How to Calculate Battery Backup for Solar System?

With the increasing popularity of solar energy, more homeowners and businesses are looking into battery storage solutions. Solar batteries allow you to store excess energy generated during sunny days to use at night or during cloudy periods, offering greater energy independence and reliability. But how do you calculate the right size of the battery to ensure it meets your needs? In this blog, we’ll walk you through the key factors you need to consider when calculating battery backup for your solar system.

What Is Battery Backup for a Solar System?

Battery backup for a solar system is a storage solution that stores the electricity generated by your solar panels for later use. It acts as a buffer during times when the solar panels are not producing power (such as at night or during cloudy weather). Properly sizing your battery backup ensures you can meet your energy needs without worrying about power outages or over-sizing your system, which could result in unnecessary costs.

Why Do You Need Battery Backup for a Solar System?

In a typical grid-tied solar system, you rely on the utility grid to provide power when your solar panels are not producing enough energy. However, if you want to ensure power during a grid outage, you’ll need a battery storage solution. Battery storage also helps maximize your solar investment by storing excess energy that can be used later, avoiding reliance on the grid and reducing electricity bills.

Key Factors to Consider When Calculating Battery Backup

1. Determine Your Daily Energy Consumption

The first step in calculating the right battery backup is to understand how much energy you use on a daily basis. This is usually measured in kilowatt-hours (kWh). Most electric bills will show your total kWh usage each month.

To calculate your daily energy consumption:

• Example: Let’s say your home uses 600 kWh per month. To find your daily consumption: Daily Consumption =600 kWh/month ÷ 30 days=20 kWh/day

This means you need a battery backup system capable of providing at least 20 kWh per day.

2. Decide on the Number of Backup Days

The next step is deciding how many days of backup power you want your battery to provide. If you live in an area prone to long periods of cloudy weather or frequent grid outages, you might want several days of backup power.

• Example: If you want your system to supply power for two days without solar input, you would multiply your daily energy usage by the number of backup days. 20 kWh/day × 2 days = 40 kWh

So, for two days of backup, you would need a battery that can store at least 40 kWh of energy.

3. Consider Battery Efficiency

Batteries are not perfectly efficient. Some energy is lost in the conversion process during charging and discharging. The efficiency of a battery is typically between 80% and 95%. This means that for every 1 kWh of energy you store, you might only be able to use 0.8 to 0.95 kWh.

• Example: If your backup requirement is 40 kWh, but your battery has an efficiency of 85%, you would need to size the battery larger to account for the energy lost in the process. 40 kWh ÷ 0.85=47.06 kWh

So, you would need a battery with a nominal capacity of approximately 47 kWh to deliver 40 kWh of usable energy.

4. Understand Depth of Discharge (DoD)

The Depth of Discharge (DoD) refers to how much of the battery’s capacity can be safely used before it needs recharging. For example, a battery with a DoD of 80% means that 80% of its total capacity can be used, and the remaining 20% is reserved to protect the battery and prolong its life.

• Example: If your battery has a recommended DoD of 80%, and you need 47.06 kWh of usable energy, you would need to consider the total battery capacity to account for the reserve energy: 47.06 kWh ÷ 0.8=58.83 kWh

So, you would need a battery with a total capacity of approximately 59 kWh to ensure you have 47.06 kWh of usable energy.

5. Match Battery Storage with Your Solar System’s Output

Your solar panels should generate enough energy to recharge your battery during the day. When calculating battery capacity, ensure your solar system can produce enough energy to meet both your daily consumption and recharge the battery.

To estimate how much energy your solar panels can produce:

• Example: Let’s assume you have a 5 kW solar system and an average of 5 hours of sunlight per day. 5 kW × 5 hours = 25 kWh/day

This means your solar system produces 25 kWh of energy each day. If you need 40 kWh for backup, your system should produce enough excess energy to charge the battery after meeting your daily consumption.

6. Choose the Right Battery Type

Different battery types have different energy densities, lifespans, and costs. Common battery chemistries for solar systems include:

• Lithium-ion (Li-ion): High efficiency, long lifespan (10-15 years), and higher upfront costs.
• Lead-acid: Lower initial cost, but shorter lifespan (5-7 years) and lower efficiency.
• Flow batteries: Still emerging, offer long lifespans, and are good for large-scale storage but are typically expensive.

7. Battery Life and Warranty

The lifespan of a battery is another important factor to consider. Batteries are typically rated for a certain number of charge/discharge cycles (e.g., 3,000 to 10,000 cycles) and years of use. The longer the battery lasts, the better your investment over time.

• Example: If a lithium-ion battery is rated for 5,000 cycles, and you use it once per day, it will last about 13.7 years. If a battery is rated for 2,000 cycles and used daily, it will last around 5.5 years.

8. System Voltage and C-Rate

Battery voltage should be compatible with your solar system's voltage. Most systems operate on 12V, 24V, or 48V. Higher voltage systems are typically more efficient for larger setups.

Additionally, the C-rate refers to how fast the battery can charge or discharge. Higher C-rates mean quicker charging/discharging without damaging the battery. Ensure your battery's C-rate is suitable for your expected load.

9. Cost of the Battery System

The cost of the battery will depend on its type, capacity, and brand. While lead-acid batteries are cheaper upfront, lithium-ion batteries are more efficient and have longer lifespans, making them a better value in the long run.

How to Calculate Battery Backup for a Solar System

To calculate the battery capacity needed for your solar system, follow this simple formula:

Battery Capacity (kWh) = [Daily Energy Consumption (kWh) × Backup Days] ÷ [Battery Efficiency × Depth of Discharge]

Let’s walk through a real-world example to demonstrate how to calculate the battery backup for your solar system.

Step 1: Identify Key Variables

• Daily Energy Consumption: 20 kWh (This is the energy your household uses in one day).
• Backup Days: 2 days (You want the battery to power your home for 2 days without solar input).
• Battery Efficiency: 85% (The battery loses 15% of the energy during charging and discharging).
• Depth of Discharge (DoD): 80% (You can safely use 80% of the total battery capacity).

Step 2: Plug Variables Into the Formula

Now that we have all the necessary variables, let’s plug them into the formula:

Step 3: Conclusion

In this example, you would need a 58.82 kWh battery to provide 20 kWh of energy per day for 2 backup days, taking into account the battery efficiency and DoD.

Final Considerations

• If you want more backup days or a larger buffer, you can adjust the "Backup Days" in the formula accordingly.
• It's important to choose a battery with a high efficiency and good DoD to get the most usable energy from your storage system.

By using this formula and example, you can quickly calculate the battery capacity needed for your solar system and ensure you’re prepared for any period without solar generation.

Are you ready to calculate the perfect battery backup for your solar system? Reach out to our experts at Shielden to discuss the best energy storage solutions for your home or business. Let’s make sure you’re prepared for whatever the weather (or the grid) throws your way!