Battery Backup Size Calculator

How to use this calculator

1. Enter average watt load and hours of runtime you need.
2. Set inverter efficiency and an optional surge multiplier.
3. Review required Wh/kWh to choose a battery or multi-battery setup.

Inputs explained

Average load

Typical continuous watts drawn by the devices you’ll run.

Hours needed

How long you want the backup to last at that load.

Inverter efficiency

Losses through the inverter; 85–95% is common.

Surge factor

Headroom for startup surges; set >1.0 if loads spike (motors/compressors).

When to use it

• Sizing a home battery for outages.
• Estimating portable power station capacity for camping/job sites.
• Planning rack batteries for small office or network gear backup.

Tips & cautions

• List and sum your critical loads (fridge, lights, networking) to set the watt load accurately.
• If devices have high startup surge, increase surge factor or size inverter accordingly.
• Temperature and battery chemistry affect usable capacity; add margin for cold conditions.
• Assumes constant average load; real loads fluctuate.
• Does not include depth-of-discharge limits—if using batteries with recommended DoD (e.g., 80%), divide capacity accordingly.
• Ignores charging time and solar/generator input; this is storage sizing only.

Worked examples

Deep dive

FAQs

Do I need to account for depth of discharge?

Yes. If your battery recommends 80% DoD, divide required kWh by 0.8 to size the pack.

What about solar or generator recharge?

Not modeled here. Add input sources separately to extend runtime.

How do I handle variable loads?

Use an average load for a rough estimate. For precision, model time-varying loads and sum Wh.

Does surge factor size inverter or battery?

It inflates required capacity for headroom. Also ensure your inverter is rated for surge wattage.

Is this for AC or DC loads?

Assumes AC load through an inverter; adjust efficiency if using DC directly.

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