Solar + Battery Storage for Romanian Businesses: BESS Sizing for Self-Consumption and Export Constraints

Romanian commercial and industrial (C&I) businesses are rapidly deploying solar PV systems to reduce operational electricity costs. However, peak solar generation frequently exceeds onsite power demand during mid-day hours. When local distribution networks impose strict grid export limits, uncaptured surplus solar power must be curtailed, resulting in direct financial loss. Installing a Battery Energy Storage System (BESS) allows facilities to absorb excess generation and shift it to high-demand periods. 

Achieving optimal project economics requires accurate system sizing: the right BESS size is determined by how much solar surplus needs to be stored, how quickly it occurs, and how much electricity the site is allowed to export—not simply by the size of the PV system.

What Romanian Businesses Need Before Sizing a BESS

Commercial BESS sizing starts with three site-specific inputs: PV generation profile, site load, and grid export limits. Relying on annual electricity consumption or total PV capacity alone leads to improperly configured storage systems.

Solar PV Generation

Installed PV capacity in kilowatt-peak (kWp) is only a starting point. Engineering teams require production curves showing when generation occurs throughout the day and year. Peak generation typically coincides with mid-day hours, but seasonal radiation variations in Romania significantly impact daily surplus volumes.

Business Load Profile

A facility consuming 1,000 megawatt-hours (MWh) annually can require vastly different battery storage sizing depending on its operational schedule. Daytime-heavy operations absorb most solar output directly, whereas evening-heavy operations produce large mid-day surpluses. Recent C&I BESS sizing guidance, such as the Econo Solar sizing methodology, emphasizes using 12 months of 15-minute or hourly interval consumption data to analyze exact supply-demand overlap rather than relying on monthly utility bills.

Grid Export Limit

For a Romanian project, the applicable grid connection and export conditions should be checked in the project's connection documentation, including the Aviz Tehnic de Racordare (ATR) where applicable, together with the requirements of the relevant distribution operator and current National Energy Regulatory Authority (ANRE) rules. A Romanian PV-BESS project in Craiova provides a practical example of this approach: the control system is designed to keep combined PV and BESS export within the power authorized in the project's ATR.

How to Calculate the Required BESS Power (kW)

Determining BESS power (kW) establishes how fast the battery charges or discharges. In an export-limited solar setup, battery charging power must be sufficient to absorb the portion of PV generation that cannot be consumed onsite or exported.

The preliminary calculation formula for required battery charging power is:

Required BESS Charging Power

                                                    ≈ max (0, PV Generation-- Onsite Load-- Allowed Grid Export)

This estimates the battery charging power needed to absorb otherwise unexportable PV at a given operating point.

Worked Calculation Example

Consider a commercial facility operating under the following conditions:

• Solar PV Output: 500 kW
• Onsite Business Load: 250 kW
• Grid Export Limit: 100 kW

Required Power ≈ max (0,500 KW - 250KW- 100 KW) = 150 KW

Under this specific operating condition, approximately 150 kW of battery charging power is required to absorb the un-exportable surplus.

This is a preliminary point calculation. Final Power Conversion System (PCS) power ratings should be checked against full 15-minute interval profiles, battery operating envelopes, inverter response times, and system thermal limits.

How to Calculate BESS Energy Capacity (kWh)

While power (kW) dictates the rate of charging, BESS capacity (kWh) measures the total volume of energy stored. To determine required usable storage capacity, multiply required charging power by surplus duration.

The formula for required usable battery capacity is:

Usable Capacity (KWh) ≈ Require BESS Power (KW) × Surplus Duration (hours)

Using the previous example, if the 150 kW un-exportable surplus lasts for 3 consecutive hours during peak sunlight:

150 KW × 3 hours = 450 KWh usable energy

Note: For a real project, the calculation should be performed across the full interval profile rather than assuming a constant surplus for several hours.

Usable vs. Nominal Capacity

The selected nameplate (nominal) battery capacity must be higher than 450 kWh due to three operational factors:

• Depth of Discharge (DoD): The usable battery capacity is lower than the nameplate capacity because the system normally operates within defined state-of-charge (SOC) limits.
• System Efficiency: Round-trip conversion losses occur across the inverter, transformer, and battery cells.
• Capacity Degradation: Battery capacity gradually fades over operating years.

Sizing Warning

Avoid fixed rules of thumb such as "1 kWp solar = 1 kWh battery." Engineering guidance from Solar Energy International (SEI) for zero-export systems warns that sizing storage strictly to maximum expected peak PV surplus creates an oversized, underutilized BESS for much of the year. Effective BESS sizing for solar self-consumption focuses on daily surplus profiles rather than short annual generation spikes.

How Grid Export Limits Change BESS Sizing in Romania

Grid connection conditions directly dictate how solar battery storage is sized for Romanian commercial sites. Projects must confirm connection terms from approved ATR documentation and ANRE prosumer framework rules.

Grid constraints shape BESS sizing across three core scenarios:

Scenario 1: Export Capacity is Sufficient

If surplus solar can be exported without a meaningful constraint, less battery capacity may be needed to manage PV curtailment. The BESS can instead be sized around the site's self-consumption goals and other operating requirements.

Scenario 2: Export is Limited

If a site generates 300 kW of solar surplus but has an ATR grid export limit of 100 kW, 200 kW of surplus power cannot enter the grid. This energy must be consumed onsite, stored in a BESS, or curtailed.

Scenario 3: Zero-Export Requirement

If the permitted export limit is 0 kW, 100% of generation exceeding immediate business load must be consumed onsite, stored, or curtailed.

As demonstrated in Romanian setups such as the Craiova project, automated Energy Management Systems (EMS) dynamically throttle PV generation or adjust battery charging to maintain combined exports within approved ATR parameters.

Romanian C&I Example: 500 kWp Solar + BESS

The following table illustrates a representative sizing model for a commercial project operating with grid injection constraints in Romania.

*This assumes the PV system reaches approximately its rated output during the modeled interval.

Step-by-Step Calculation

1.Calculate Solar Surplus:

500 KW - 250 KW = 250 KW

2.Calculate Required BESS Power:

max (0,250 KW surplus  - 100 KW export limit ) = 150 KW charging power

3.Calculate Usable Energy Capacity:

150 KW × 3 hours = 450 KWh usable energy

Engineering Interpretation

This calculation indicates a need for roughly 450 kWh of usable storage. The nominal battery capacity would then need to be selected above this level based on the system's usable DoD, efficiency, and required end-of-life capacity. A system around 150 kW / 500 kWh may therefore be considered as one preliminary configuration, subject to detailed simulation.

For example, Ultimati Energie's ULTIBLOCK-TL261 is a liquid-cooled integrated C&I energy storage system with a 261 kWh nominal energy capacity, providing one example of the type of integrated BESS hardware that can be considered after the required power and energy capacity have been established.

This calculation serves as an illustrative engineering model rather than a universal project specification. Actual BESS sizing in Romania should be based on full interval PV and load data alongside confirmed grid connection terms.

Why PV Capacity Alone Is Not Enough

Sizing a commercial BESS solely on installed PV array capacity leads to inefficient equipment configuration. Storage requirements depend on how onsite load interacts with solar production and grid constraints.

Consider these four operational variables:

• High Daytime Consumption: Less solar surplus potentially smaller BESS required.
• Low Daytime Consumption: More solar surplus potentially larger BESS required.
• Higher Export Allowance: Less surplus needs to be stored smaller BESS required.
• Lower Export Allowance: More surplus needs storage or curtailment larger BESS required.

A 500 kWp solar system does not automatically need a 500 kWh BESS. The correct BESS size comes from the site's hourly energy balance, not a fixed PV-to-battery ratio.

What Data Does Ultimati Energie Need for Final BESS Sizing?

To move from preliminary estimates to a formal project design, Ultimati Energie requires specific technical parameters to evaluate the required power and storage capacity for your site.

Data Checklist for BESS Sizing

Prepare the following project information:

• Installed solar PV capacity (kWp) and estimated generation profile
• 12 months of electricity consumption data (15-minute or hourly interval load profiles)
• Peak and typical daytime business load (kW) 
• Grid connection capacity and permitted ATR export limit (kW) 
• Facility operating schedule (shifts per day, weekend operations, seasonal shutdowns) 
• Target solar self-consumption goals

With this information, Ultimati Energie can assess the required BESS power and usable energy capacity for your project rather than applying a generic battery-to-PV ratio.

Once the preliminary BESS size is known, the next step is to evaluate whether the system can generate an attractive return under Romania's electricity prices, export conditions, and operating profile. Read our complete guide on how to build a profitable commercial battery storage project in Romania. You can also explore our C&I energy storage case studies to see how BESS systems are configured for different commercial and industrial applications.

Planning Solar + BESS for a business in Romania? Contact Ultimati Energie to discuss your project's PV profile, load data, and grid export conditions.

Frequently Asked Questions

How big should a BESS be for a 500 kWp solar system?

There is no fixed answer. The required BESS size depends on onsite consumption, solar surplus, permitted export limits, and surplus duration.

How do I calculate BESS size for solar self-consumption?

First, calculate solar surplus by subtracting onsite load from PV output. Next, subtract the allowed grid export limit to determine required BESS power (kW). Finally, multiply BESS power by surplus duration to calculate usable energy capacity (kWh).

Can a BESS help with a grid export limit?

Yes. A BESS absorbs excess PV generation that cannot be exported to the grid, subject to system control architecture and battery operating limits.

Does a larger solar system always need a larger battery?

No. Onsite load profile and grid export constraints often impact storage sizing more than installed solar PV capacity alone.