How long will a 100Ah battery last?

Divide usable watt-hours by the load. A 12 V 100 Ah lead-acid battery used to 50% gives 600 Wh; at 90% inverter efficiency a 100 W load runs about 5.4 h before Peukert losses (about 4.9 h with Peukert k = 1.15). A 12.8 V 100 Ah LiFePO4 used to 90% gives 1,152 Wh and runs the same 100 W load about 10.4 h.

What is the battery runtime formula?

Runtime (hours) = capacity (Ah) × DoD × efficiency ÷ load (A). With a load in watts: runtime = Ah × V × DoD × efficiency ÷ W. Example: 3000 mAh at 85% efficiency, 100% DoD and a 500 mA load gives 3.0 × 0.85 ÷ 0.5 = 5.1 h.

What is Peukert's law?

Peukert's law describes how lead-acid capacity falls as discharge current rises: t = H × (C ÷ (I × H))^k, where C is the rated capacity at H hours (usually 20 h), I the current and k the Peukert exponent. Typical k is 1.2–1.3 for flooded, about 1.2 for gel, 1.1–1.15 for AGM and 1.02–1.05 for LiFePO4. Victron battery monitors default to 1.25 for lead-acid and 1.05 for lithium when the datasheet gives no value.

What depth of discharge should I use?

Lead-acid batteries last longest when discharged to about 50%, so 50% DoD is the usual design figure. LiFePO4 tolerates 80–100% DoD for thousands of cycles; 80–90% is a common design value. Lithium-ion (NMC) packs are often limited to 80–90% by their battery management system.

C-rate is the discharge current divided by the capacity in Ah. 1C empties the battery in about one hour, 0.5C in two hours and 0.05C (C/20) in twenty hours. Lead-acid capacity is usually rated at C/20, so runs at higher C-rates deliver less than the label figure.

Why does the 25% row show no runtime with lead-acid presets?

The state-of-charge table counts only the charge above the depth-of-discharge cutoff. With 50% DoD the cutoff is 50% state of charge, so a battery starting at 25% is already below it and should be recharged rather than used.

Battery Discharge Calculator

Runtime (h) = Ah × V × DoD × efficiency ÷ load W, or Ah × DoD × efficiency ÷ load A. A 100 Ah 12.8 V LiFePO4 at 90% DoD and 90% efficiency runs a 100 W load for 100 × 12.8 × 0.9 × 0.9 ÷ 100 = 10.4 h. For lead-acid, Peukert's law shortens runtime at high current.

Battery Parameters

Chemistry preset

Battery Capacity

Nominal Voltage (V)

Load

Efficiency: 85%

50% (lossy) 100% (ideal)

Depth of Discharge (DoD)

% of capacity you will use

Apply Peukert's law (lead-acid)

Exponent k

Rated hours H

Discharge Time

—

hours : minutes

C-Rate

—

discharge rate

Battery current	—
Stored energy	—
Usable energy (DoD)	—
Runtime without Peukert	—
Runtime with Peukert	—

Usable capacity —

0% Rated capacity

Runtime by State of Charge

Runtime from each starting charge down to the DoD cutoff

Starting Charge Usable Runtime

100% — —

75% — —

50% — —

25% — —

Rated Capacity

—

Usable Capacity

—

DoD Applied

—

Battery chemistry presets

Typical design values. Use your datasheet when it gives a Peukert exponent or cycle-life curve.

Chemistry	Volts	DoD	Peukert k	100 Ah, 100 W	Notes
LiFePO4 (12.8 V)	12.8	90%	1.05	10.4 h	3.2 V/cell, 2,000–6,000 cycles at 80–100% DoD
Li-ion / LiPo cell (3.7 V)	3.7	90%	1.05	3.0 h	3.6–3.7 V/cell, BMS usually limits DoD
AGM lead-acid (12 V)	12	50%	1.15	4.9 h	2 V/cell, rated at 20 h; deep cycles shorten life
Gel lead-acid (12 V)	12	50%	1.20	4.8 h	2 V/cell, rated at 20 h, low current preferred
Flooded lead-acid (12 V)	12	50%	1.25	4.6 h	2 V/cell, rated at 20 h, needs watering

100 Ah at the listed voltage and DoD, 90% inverter efficiency, 100 W load; lead-acid rows include Peukert's law at the 20-hour rate.

Worked examples

12 V 100 Ah AGM, 100 W through a 90% inverter. Battery current = 100 ÷ (12 × 0.9) = 9.26 A. Without Peukert: 100 × 0.5 ÷ 9.26 = 5.4 h. With Peukert (k = 1.15, H = 20): 20 × (100 ÷ (9.26 × 20))^1.15 × 0.5 = 4.9 h.

12.8 V 100 Ah LiFePO4, same load. 100 × 12.8 × 0.9 × 0.9 ÷ 100 = 10.4 h at 90% DoD, about twice the AGM figure from the same Ah rating.

3000 mAh 18650 cell, 500 mA load. 3.0 Ah × 0.85 ÷ 0.5 A = 5.1 h. Battery current is 0.5 ÷ 0.85 = 0.59 A, a C-rate of 0.196C.

Sources: Victron SmartShunt manual (Peukert defaults) · Peukert's law

Summary

Battery runtime depends on usable energy and how fast the load draws it. This calculator takes capacity in mAh, Ah or Wh, the battery voltage, and a load in mA, A or W, then applies depth of discharge (how much of the battery you allow yourself to use) and system efficiency (inverter, converter and wiring losses). For lead-acid batteries it can apply Peukert's law, which reduces usable capacity at discharge rates above the rated 20-hour rate. Chemistry presets fill typical voltage, depth of discharge, efficiency and Peukert values for LiFePO4, lithium-ion, AGM, gel and flooded batteries.

How it works

Pick a chemistry preset or enter your own values. Presets set nominal voltage, recommended depth of discharge, efficiency and the Peukert exponent.
Enter capacity in mAh, Ah or Wh, and the nominal voltage (needed for Wh capacity and watt loads).
Enter the load in mA, A or W. Battery current = load W ÷ (V × efficiency), or load A ÷ efficiency.
Runtime = Ah × DoD ÷ battery current, which equals Ah × V × DoD × efficiency ÷ W.
With Peukert on: runtime = H × (Ah ÷ (I × H))^k × DoD, where H is the rated discharge time (20 h for most lead-acid) and k the Peukert exponent.
The state-of-charge table shows runtime from 100%, 75%, 50% and 25% charge down to the depth-of-discharge cutoff.

Use cases

Find how long a 100 Ah LiFePO4 or AGM battery runs a fridge, CPAP or trolling motor.
Size an RV, van or off-grid battery bank for an inverter load in watts.
Estimate how long an 18650 cell or a LiPo pack powers a flashlight, sensor or drone.
Check that a UPS battery covers the required backup window.
Compare lithium and lead-acid batteries of the same Ah rating.
Check the C-rate against the battery's rated maximum discharge.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu