How is battery backup time calculated?

Hours ≈ (Voltage × Amp-hours × Efficiency × Usable depth-of-discharge) ÷ Load in watts.

Why are ideal and real-world results different?

Losses in wiring and inverter plus limited depth-of-discharge reduce usable energy. Real-world runtime is typically 60–85% of the ideal value.

Can I share my calculation?

Yes. Use the Copy Result Link button. The URL encodes your inputs so others can open the same result instantly.

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August 20, 2025

TOOL-Battery Backup Time Calculator

Battery Backup Time Calculator (Deep Green) — 12V, Ah, Load (W)

Battery Backup Time Calculator

Deep-green · White fonts · Result-oriented

Inputs

Battery Voltage (V)

Battery Capacity (Ah)

Total Load (W)

System Efficiency (%) (inverter + wiring)

Usable Depth of Discharge (%)

Label (optional)

Tip: Ideal = (V × Ah) ÷ Load. Real-world also applies efficiency & DoD.

Results

Enter values and hit Calculate.

–.– h

Ideal estimate

–.– h

Real-world estimate (efficiency + DoD)

Usable Energy

– Wh

Current Draw

– A

Result Link

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Quick FAQ

How do I match the example 12V, 225Ah, 300W?

Use V=12, Ah=225, Load=300, Efficiency=100%, DoD=100% to get ≈ 9 hours (ideal). With 85% efficiency & 80% DoD, the realistic estimate is ≈ 6.12 hours.

What is DoD?

Depth of Discharge is the portion of capacity you plan to use. Many lead-acid batteries are kept to 50–80% DoD to extend life; lithium often allows more.

What affects runtime?

Load wattage, inverter efficiency, temperature, battery age, discharge rate (Peukert effect for lead-acid), and actual DoD.