So, you’re sold on solar — or at least flirting with it. Then comes the big question: how much battery do I really need?
Too small, and you’re in the dark when Netflix finally gets good. Too big, and your wallet groans louder than your generator. This guide breaks down the basics so you can right-size it without the guesswork.
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Why battery sizing matters
- Undersized battery: shuts down early, frustration guaranteed.
- Oversized battery: money tied up in unused capacity.
- Right-sized battery: covers essentials, recharges daily, and doesn’t break the bank.
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Step 1: Define your “must-run” loads
Think of what you can’t live without during an outage or cloudy stretch:
- Refrigerator/freezer
- Lights
- Wi-Fi & devices
- A few outlets for charging
- Fans or one small AC (if you’re in the tropics, you know)
👉 Use the Appliance Cost Calculator to see exactly what each device eats up.
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Step 2: Estimate daily energy needs
- Add up total watts for your must-run list.
- Multiply by hours of use.
- Divide by 1,000 to get kWh.
Example:
- Fridge (150 W avg × 24 hrs) = 3.6 kWh
- Lights (80 W × 6 hrs) = 0.48 kWh
- Wi-Fi & devices (40 W × 10 hrs) = 0.4 kWh
- Total ≈ 4.5 kWh/day
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Step 3: Pick your backup window
How long should that battery carry you?
- 8–12 hours: overnight security.
- 24 hours: ride out full-day outages.
- 48+ hours: rural areas or storm zones.
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Step 4: Factor in battery efficiency
- Lithium iron phosphate (LFP): ~90–95% efficiency, deep discharge possible.
- Lead-acid? Don’t go below 50%.
- Always add ~20% margin for safety.
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Step 5: Don’t forget solar recharge
Panels refill your battery by day.
If your system can fully recharge in daylight, you can get by with less storage capacity.
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Example Walkthrough
- Essentials = ~4.5 kWh/day
- Add 20% buffer = ~5.5 kWh
- With efficiency margin → a 6 kWh battery covers daily essentials comfortably.
Stack additional modules if you want more breathing room.
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Financing & ROI angle
Batteries aren’t just about money. They’re about control and comfort.
But if you want to run the numbers, plug battery cost into the Solar ROI Calculator. You’ll see how adding storage shifts your payback curve.
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Common pitfalls
- Ignoring surge loads (fridges/AC spike at startup).
- Forgetting future appliances.
- Overbuying because “bigger is better.”
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Your next steps
- Run your must-run loads through the Battery Sizing Calculator.
- Double-check appliance consumption with the Appliance Cost Calculator.
- Model payback with the Solar ROI Calculator.