How to Design a 3-Day Solar Backup System — Step-by-Step
Whether it’s storm season, grid outages, or off-grid camping — a reliable solar system with 3 days of battery backup can be a total lifesaver. Here’s exactly how to build one from scratch.
📋 Step 1: Calculate Your Daily Load
List the essential devices you’ll run during backup. Use our Load Calculator to speed it up.
Example:
- 4 LED lights × 10W × 8h = 320Wh
- 1 fan × 50W × 8h = 400Wh
- Router + laptop = 300Wh
- Total Daily Load ≈ 1,020Wh
🔋 Step 2: Multiply by 3 Days
Backup Load = Daily Load × 3
= 1,020Wh × 3
= 3,060Wh
Now divide by battery voltage (say, 12V) to get total amp-hours:
Ah needed = 3,060 ÷ 12 = 255Ah Add 20% buffer → ~306Ah
Use our Battery Sizing Tool to do this automatically.
☀️ Step 3: Solar Panel Requirement
To recharge the system after a 3-day outage, we need to restore 3,060Wh in one day (if possible):
Panel Wattage = 3,060 ÷ 5h (avg sunlight) = ~612W Round up → Use 800W (for buffer & cloudy conditions)
⚙️ Step 4: Choose the Right Inverter
Match your daily peak power. For small essentials, a 1000–1500W inverter is enough.
Use our Inverter Sizing Tool to choose accurately.
🧰 Sample 3-Day System Design
- Panels: 4 × 200W = 800W total
- Batteries: 2 × 12V 150Ah AGM = 300Ah
- Controller: 40A MPPT
- Inverter: 1500W pure sine wave
Total estimated cost: $1,200 – $1,500 (as of 2025)
🧠 Pro Tips
- Use lithium batteries if budget allows (smaller size, deeper DoD)
- Run essential loads only (no ACs, heaters, irons)
- Test discharge once every 3–6 months
✅ Final Thoughts
A well-designed 3-day solar backup setup means peace of mind — during power cuts, storms, or grid failure. Plan smart, size right, and invest in good batteries. Your future self will thank you.