How to Determine the Right Size Solar Battery for Your Needs

Home energy storage · Jan 25, 2024

As the popularity of solar energy continues to grow, homeowners are increasingly considering adding solar batteries to their homes. A home energy management system that links solar production and battery storage is a great way to store excess energy generated by your solar panels and use it when the sun is not shining.

However, choosing the right size and capacity for solar batteries can be challenging. In this article, we will discuss some key factors to consider when selecting a solar battery system for your home.

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The Basics of Solar Battery Sizing

Solar battery sizing is a crucial aspect of designing a reliable and efficient home energy management system. It involves determining the appropriate size and capacity of batteries to store energy generated by solar panels, based on household needs. The goal is to ensure a continuous and reliable power supply, especially during periods of low sunlight or at night.

The fundamental concepts of solar battery sizing revolve around optimizing the storage capacity to meet the energy demands of your home. This process considers factors such as energy consumption patterns, solar panel output, and the desired level of energy independence.

Solar battery sizing is significant because it directly impacts the overall performance and cost-effectiveness of the system. An undersized battery may lead to insufficient energy storage for outages or time of use (TOU) load shifting, resulting in a lack of power, or the need to use high-priced utility power during peak periods. On the other hand, an oversized battery may incur unnecessary costs and take up additional space without providing substantial benefits.

Key terminologies associated with solar battery sizing include:

Kilowatt-hour (kWh): A unit of energy measurement, representing the amount of energy consumed or produced over one hour. It is used to quantify the energy storage capacity of solar batteries.

Capacity: Refers to the total amount of energy that a solar battery can store. It is typically expressed in kWh and is a crucial factor in determining how long the system can provide power without additional solar input.

Depth of Discharge (DoD): This metric indicates the percentage of a battery's total capacity that can be safely utilized before recharging is required. For example, a battery with a 50% depth of discharge can only use half of its total capacity before needing to be recharged.

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Factors to Consider When Sizing a Solar Battery

Several factors need to be carefully considered to ensure an efficient and tailored energy storage solution.

Calculating Your Electricity Load

Accurately calculate your electrical load by analyzing historical energy bills and understanding peak usage periods. This information helps determine the size of the battery storage required to meet your daily energy demands and avoid shortages during high-consumption periods.

Determining the Size and Production of Your Solar Panel System

Coordinate the sizing of your solar battery with the capacity and production of your solar panel system. The solar panels generate electricity that powers the home and charges the battery, so the sizing should be proportional to ensure efficient utilization of the solar energy harvested.

Flat vs. Variable Electricity Rates

Consider the pricing structure of your electrical grid rates. If you have variable rates with peak and off-peak periods, sizing the solar battery to accommodate energy storage during off-peak times can lead to cost savings. Understanding your rate structure helps optimize the solar battery for maximum economic benefit.

Depth of Discharge

Evaluate the depth of discharge (DoD) that is acceptable for your chosen battery technology. Deeper discharges offer more usable capacity but may impact the battery's lifespan. It's crucial to strike a balance between maximizing capacity and preserving the battery's longevity.


Factor in the overall costs associated with the solar battery system, including the initial investment, installation, and maintenance expenses. Consider the total cost of ownership over the lifespan of the system to assess its long-term economic viability. Balancing costs with performance ensures a cost-effective and sustainable solution.

solar panels

Choosing the Right Size Solar Battery

Choosing the correct size solar battery involves considerations beyond meeting basic energy needs. It encompasses factors such as cost savings through load shifting, backup options for essential systems, and the potential for whole-home backup solutions.

Cost Savings from Load Shifting

One of the key advantages of sizing a solar battery appropriately is the potential for cost savings through load shifting. By storing excess solar energy during periods of high solar production and using it during peak demand times or when electricity rates are higher, you can optimize your energy consumption patterns. This strategic use of stored energy can lead to significant reductions over time in electricity costs.

Backup Options for Essential Systems

Sizing a solar battery to provide backup power for essential systems ensures continued functionality during grid outages. Critical appliances such as refrigerators, air conditioners, and communications devices can be powered by stored energy, offering a reliable source of electricity during emergencies. Choosing the right size ensures that the battery can support these essential systems for the required duration.

Whole-home Backup Solutions

For those seeking comprehensive energy independence, sizing a solar battery for whole-home backup becomes essential. This involves determining the total energy consumption of the entire household and selecting sufficient capacity to sustain the entire home during power outages. Whole-home backup solutions provide peace of mind and an uninterrupted power supply, making them suitable for residences and small businesses alike.

Take the FranklinWH home energy management system, for example. The FranklinWH system was designed to meet the whole home's energy needs with a robust 13.6 kWh storage capacity per battery, which multiple batteries can expand to 204 kWh per intelligent controller. It can seamlessly power your home without you even noticing a power outage.

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Calculating the Number of Batteries Needed

Calculating the number of batteries needed for an energy storage system is a crucial step in ensuring optimal performance and reliability.

First, you understand your energy consumption data from utility bills, considering both daily and seasonal variations. Online calculators and software tools designed for solar battery sizing can assist in making accurate estimates.

Then, you can calculate the needed battery storage capacity through this basic formula:

Battery storage capacity=(Total Daily Energy Consumption)/(DoD × Days of Autonomy)

Where: Total Daily Energy Consumption is in kWh, DoD is the chosen depth of discharge (expressed as a decimal), and Days of Autonomy is the desired number of days the batteries should sustain the load without solar input.

This capacity will determine how many batteries are needed depending on the size of each vendor’s battery storage.

Optimizing Home Energy Management Systems

Optimizing a home energy management system to integrate solar and batteries is essential for maximizing efficiency, cost-effectiveness, and overall performance. This involves implementing strategies to enhance energy production, considering different system architectures, and selecting appropriate battery types.

Maximizing System Efficiency

Achieving optimal efficiency involves several tactics. Proper placement and orientation of solar panels, regular maintenance, and using energy-efficient appliances contribute to increased energy production and reduced waste. Intelligent home energy management systems and load-shifting practices also play a crucial role in optimizing overall system efficiency.

AC- vs. DC-Coupled Solar Battery Systems

AC-coupled systems involve separate inverters for solar panels and batteries, offering flexibility and scalability. On the other hand, DC-coupled systems share a single inverter, potentially increasing efficiency but limiting flexibility. Choosing between the two depends on specific system requirements, space constraints, and budget considerations.

Battery Types and Longevity

Explore different battery types and factors affecting longevity and performance. Despite being cost-effective, lead-acid batteries may have shorter lifespans and lower energy density than lithium-ion batteries. Factors such as temperature, depth of discharge, and charging cycles impact battery longevity. Understanding these considerations helps select the right battery chemistry for the intended application.

Advanced Battery Management Systems (BMS)

The BMS monitors and controls various parameters, such as temperature and state of charge, of operating batteries to ensure safe and efficient operations. A robust BMS enhances the overall reliability and longevity of the battery system. Premium home energy management solutions typically provide an advanced BMS by default for optimized battery performance.

Integration with Smart Technologies

Discuss the integration of solar and battery systems with smart technologies. Smart inverters, energy monitoring systems, and home automation tools enable real-time monitoring and control, allowing users to make informed decisions for optimal energy utilization. Integrating the grid and emerging technologies, such as virtual power plants, further enhances system efficiency.

The FranklinWH home energy management system integrates solar, grid, battery storage, and generators, managing multiple home energy sources intelligently for optimized energy efficiency and increased solar ROI.

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Special Considerations for Off-Grid Solar Energy Systems

Off-grid solar energy systems present unique challenges and considerations compared to grid-tied systems. The off-grid systems operate independently, without a connection to the utility grid, making it crucial to address specific needs and challenges. To take good care of your off-grid systems, consider below things:

Unique Needs of Off-Grid Systems

Off-grid solar energy systems are used in locations where a connection to the utility grid is unavailable. These systems should generate, store, and entirely manage their power. Designing a system that can consistently meet energy demands, especially during periods of low solar availability, by ensuring an adequate energy storage capacity for energy independence is a must.

Additional Challenges and Requirements

Off-grid systems face additional challenges such as limited energy storage capacity, reliance on renewable energy sources, and the need for backup solutions during extended periods of low sunlight. Balancing the system's capacity with energy demands, implementing efficient energy management strategies, and incorporating backup power sources are critical for off-grid systems.

The FranklinWH energy management system's outstanding off-grid capability is one of its features. When the grid is not available, the FranklinWH system integrates solar, battery storage, and generators. In this way, solar will be the main energy source. The excess will be used to charge the battery and use the battery to power the house when solar is not available. If there is no solar and no power in the battery, the system will start the generator to power the house and charge the battery.

Choosing Robust and Reliable Components

Due to the self-reliant nature of off-grid systems, selecting robust and reliable components is important. This includes durable solar panels, high-capacity and long-lasting batteries, efficient charge controllers, and reliable inverters. Opting for components with proven performance in off-grid conditions ensures the system's resilience and longevity.

Proper Sizing and Energy Management

Accurate sizing of the solar array, battery bank, and other components is crucial for off-grid systems. Understanding energy consumption patterns, factoring in seasonal variations, and ensuring sufficient storage capacity are essential steps. Implementing effective energy management strategies, such as load prioritization and load shedding, contributes to optimized system performance.

The FranklinWH home energy management system supports three modes to manage energy supply and battery storage. They are backup mode, self-consumption mode, and time-of-use mode. Homeowners can set different periods to charge and discharge the battery according to mode and need, increasing energy efficiency and making the most use of solar energy.

Maintenance and Monitoring

Off-grid systems require regular maintenance and monitoring due to their reliance on standalone components. Regular checks on battery health, cleaning solar panels, and monitoring system performance help prevent unexpected failures. Additionally, having a reliable monitoring system in place allows for real-time assessment and timely response to any issues that may arise.

FranklinWH smartphone app provides the real-time energy status of the home so that homeowners can monitor and control the system anytime, anywhere, experiencing a hassle-free experience.

off grid battery system


1. How do I calculate the size of the solar battery I need?

To calculate the size of the solar battery you need, use the formula:

Battery storage capacity=(Total Daily Energy Consumption)/(DoD × Days of Autonomy)


  • Total Daily Energy Consumption is in kilowatt-hours (kWh),
  • DoD is the chosen depth of discharge (expressed as a decimal),
  • Days of Autonomy is the desired number of days the batteries should sustain the load without solar input.

2. What factors should I consider when selecting a solar battery size?

Electrical Load: Calculate your daily electricity load to determine the needed battery storage capacity.

Solar Panel System Size: Coordinate the battery size with the capacity and production of your solar panels.

Rate Structure: Consider electricity grid rate structures for cost-effective load shifting.

Depth of Discharge: Evaluate acceptable depth of discharge to balance capacity and battery lifespan.

Costs: Factor in initial investment, installation, and maintenance costs for a cost-effective solution.

3. Can one solar battery power my entire home?

The ability of one solar battery to power an entire home depends on factors such as the home's energy consumption, solar panel system size, and battery capacity. Multiple batteries may be needed for sustained power during periods without sunlight or in the event of a power outage, especially with smaller-capacity batteries.

4. How does my daily energy usage affect the size of the solar battery?

Your daily energy usage directly influences the size of the solar battery needed. Higher energy consumption requires a larger battery capacity to store sufficient energy for periods without sunlight.

5. What is the difference between AC- and DC-coupled solar battery systems?

The main difference between AC- and DC-coupled solar battery systems is their architecture. AC-coupled systems have separate inverters for solar panels and batteries, offering flexibility. DC-coupled systems share a single inverter, potentially increasing efficiency but limiting flexibility. The choice depends on specific system requirements, space constraints, and budget considerations.

6. What special considerations are there for sizing batteries in off-grid solar systems?

Special considerations for sizing batteries in off-grid solar systems include addressing unique energy needs, accommodating additional challenges such as extended periods without sunlight, and choosing robust and reliable components tailored to self-reliant operation. Accurate sizing, proper energy management, and regular maintenance are crucial for optimal performance in off-grid systems.

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Optimizing solar and battery systems involves tailoring the system to specific needs and considering factors such as energy consumption, system architecture, and component reliability.

Special considerations for off-grid systems highlight the need for self-sufficiency and robust components. Accurate calculations and thoughtful planning ensure efficient energy utilization, cost savings, and a reliable power source in diverse solar energy applications.

The FranklinWH home energy management system is designed to meet various household needs, empowering homeowners with energy security in a changing environment.

Still have questions? Let's connect with an energy expert!

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