How Much Battery Backup Do I Need for My Solar System?
When designing a reliable solar power system, battery backup capacity is a crucial element that determines how effectively your system can supply electricity during grid outages or at night. Choosing the right battery backup for solar systems not only ensures energy reliability but also optimises performance and return on investment. In this guide, we explore in-depth how to accurately determine how much battery backup you need for your solar setup.
Factors That Affect Battery Backup Requirements
Your Daily Energy Consumption
Before figuring out battery size, you need to know how much energy you use daily. This is usually measured in kilowatt-hours (kWh).
How to Calculate Your Daily Usage
Check your utility bills. Most will list your average daily or monthly usage. Divide your monthly usage by 30 to get daily numbers. Let’s say you use 600 kWh a month—your daily usage is 20 kWh.
Type of Solar System: Grid-Tied vs Off-Grid
Grid-tied systems use the grid as backup and need less battery.
Off-grid systems must store enough power to get you through nights and bad weather—so more battery capacity is required.
Depth of Discharge (DoD) and Efficiency
Not all battery capacity is usable. Most batteries are rated for a specific DoD—say 80%. That means you should only use 80% of the battery to preserve its lifespan.
Also, efficiency losses (usually around 90-95%) during charging/discharging should be factored in.
Battery Type and Storage Capacity
A 5kWh lithium battery gives you a different experience than a 5kWh lead-acid one. Why? Because of cycle life, DoD, efficiency, and maintenance needs.
Number of Backup Days Desired
Want to store power for one night, two days, or even three cloudy days in a row? The more backup days you want, the more storage you need.
Calculating Your Battery Backup Needs
Step-by-Step Battery Sizing Formula
Here’s a simple formula:
Battery Capacity (kWh) = Daily Usage × Number of Days × 1.1 (efficiency buffer) ÷ DoD
Example Calculation
Let’s say:
Daily usage = 10 kWh
Backup for 2 days
DoD = 80% (0.8)
Efficiency buffer = 1.1
Then:
Battery size = 10 × 2 × 1.1 ÷ 0.8 = 27.5 kWh
So you’d need roughly 28 kWh of usable battery storage.
Tools and Apps That Help You Estimate
There are free online calculators (like Outback, Tesla’s Powerwall estimator, or Solar.com tools) that give you ballpark figures based on your load profile.
Best Battery Types for Solar Backup
Lithium-ion Batteries
The most popular for home use. They’re compact, have high DoD (~90–95%), longer lifespans, and require little maintenance. Brands like Tesla Powerwall and BYD fall here.
Lead-Acid Batteries
Cheaper upfront but bulky and require regular maintenance. Lower DoD (~50%) means you need more to store the same usable energy.
How to Optimize Your Battery Backup
Use Energy-Efficient Appliances
Why store energy for an old fridge that guzzles power? Swapping out power-hungry appliances reduces your backup needs significantly.
Monitor Your Battery Usage
Many modern inverters have smart apps that show you battery status, usage trends, and even suggest when to charge or discharge. Use them.
Integrate Smart Energy Management
Smart home systems let you schedule energy use—running your water heater during peak sun, for example—and reduce unnecessary drain.
Conclusion
To determine how much battery backup you need for your solar system, start by calculating your daily usage, then adjust for autonomy, battery type, and DoD. Match the result with a solar array that can replenish your battery daily, and ensure system components—especially the inverter—are compatible and scalable.
A well-sized battery bank maximises self-reliance, protects against power outages, and delivers long-term savings.
At Voltmatic, we specialise in custom solar and battery backup solutions tailored to your energy goals. Whether you’re powering a home, farm, school, or business, we’ll help you size it right the first time.
Contact us today for a free solar consultation and let’s build a system that keeps your power on—day and night.
Phone Number:0759493610
Email: info@voltmaticenergysolutions.co.ke