By Chandrajit Manhare — Founder, Solar Power Simplified · Last Updated: July 12, 2026
“How big a battery do I need?” is the #1 question I get about home backup. Instead of guessing, tick the appliances you’d actually run during an outage and this tool shows your kWh requirement and how many batteries that means — in Powerwall-class (13.5 kWh) units or smaller 5 kWh modules.
Battery Backup Calculator
Tick what you’d run in an outage — see exactly how much battery you need.
Includes ~15% inverter & conversion losses. Fridge and pump duty cycles are averaged — real usage varies. For solar recharging during multi-day outages, pair with panels.
How This Calculator Works
Each appliance uses its typical average running wattage (a fridge cycles on and off, so 150 W averaged — not its 600 W surge). We multiply your total load by your backup hours, then add 15% for inverter and conversion losses. That’s your usable-kWh requirement — compare it against usable capacity, not the marketing number on the box.
Two honest caveats: surge power matters as much as energy (that’s what the heavy-load warning is about), and running batteries to 0% regularly shortens their life. Size with ~20% headroom if you can.
Frequently Asked Questions
How long will a 13.5 kWh battery run my house?
Running just the essentials (fridge, lights, Wi-Fi, phones — about 285 W), roughly 40+ hours. Add a window AC and it drops to about 9–10 hours. That’s exactly why this calculator asks what you’ll run, not just your home’s size.
Can the battery recharge from solar during an outage?
Yes — if you have panels and a battery system designed for off-grid operation (most modern ones are). That effectively stretches your backup indefinitely for essential loads. See my running a house 100% on solar deep-dive.
Which battery should I buy?
Depends whether you want whole-home backup, modular growth, or DIY-friendly gear — I ranked the top options in my best home solar batteries guide.
More free tools: Solar Cost Calculator · How Many Panels Do I Need?
