How Long Will a 100Ah Battery Last?

The Quick Answer

How long a 100Ah battery lasts depends on what you're running. Here's the quick math: a 12V 100Ah battery holds 1,200 watt-hours of energy. Divide that by your load's wattage to get hours of runtime.

Running a 100-watt load? You get 12 hours. Running a 50-watt load? You get 24 hours.

But there's a catch - battery type matters. Lithium batteries let you use nearly 100% of that capacity. Lead-acid? You can only safely use 50%, cutting your runtime in half.

What Does 100Ah Actually Mean?

Amp-hours (Ah) measure how much current a battery can deliver over time. A 100Ah battery can theoretically provide:

• 100 amps for 1 hour
• 10 amps for 10 hours
• 1 amp for 100 hours

But most of us think in watts, not amps. To convert, multiply amp-hours by voltage:

This is your energy budget. Every device you run draws from this pool until it's empty.

The Runtime Formula

Calculate battery runtime with this simple formula:

For a 12V 100Ah battery running a 60-watt device:

Want to skip the math? Use our Battery Runtime Calculator to get instant results for any device and battery combination.

Runtime Reference Table

Here's how long a 12V 100Ah battery will run common devices. These assume a lithium battery with 100% depth of discharge.

Note: Actual runtime varies based on battery condition, temperature, and inverter efficiency.

Why Battery Type Matters

Not all 100Ah batteries are equal. The chemistry determines how much you can actually use:

Lead-Acid Batteries (AGM, Flooded, Gel)

• Usable capacity: 50% (600Wh from a 12V 100Ah)
• Discharging below 50% dramatically shortens lifespan
• Cheaper upfront, but fewer cycle life (300-500 cycles)
• Heavier (60-70 lbs for 100Ah)

Lithium (LiFePO4) Batteries

• Usable capacity: 80-100% (960-1,200Wh from a 12V 100Ah)
• Can be discharged to 20% or lower without damage
• More expensive upfront, but 2,000-5,000 cycle life
• Lighter (25-30 lbs for 100Ah)

For more on battery voltage and state of charge, check our LiFePO4 Voltage Chart.

Real-World Examples

Example 1: Emergency Power Outage

You need to run these during a 12-hour outage:

• 12V fridge: 50W x 12h = 600Wh
• Phone charging (2 phones): 20W x 4h = 80Wh
• LED lights: 30W x 6h = 180Wh

Total needed: 860Wh

A 100Ah lithium battery (1,200Wh) handles this with 340Wh to spare. A lead-acid battery (600Wh usable) would fall short.

Example 2: Overnight Camping

Running overnight (10 hours):

• 12V fridge: 50W x 10h = 500Wh
• CPAP machine: 40W x 8h = 320Wh
• Phone charging: 10W x 2h = 20Wh

Total needed: 840Wh

Again, lithium handles it. Lead-acid would need careful management or a second battery.

Plan your own power needs with our Battery Bank Sizing Calculator.

Factors That Reduce Runtime

The calculations above are ideal scenarios. Real-world runtime is often lower due to:

Inverter Efficiency

If you're converting 12V DC to 120V AC, inverters lose 10-15% to heat. Factor this into your calculations by dividing your usable watt-hours by 0.85.

Temperature

Cold reduces battery capacity. At 32°F (0°C), expect 10-20% less capacity. At 0°F (-18°C), you might lose 30-40%. LiFePO4 batteries have built-in protection that prevents charging below freezing.

Battery Age

All batteries lose capacity over time. After 500 cycles, a lead-acid battery might only hold 60-70% of its original capacity. Lithium batteries hold up better, often retaining 80% capacity after 2,000 cycles.

Discharge Rate

High-current draws reduce effective capacity. Drawing 100A from a 100Ah battery won't give you a full hour due to voltage sag and increased resistance.