the surplus is, when it arises and how much energy is needed in the evening. If your solar plant is still at the planning stage, the generation estimate from its design is the starting point, and both can be designed together.
- Capacity in kWh sets how much surplus can be stored. Storage much larger than the typical surplus will sit half empty on most days.
- Power in kW sets how fast it can charge during peak generation and how much it can deliver in the evening.
- The season changes the balance. Summer surpluses are large, winter ones small or absent, so in winter the storage can take on other work, such as shaving demand peaks or charging in cheap quarter-hours.
Our modular storage starts with a base module of 243 kWh / 100 kW and scales to 6 MWh in one system, so power and capacity can be built up module by module to match what the data show. For charging around market prices, see our article on fifteen-minute power prices.
Backup and island operation
Storage can also act as backup. If the grid fails, it takes over within 6 ms, so sensitive machinery, servers or refrigeration keep running. How long it can do so depends on the storage capacity and on what needs to stay on.
Standard solar systems disconnect during a grid outage for safety reasons. If the storage can form its own island grid, the solar plant can keep generating and recharge the batteries during the day, so the site can run on its own for longer. Island operation has to be designed in from the start, though: deciding which circuits stay live and ensuring safe separation from the distribution grid.
Financing and grants
You do not have to buy storage outright. We offer three models:
| Model | How it works | Suits |
|---|---|---|
| Rent | A flat fee based on rental length and number of cycles. If you later decide to buy, the rent you have paid counts towards the price. | trials and pilot projects |
| Lease | You pay from the savings; we keep ownership of the storage. | companies and municipalities without spare capital |
| Purchase | The storage is yours, along with all savings and revenue. | long-term operation |
Grant calls for energy storage open regularly in the Czech Republic, for example under the Modernisation Fund. Conditions, support levels and eligible costs vary from call to call, so it pays to check them before you place an order. We can help you choose a programme and prepare the application.
See how storage carries midday solar into the evening on the sample day on our energy storage page.