How to size PV systems and batteries
Source：pv magazine
Batteries can make solar arrays even more independent from the grid and contribute to more comprehensive consumption management while bringing savings and improved energy security. To capture such opportunities, however, the elements of solar-plus-storage systems must be appropriately sized according to consumer profiles.
To discuss this topic, pv magazine Brasil invited Natália Maestá, CEO of the integration company Fonte Solar; Andrey Oliveira, director of products and solutions at Huawei Digital Power Brasil; and Natanea Guimarães, senior market intelligence analyst at Greener to form a How to size photovoltaic systems with batteries panel at the Intersolar South America 2025 fair.
Here, pv magazine Brasil summarizes their thoughts.
Differing load profiles
Technical training is essential to allay consumer doubt and ensure the solution offered will deliver the promised results, the panel agreed. “It’s common to find customers who say that storage doesn’t work or that it’s expensive,” said Maestá. This perception, she said, “usually stems from flaws in preparation and training, as well as incorrect load assessments and a lack of clear priority definition.”
Huawei’s Oliveira said around 40% of solar plants do not deliver as expected, as a consequence of a lack of training at the start of projects.
Sizing projects requires a different approach for the solar array and battery energy storage system (BESS), said the Huawei executive. “With PV, we look at the solar source’s generation and, from there, determine how to meet the load, without necessarily considering its behavior. With BESS, the process is reversed. First, we need to understand the load, then do the complete sizing and define how the system will behave to meet it.”
Oliveira noted there are two factors to consider when sizing the load: steady state, which is commonly analyzed; and transient loads, which are especially important when sizing a BESS that will operate alone, for example as a backup. In the latter case, it is crucial to evaluate transient load elements – starters, frequency inverters, soft starters – to ensure a system can meet needs.
“These transients were never considered in traditional photovoltaic projects but they are essential in storage,” said Oliveira. “Without electrical engineers and qualified professionals, the risk isn’t just poor performance: in many cases, the equipment won’t even turn on.”
Customer ‘needs,’ not ‘wants’
Many customers want to be completely disconnected from the grid, said Fonte Solar’s Maestá, adding, “However, what the customer wants doesn’t always correspond to what they actually need. In these cases, the technical team performs detailed sizing, with generation and consumption spreadsheets, and presents different proposals to meet the demand.”
Rural producers, for instance, “often face power outages lasting only a few hours but because they’ve suffered production losses in the past, they end up requesting storage systems capable of ensuring complete independence from the grid,” the CEO said. “Technical analysis, however, shows that there’s no need for complete disconnection. Proper sizing takes into account the average number of hours without power, providing energy security and reducing losses without generating disproportionate costs.”
As a result, Fonte often offers two or three alternative options in addition to the system requested, simulating different autonomy and investment scenarios. “This approach allows the client to better understand their options and realize that the economically viable solution is often not what they initially imagined,” said Maestá. Detailed diagnosis and personalized proposals help align expectations with needs.
Companies often focus on cost reduction via energy arbitrage without considering revenue loss during interruptions, Huawei’s Oliveira said, adding, “Some industries cannot tolerate power fluctuations [and require] solutions like UPS [uninterruptible power supply] systems, capable of operating in seconds and ensuring continuous power supply until the battery is up and running. This demonstrates that energy security depends directly on the application and can go beyond backup, including arbitration; consumption reduction; and support for transient events.”
There are technical challenges to overcome, noted Oliveira, such as the capacity of storage systems to generate reactive energy of different magnitudes in each phase, something current equipment does not yet address satisfactorily.
Payback periods
The country offers significant potential, with viable business models for customers with varied profiles. Greener’s Guimarães highlighted the residential, commercial and industrial (C&I), and agribusiness and microgrids market segments.
“In the residential market, the central motivation is still not financial, but rather energy reliability,” said the analyst. “Given the fragility of the grid, which records an average of 14 hours of outages annually – and can reach 30 or 36 hours, depending on the region – consumers are already seeking solutions to ensure continuity of supply. The recent blackout in São Paulo illustrates this demand well.”
Storage offers C&I clients load shifting and peak shaving, to reduce costs, as well as backup power.v With Group A tariffs differentiated between peak and off-peak periods, the economic incentive is clear.
Energy availability is the challenge for agribusiness. Many regions no longer have sufficient transmission infrastructure to meet demand. In that context, storage systems combined with diesel generators and photovoltaic plants form hybrid microgrids capable of providing stable and predictable power for critical operations.
If storage equipment costs continue to fall at the current rate, Guimarães said, agribusiness systems will become even more attractive.
“According to our study, the cost of hybrid systems is expected to drop by around 20% by 2030, mainly due to the reduction in battery prices,” said the analyst. “This trend has a direct impact on project payback. In simulations of microgrids with reduced diesel use, for example, the payback time is only 2.4 years, considered highly competitive. In the residential segment, the payback period is still longer but follows a curve similar to that of solar energy in its early stages, when the payback was between five and six years, gradually decreasing as the market matured.”
Developing technology
Fonte Solar, active in storage for 10 years, has developed a sizing spreadsheet based on theory and validated in practice. The company uses the tool along with generation software, which remains indispensable. “We often see backup systems without sufficient generation, without a suitable inverter, or with motors that cause current spikes,” said CEO Maestá.
Huawei’s Oliveira said there are few energy simulation software tools based on real BESS use. He added, “For example, when sizing an irrigation system in agribusiness, it’s possible to calculate the optimum LCOE [levelized cost of energy] by comparing a diesel generator, solar generator, and BESS. However, when the BESS operates alone, without diesel, it enters VSG (virtual synchronous generator) mode. In this case, most equipment reduces capacity from 100% to around 70%, acting as a buffer to meet variable loads. So when we talk, again, about transients or other specific conditions, there’s still a lack of software that incorporates these factors into the sizing process.”
Battery sizing tips
- Make a detailed load diagnosis: Identify hourly profile, seasonality, and transients such as motor starting, frequency inverters, etc.
- Define customer priorities: Is it savings, energy security, or both?
- Simulate scenarios: Present autonomy alternatives and pricing strategies including peak shaving and time shifting.
- Use support tools: Proprietary spreadsheets and simulation software help but validate them with real equipment data.
- Consider the electrical infrastructure: Retrofits have limitations; new systems can be designed for future expansion.
- Include all costs in the budget: Factor in integration, protection, UPS, network adjustments, and maintenance expense.
- Plan for future expansion: Anticipate physical space, connections, and inverter compatibility.
- Align expectations: Does the client need autonomy during critical periods rather than total independence?