How Big Should a Battery Storage System Be? How to Calculate the Perfect Storage Size for Your Photovoltaic System

Did you know that electricity costs in Germany have increased by over 30% in recent years? At the same time, valuable solar power from many photovoltaic systems remains unused simply because the storage system is missing or incorrectly sized.

With the right storage size, your customers can maximize self-consumption and sustainably reduce energy costs. As an installer, you have the crucial influence—and we at Ultimati Energy are here to help you find the perfect storage solution.

How big should a battery storage system be? Let’s calculate it together—with practical examples and clear formulas.

Areas of Application for Battery Storage in Photovoltaics: What Size Does Your Customer Need?

A battery storage system makes photovoltaics truly worthwhile—but its size depends on how your customers intend to use it. Let’s take a look:

Increasing Self-Consumption – Using More Solar Power

A household with an annual consumption of 5,000 kWh and a 4 kWp PV system (yielding 4,000 kWh) wants to achieve 70% self-consumption.

• During the day, 3 kWh often remain unused.
• A storage system with 90% efficiency would need to store about 3.3 kWh (3 kWh ÷ 0.9).
• Result: More independence from the grid and lower electricity costs.

Ensuring Backup Power – Bridging Power Outages

A customer wants to secure critical appliances like lighting and a refrigerator during a power outage:

• Required power:2 kW for 4 hours = 8 kWh storage capacity needed
• Solution: A battery storage system with at least 8 kWh capacity to reliably supply essential loads.

Load Shifting – Using Electricity at a Lower Cost

A small business consumes 2 kWh in the evening when electricity prices are high. With a 2 kWh storage system, they can use cheap solar power generated during the day, reducing energy costs.

Each customer has different needs—and as an installer, you can impress them with precise calculations! At Ultimati Energy, we support you in ensuring your customers get exactly what they need.

Factors Affecting Battery Storage Size

Solar Panel Output

The energy your photovoltaic system produces is the first piece of the puzzle. Larger solar arrays generate more electricity, which can charge bigger batteries and reduce grid reliance.

Daily Energy Consumption

As discussed, your household’s daily kWh usage sets a baseline for storage needs. If you consume 30 kWh per day, a battery with a capacity of 20–25 kWh may cover most of your nighttime usage, depending on how much energy you want to store for emergencies.

Depth of Discharge (DoD)

Batteries have a recommended DoD, which is the portion of energy that can be safely used. For example, if a battery has a 10 kWh capacity with an 80% DoD, only 8 kWh is usable. This factor must be included in your calculations.

Battery Efficiency

Some energy is lost during charging and discharging. Typical lithium-ion batteries have 90–95% efficiency. Accounting for this ensures your storage system meets real-world energy needs.

Autonomy Days

Consider how many days you want your battery to last without solar input. This is especially important in regions with long periods of cloudy weather or frequent outages.

Storage Technologies and Sizing: What’s the Best Choice?

The choice of storage technology affects not only the size but also the efficiency and lifespan of the system. Which technology fits the right size? And how do you calculate the correct battery storage size? Here are the options—clearly explained:

Lithium-Ion vs. Lead-Acid – Which Is Better?

Recommendation:

• Lithium-Ion is the best choice for households and businesses prioritizing efficiency and longevity.
• Lead-Acid can be a cheaper alternative for backup power solutions but requires more space and maintenance.

Formula for Calculating Battery Storage Size:

Storage Size (kWh) = Daily Consumption × Days of Autonomy ÷ Depth of Discharge ÷ Efficiency

Example:

A household consumes 10 kWh per day and wants to be self-sufficient for 1 day. With 80% DoD (Depth of Discharge) and 90% efficiency, the required storage size is:

10 kWh ÷ 0.8 ÷ 0.9 = 13.9 kWh

With Ultimati Energy, you’ll find the right technology—we help you calculate the storage size and convince your customers.

Legal and Financial Benefits in Germany

Beyond technology, economic and legal aspects also interest your customers:

• Grid Fee Exemption: Small storage systems help save fees.
• Subsidy Programs: KfW and other government programs offer attractive grants.
• Rising Demand: Since 2022, Germany has increased support for PV storage—demand is growing.

Ultimati Energy: Your Partner for the Right Storage Solution

How do you find the perfect storage size for your customers? We help you—personally and on-site.

• Local Consultation in Germany: We visit and analyze what your customers need.
• Fast Inventory and Logistics: Our battery storage systems are immediately available.
• Technical Support: From installation to maintenance, we ensure everything runs smoothly.

We help you choose a battery storage system that truly benefits your customers—reliably and without detours. Contact us, and let’s take your projects to the next level together!

Conclusion: The Right Storage Size Maximizes Savings

How big should a battery storage system be? The answer depends on your customers' individual needs. With precise calculations and the right technology, you can provide real added value.

Frequently Asked Questions (FAQ) About Battery Storage Size

How long does a battery storage system last?

• Lithium-Ion:10-15 years, depending on usage.
• Lead-Acid:3-5 years, requiring regular maintenance.

How do I find the right battery storage size for my customers?

Use our practical formulas like Energy (kWh) = Daily Consumption × Days of Autonomy, or request a custom consultation from Ultimati Energy.

Can a battery storage system be expanded later?

Yes, many modern lithium-ion storage systems are modular and expandable. It’s important that the Battery Management System (BMS) is compatible. Alternatively, a second battery with its own inverter can be added. Anyone planning to expand capacity later should choose expandable systems from the start!