How long will a 100 amp hour battery last?

[DiMiranda]

A 100Ah battery's runtime ranges from 1 hour to 50 hours, dictated by your appliance's wattage and the battery's type. For a common 12V 100Ah LiFePO4 battery with 1,280 watt-hours of usable energy, a 100W TV runs about 10-12 hours, while a 1000W microwave lasts roughly 1-1.5 hours. The core calculation divides usable battery energy by your device's power consumption.

Accurate estimation requires understanding key variables. The primary formula is: Runtime (hours) = (Battery Voltage × Amp Hours × Usable Depth of Discharge) / Load Power (Watts). This highlights the critical impact of battery chemistry on usable capacity.

Appliance Example (Watts)	Estimated Runtime (12V 100Ah LiFePO4)	Estimated Runtime (12V 100Ah AGM, 50% DoD)
LED Light (10W)	~128 hours	~60 hours
Portable Fridge (40W)	~32 hours	~15 hours
TV (100W)	~12.8 hours	~6 hours
Trolling Motor (50A draw ~600W)	~2.1 hours	~1 hour
Microwave (1000W)	~1.3 hours	~0.6 hours

Battery Type is the Deciding Factor: A Lithium Iron Phosphate (LiFePO4) battery delivers nearly 100% of its rated capacity (100Ah usable). A lead-acid AGM battery should only be discharged to 50% to prevent damage, yielding just 50Ah of usable power—effectively halving runtime compared to lithium.

Inverter Efficiency Matters: If powering AC appliances, a power inverter adds 10-15% overhead loss. For a 100W load, the battery actually supplies 110-115W, reducing runtime proportionally.

Lifespan Context: Runtime per charge is different from total battery life. A quality LiFePO4 battery withstands 2,000 to 5,000 full cycles, often lasting 5-10 years. An AGM battery typically manages 300-500 cycles, lasting 2-3 years under similar use, making lithium more cost-effective long-term.

For safety and performance, never exceed the battery's continuous discharge rating (e.g., 100A for a 100Ah lithium battery). Always match your energy needs with the battery's specifications, factoring in chemistry and inverter losses for a reliable power system.

[DeElaina]

I live off-grid in my van, and my 100Ah lithium battery is my lifeline. Last weekend, I ran my 40W fridge for two full days and nights, plus a few hours of 100W laptop work and LED lights in the evening. By morning, I still had about 30% power left. It’s all about watching your wattage and thinking in terms of total energy used per day. For me, that means knowing my fridge uses about 1,000Wh daily, which is well within my battery’s 1,280Wh budget. The peace of mind comes from lithium’s ability to use almost all that power without harming it.

[DelVivian]

Let’s make the math simple. Forget the complex formula for a second. Think of your battery as a fuel tank. A 12V 100Ah lithium battery holds about 1.2 kilowatt-hours of “fuel.”

Now, look at your appliance label for its watts. A 60-watt light bulb? That’s 0.06 kilowatts.

Just divide the tank size by the appliance’s hunger: 1.2 kWh / 0.06 kW = 20 hours. That’s it.

If you’re using an inverter for regular household plugs, mentally set aside 15% of your tank as an “inverter tax.” So, your 1.2 kWh tank becomes a 1 kWh tank for your calculations. It’s a quick, practical way to ballpark your runtime without getting bogged down in technical details.

[Dante]

The most common mistake is comparing batteries by their Amp-hour (Ah) rating alone. A “100Ah” label tells only half the story. The real question is: how many of those amp-hours can you actually use?

With a lithium battery, you can use all 100Ah. With a standard lead-acid or AGM battery, you should only use about 50Ah to avoid severely shortening its life. This means for the same 100Ah rating, the lithium battery provides double the usable energy from the start.

Furthermore, lithium batteries maintain a higher voltage as they discharge, meaning appliances run efficiently for longer. An AGM battery’s voltage drops steadily, which can cause some devices to shut down prematurely, even if some capacity remains. When estimating runtime, the battery chemistry is your starting point.

[StAsher]

As an installer, my first question to clients is always: “What’s your total daily watt-hour need?” A 100Ah/12V lithium system (1,280Wh) is perfect for modest, managed loads.

For instance, a typical setup might be a 12V fridge (40W running 12 hours = 480Wh), LED lighting (20W for 4 hours = 80Wh), and charging phones/laptops (100W for 2 hours = 200Wh). That’s a total of about 760Wh, leaving a comfortable buffer.

My key advice: Always size for your worst-case, not your average day. Add a 20% buffer to your calculated needs. And never forget the inverter. If your total daily need is 1,000Wh from AC devices, your battery must supply roughly 1,150Wh to account for the conversion loss. This planning prevents over-discharge and ensures your system lasts for years, not just a single trip.