Battery Runtime Calculator

Frequently Asked Questions

What depth of discharge should I use?

Lithium batteries tolerate 80-100% DoD without significant lifespan impact. Lead-acid batteries last far longer when kept above 50% charge - cycling to 80% DoD regularly can cut cycle life in half or more.

Do I need to enter inverter efficiency for DC loads?

No. Set efficiency to 100% for direct DC loads such as 12V lights, fans, or USB devices. Only AC loads powered through an inverter require the efficiency derating.

Why is my real runtime shorter than this estimate?

Temperature, battery age, and high discharge rates all reduce usable capacity. This calculator uses nominal ratings - treat results as an optimistic upper bound and add a safety margin.

What is the difference between Ah and Wh?

Amp-hours measure charge capacity at a given voltage. Watt-hours measure energy directly. Wh = Ah x V, so a 100 Ah, 12 V battery and a 100 Ah, 24 V battery hold very different amounts of energy.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

Enter your battery capacity in amp-hours (Ah) or watt-hours (Wh), the system voltage if using Ah, the load in watts, the usable depth of discharge, and the inverter efficiency. The calculator converts everything to watt-hours, removes the energy you should not draw (depth of discharge), applies conversion losses (inverter efficiency), and divides by the load to estimate runtime.

Formula: Energy (Wh) = capacity (Ah) × voltage. Usable energy = total energy × (DoD% ÷ 100) × (efficiency% ÷ 100). Runtime (hours) = usable energy ÷ load (W).

Worked Example

A 100 Ah, 12 V battery holds 1,200 Wh total. At 80% depth of discharge and 90% inverter efficiency: usable = 1,200 × 0.8 × 0.9 = 864 Wh. Powering a 100 W load: runtime = 864 ÷ 100 = 8.64 hours, or about 8 hours 38 minutes.

The same battery with a 200 W load would last only 4.32 hours, and running below 50% DoD (as recommended for lead-acid) would cut that further to about 3.24 hours.

Tips for Longer Battery Runtime

Use lithium iron phosphate (LFP) batteries - they tolerate 80-100% DoD versus 50% for lead-acid, effectively doubling usable capacity at the same nameplate rating.

Minimize high-current draws. Doubling current often reduces effective capacity by 10-20% due to the Peukert effect in lead-acid batteries.

Keep batteries in a moderate temperature range - extreme cold sharply reduces capacity; extreme heat accelerates degradation.

For AC loads, invest in a high-quality inverter with 90%+ efficiency to reclaim lost watt-hours.

Use DC loads directly from the battery wherever possible to skip inverter losses entirely.

FAQ