How Much Solar Battery Backup Do You Really Need?
One of the most common questions when planning an off-grid or hybrid solar system is: how much backup do I need? Whether you're aiming for full autonomy or just backup during outages, this guide will teach you exactly how to calculate your required solar battery backup size.
🔋 What Does Backup Mean in Solar?
Backup refers to the number of hours or days your system can run purely on stored energy — when the sun isn't shining, or the grid is down.
🧮 Key Factors in Sizing Backup
- Daily Load: Your total energy consumption in Wh or kWh
- Autonomy Days: How many hours/days you want backup for
- Battery Type: Lead-acid, Lithium, or Gel
- Depth of Discharge (DoD): Max usable % of battery
🔢 The Formula
Battery Bank Size (Wh) = Daily Load × Autonomy Days ÷ DoD Battery Ah = Wh ÷ Voltage
Example:
• Daily Load = 3000Wh
• Desired Backup = 2 days
• DoD = 80% (for lithium)
• Voltage = 24V
• Wh = (3000 × 2) ÷ 0.8 = 7500Wh
• Ah = 7500 ÷ 24 = 312.5Ah
So you'd need approx. 2 × 150Ah batteries at 24V (or 4 × 12V 150Ah in series-parallel).
🧠 Backup Scenarios
✔ Basic Backup (Lights + Fans)
- Daily Load: ~1000Wh
- Backup Time: 6–8 hours
- Recommended: 1 × 12V 150Ah battery
✔ Partial Home Backup (TV, fridge, lights)
- Daily Load: ~3000Wh
- Backup Time: 12–24 hours
- Recommended: 2 × 12V 150Ah batteries
✔ Full Home Backup (everything)
- Daily Load: 5000–7000Wh
- Backup Time: 1–2 days
- Recommended: 4–6 lithium batteries (48V)
🔄 Recharge Time
Backup only works if you can recharge in time. Make sure your solar panel array is big enough to recharge the battery bank fully by midday.
🔋 Best Battery Types for Backup
- Lithium-Ion: Ideal for long-term backup and space-saving
- AGM/Gel: Good mid-range option, maintenance-free
- Flooded Lead-Acid: Budget friendly, but needs regular maintenance
🔧 Tips to Maximize Backup
- Use energy-efficient appliances
- Turn off non-essential loads during outages
- Schedule high-consumption tasks during sunny hours
- Use MPPT charge controller for efficient charging
🧰 Tools You Can Use
💡 Final Thoughts
Backup isn’t about going completely off-grid — it’s about energy security. Once you know your daily usage and how long you want backup, you can size your battery bank with confidence. For best results, use our free solar calculator to generate a full setup plan.