Planning a road trip, setting up a solar system, or powering your off-grid setup? Then you’ve probably asked yourself: “How long will a 100Ah battery last?”.
The answer isn’t as simple as a single number. The battery runtime depends on what you’re powering, how you’re using it, and even the type of battery you choose.
In this easy-to-follow guide, we’ll help you explore how to calculate battery runtime using simple formulas. You’ll learn about the factors that affect battery lifespan. Additionally, we’ll share expert tips on how to extend your battery’s life so that you get the most out of every charge.
Let’s get started. No guesswork, just practical insights.
What Does Ah Mean in a Battery?
Before we can figure out how long your 100Ah battery will last, let’s take a moment to understand what Ah means.
Understanding Amp-Hours (Ah) – The Core of Battery Capacity
When you see “Ah” on a battery spec sheet, it stands for Ampere-Hour. This is a unit of measurement that tells you how much electrical current a battery can provide over a specific period. You can think of Ah as the fuel tank of your battery. The bigger the tank (or the higher the Ah rating), the more energy your battery can store and supply.
For example, a 100Ah battery can provide:
- 100 amps for 1 hour
- 10 amps for 10 hours
- 1 amp for 100 hours
These are ideal, theoretical numbers, though. In real life, factors like temperature, the type of load, and battery efficiency can affect how long the battery lasts. So, we’ll need to consider a few more things when estimating real-world performance.
How Ah Relates to Battery Capacity and Runtime
To estimate how long a battery will run, we first need to understand battery capacity. Battery capacity tells us how much electrical charge the battery can store, and it’s usually measured in Ah (Ampere-Hours) or mAh (Milliampere-Hours). For reference, 1Ah = 1000mAh.
For example:
A 100Ah battery can provide 100 amps for 1 hour or 10 amps for 10 hours.
Using the formula:
- 100Ah ÷ 100A = 1 hour
- 100Ah ÷ 10A = 10 hours
When purchasing a battery, must match your power requirements with the correct Ah rating to ensure you get the runtime you need.
But even with the right Ah rating, your battery’s performance can vary. That’s because real-world conditions affect how long it actually lasts. Let’s go those conditions first.
8 Real-World Factors Affecting Battery Runtime
Before jumping onto the battery runtime formulas, let’s discover the key factors that affect the 100Ah battery lifespan:
1. Battery Condition
Think of a battery like a car engine. It works great when new. But over time, wear and tear reduce performance. A brand-new 100Ah lithium battery will give you close to its full capacity. But if it’s a few years old or hasn’t been maintained properly, the available capacity might drop to 80Ah or even lower.
2. Higher Voltage = Longer Runtime (With the Same Current)
Power isn’t just about amps. It’s about volts too. The formula is simple:
Power (W) = Voltage (V) × Current (A)
This means that with the same amp draw, a higher-voltage battery can deliver more power. For example:
| Tool | Battery | Total Energy | Run Time |
| 20W Electric Trimmer | 12V 100Ah | 1200Wh | 60 hours |
| 20W Electric Trimmer | 24V 100Ah | 2400Wh | 120 hours |
So, if you’re running a 20W gardening tool, a 24V battery will give you double the runtime compared to a 12V one of the same amp-hour rating. That’s why TOPO’s 24V lithium battery packs are a popular choice for higher-efficiency projects.
3. Loads Characteristics
Just like heavy lifting tyres you out faster, high-power devices drain your battery more quickly. A 100W lawn mower will eat up power twice as fast as a 50W model.
Let’s break it down using a 24V 100Ah lithium battery (2400Wh):
- 50W mower: 2400Wh ÷ 50W = 48 hours
- 100W mower: 2400Wh ÷ 100W = 24 hours
So, when you’re estimating run time, always check how many watts your devices are using. Lower power = longer playtime.
4. Discharge Rate
How quickly a battery discharges affects performance too. LiFePO4 batteries can handle higher discharge rates better than lead acid batteries. This means they can power bigger loads without losing efficiency and without getting damaged.
So, if you’re planning to run power tools or appliances that need more juice, a LiFePO4 battery is your best bet for reliable and consistent performance.
5. Temperature
Batteries hate extreme temperatures. Cold weather slows down chemical reactions inside the battery, while very high heat can degrade it faster.
For example:
- At room temperature, a 100Ah lithium battery might last a full 20 hours on a 5A load.
- At -10°C, it could run for only 10–12 hours under the same conditions.
We recommend storing and using batteries in moderate temperatures whenever possible. And if you’re using batteries in winter or summer conditions, always factor that into your runtime estimates.
6. Self-Discharge While in Storage
Even when not in use, batteries slowly lose charge over time. This is called self-discharge. Here’s how it compares:
- Lithium batteries: About 2% per month
- Lead acid batteries: Up to 4% per week
If you’re storing batteries for a long time, lithium is clearly the more reliable choice. It holds a charge longer and will be ready when you need it—perfect for solar setups or backup power systems.
7. Power Surges: When Devices Draw Power in Spikes
Not every load is steady. Appliances with compressors, motors, or heating elements may require a burst of power at startup, even if their average wattage is low. For instance, a camping fridge might use 40W continuously but need 120W for a few seconds every time the compressor kicks in.
These spikes can affect your total battery run time and performance, especially if you’re running multiple devices at once.
8. Device Efficiency
A device’s condition and efficiency affect how much energy it actually uses. Older appliances that haven’t been maintained mostly waste more energy than they should.
That wasted energy = faster battery drains.
Now that you know what can affect your battery’s performance in the real world, let’s look at how to actually calculate the battery runtime with some simple formulas.
The Basic Formula to Calculate How Long a Battery Will Last
If you need to figure out how long a battery will power a device, here’s a simple way to do it. You can calculate it using these formulas:
Battery Runtime (hours) = Battery Capacity (Ah) ÷ Load (Amps)
For example, if your load current is 20A, then a 100Ah battery can last for 5 hours:
- 100Ah ÷ 20A = 5 hours
Battery Runtime (hours) = Battery Energy (Wh) ÷ Load (W)
This is useful if you know the load’s power requirements (in watts) but not the current draw.
Example #1:
If you connect a 100Ah 12V battery to a 40W refrigerator, you can expect it to run for around 30 hours:
- Battery Capacity (Wh) = 12V × 100Ah = 1200Wh
- Battery Runtime = 1200Wh ÷ 40W = 30 hours
Example #2:
Using a 100Ah 24V battery to power a 40W refrigerator will extend the runtime to about 60 hours:
- Battery Capacity (Wh) = 24V × 100Ah = 2400Wh
- Battery Runtime = 2400Wh ÷ 40W = 60 hours
Example #3:
If the 100Ah 24V battery powers a 75W electric heater, it will run for approximately 32 hours:
- Battery Capacity (Wh) = 24V × 100Ah = 2400Wh
- Battery Runtime = 2400Wh ÷ 75W = 32 hours
Use an Amp-Hour Calculator for Quick Results
Don’t feel like doing the math yourself? No worries! An amp-hour calculator can make this process much easier. Just input the battery capacity and your device’s power draw, and you’ll get an estimate in seconds.
For example:
- Battery: 100Ah LiFePO4
- Load: 2A (like a 12V fan)
Using the formula:
100Ah ÷ 2A = 50 hours
That means your fan can run for about two full days without needing a recharge!
How to Calculate Battery Runtime | 4 Easy Steps
Let’s get into a more detailed, step-by-step approach to calculate how long a 100Ah battery will last, especially when you want more accurate results considering additional factors.
Here’s the process to get a more precise estimate:
Step #1. Convert Amp-Hours to Watt-Hours
First, we need to convert the battery capacity from amp-hours (Ah) to watt-hours (Wh). This step is important because watt-hours measure energy consumption more directly.
To do this, multiply the battery capacity by the voltage. Most deep-cycle batteries are rated at 12V.
Watt-hours (Wh) = Battery Capacity (Ah) × Battery Voltage
For example, for a 100Ah battery:
- Wh = Ah × Voltage
- Wh = 100 × 12
- Wh = 1200 watt-hours
This means that your 100Ah battery can provide 1200 watt-hours of energy.
Here’s how it looks with a 100Ah battery:
| Battery Voltage | Battery Capacity | Total Energy (Wh) |
| 12V | 100Ah | 1,200Wh |
| 24V | 100Ah | 2,400Wh |
| 48V | 100Ah | 4,800Wh |
Step #2. Consider Depth of Discharge (DoD)
The Depth of Discharge (DoD) is important to know how much of your battery you can safely use without damaging it. For lead-acid batteries, the DoD is usually 50%, meaning you should only use half of the battery’s capacity. However, for LiFePO4 batteries, you can use up to 100% of the capacity.
So, here’s how it looks for different battery types:
- For Lead-Acid Batteries:
Available Capacity = 1200 Wh × 0.50
Available Capacity = 600 Wh
- For LiFePO4 Batteries:
Available Capacity = 1200 Wh × 1.00
Available Capacity = 1200 Wh
This is the energy you have available to use before needing to recharge.
Step #3. Account for Inverter Efficiency
Most devices run on AC power, while your battery supplies DC power. To convert DC to AC, you’ll need an inverter. However, inverters aren’t perfect and have efficiency losses, usually around 95%.
So, here’s how it works:
- For Lead-Acid Batteries:
Net Capacity = Available Capacity × Efficiency
Net Capacity = 600 Wh × 0.95
Net Capacity = 570 Wh
- For LiFePO4 Batteries:
Net Capacity = 1200 Wh × 0.95
Net Capacity = 1140 Wh
This tells you the actual usable energy after considering inverter efficiency.
Step #4. Calculate Battery Runtime
Finally, you’re ready to calculate the runtime of your battery. To do this, you need to know the total load (in watts) of the devices you’re powering.
For example, let’s say you’re running a 50W lamp and a 50W speaker (total load = 100W).
Here’s how it works for each battery type:
- For Lead-Acid Batteries:
Runtime = Net Capacity ÷ Total Load
Runtime = 570 Wh ÷ 100 W
Runtime = 5.7 hours
- For LiFePO4 Batteries:
Runtime = Net Capacity ÷ Total Load
Runtime = 1140 Wh ÷ 100 W
Runtime = 11.4 hours
As you can see, the LiFePO4 battery lasts much longer due to its ability to use more of its capacity.
Practical Scenarios: Estimated Battery Runtime for Everyday Uses
| Application | Power Consumption | Runtime with 12V 100Ah Battery |
| 12V LED Light | 10 watts | 120 hours |
| Phone Charging | 5 watts | 240 hours |
| 12V Portable Fan | 30 watts | 40 hours |
| 12V Refrigerator | 60 watts | 20 hours |
| Laptop Charging | 90 watts | 13 hours |
| 500W Inverter (Mixed Load) | 500 watts | 2.4 hours |
| Electric Golf Cart | 300 watts | 4 hours |
| RV Appliances | 250 watts | 4.8 hours |
Note: Actual runtime may vary based on efficiency, inverter losses, battery age, and temperature conditions.
Tips for Extending Battery Life
Want to squeeze more runtime from your 100Ah battery? These simple, practical habits can help you get the most out of every charge, whether you’re powering an RV, off-grid cabin, or solar setup.
- Cut down on power-hungry devices
Use energy-efficient appliances, turn off anything non-essential, and keep your gear well-maintained to reduce unnecessary drain.
- Keep the temperature in check
Store and use your battery in shaded, well-ventilated areas, and choose self-heating lithium batteries if you’re operating in cold weather.
- Charge and discharge the right way
Avoid letting your battery run flat, use a compatible charger (especially for LiFePO4), and unplug it once fully charged to prevent overcharging.
- Use an efficient inverter for AC devices
Choose a high-efficiency inverter to reduce energy loss when converting DC to AC, helping your battery last longer.
- Add more batteries for bigger jobs
Connect 100Ah batteries in parallel to double capacity and runtime, making sure all batteries are the same type, age, and brand for best performance.
- Recharge with solar power when possible
Add a solar charging setup to top up your battery during the day, especially when you’re off-grid or camping.
Tired of weak power banks and unreliable batteries?
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Our TOPO lithium power systems are built tough for Australian conditions. Whether you’re out on the road, off-grid camping, or need a powerful backup for your tools or tech, our 100Ah LiFePO4 battery systems are ready to go.
Here’s what sets us apart:
- Built-in FAST charging from vehicle, solar, or mains power – even all at once.
- Steel-cased rugged exterior for battery protection & durability.
- Built-in Battery Management System (BMS) for efficient, safe, and long-lasting performance.
- Zero Maintenance with LiFePO4 chemistry to get years of reliable use without the fuss.
So, don’t settle for average! Power up with TOPO’s lithium power systems today
FAQs About 100Ah Battery Lifespan
- How long does a 100Ah battery last?
It depends on how much power you’re using. For example:
100Ah ÷ 5A = 20 hours
That means if your device draws 5 amps, your battery will last about 20 hours. Always check your devices’ power draw to estimate usage time and avoid unexpected shutdowns.
- Can I use a 100Ah battery for solar power storage?
Absolutely. A 100Ah LiFePO4 battery is an excellent option for solar systems. It’s efficient, long-lasting, and can be recharged daily using your solar setup, perfect for off-grid living, campers, or tiny homes.
- What is the average lifespan of a 100Ah battery?
With proper care, LiFePO4 100Ah batteries can last over 2,000–5,000 charge cycles, mostly more than 10 years. That’s much longer than traditional lead-acid batteries, making them a smart, long-term investment.
- How long will a 100Ah battery run a fridge?
That depends on the fridge’s power draw. If your fridge uses around 1 amp per hour (common for efficient 12V fridges), a 100Ah battery could run it for up to 100 hours. If it draws more, like 5 amps, you’re looking at around 20 hours of runtime.
- Can I run a TV on a 100Ah battery?
Yes, a 100W TV could run for about 11–12 hours on a 100Ah lithium battery.