A 100Ah battery is a popular choice for off-grid power, backup systems, and solar energy storage. But how long will it last? The answer depends on factors such as voltage, power consumption, and efficiency losses. In this guide, we'll walk you through the calculations needed to estimate 100ah battery backup time.
Factors Affecting 100Ah Battery Runtime
100Ah meaning on a battery
The "100Ah" rating indicates the battery can theoretically deliver 100 amperes for one hour, or proportionally less current for longer periods (e.g., 10A for 10 hours). In practice, factors like discharge rate, temperature, and battery age affect actual performance, making real-world runtime calculations more complex than simple division.
Battery types and DoD
Different battery chemistries have varying Depth of Discharge (DoD) limits that significantly impact usable capacity. Lithium batteries can safely discharge 80-90% of their rated capacity, providing 80-90Ah of usable energy from a 100Ah battery. Lead-acid batteries should only be discharged to 50% DoD to prevent damage, offering just 50Ah of usable capacity. This difference dramatically affects actual runtime calculations.
Loads running with 100Ah battery
The type and power consumption of connected devices directly determine battery runtime. Resistive loads like LED lights provide consistent power draw, while motor-driven appliances like refrigerators have variable consumption with high startup surges.
Steps to calculate how long will a 100Ah battery last
In theory, the battery backup time can be calculated by dividing the battery capacity by the total load current (Ah ÷ A). For example, a 100Ah battery with a 5A draw will last approximately 20 hours under ideal conditions.
However, in a real-world power system—especially one that includes household appliances—the current consumption is often unstable, and some devices may not specify their exact current rating in the specifications. In such cases, the battery runtime can be estimated based on the load power (W).
Below, we provide the specific steps to help you calculate the battery backup time.
Step1 - Calculate how many kwh in a 100Ah battery
The first step is to convert the battery's Ah capacity into Wh, making it easier to calculate how long will a 100ah battery last in hours based on the load’s rated power.
- Total energy (Wh) = Battery capacity (Ah) × Battery voltage (V)
For example, a 12V 100Ah battery provides 1200Wh, while a 24V 100Ah battery offers 2400Wh, and a 48V 100Ah battery delivers 4800Wh.
Since fully discharging a battery shortens its lifespan, it’s recommended to stay within a specific Depth of Discharge (DoD). Lithium batteries typically allow 80-90% usage of their capacity, while lead-acid batteries are generally limited to 50%.
The formula below estimates the usable energy while accounting for the Depth of Discharge (DoD).
Available energy (Wh) = Battery capacity (Ah) × Battery voltage (V) × DoD%
Step2 - Plan what you will run off a 100Ah battery
Next, calculate the total wattage of the devices you plan to run. Every appliance has a specific power rating in watts (W), which indicates how much energy it consumes per hour. For example, a 100W TV running for 5 hours will use 500Wh (100W × 5h).
Adding up the power of all devices helps estimate how long your battery can sustain them.
Additionally, inverter efficiency and power surge consumption can impact battery runtime. To ensure reliable performance, you can add a margin to account for these factors.
Step3 - Work out how long will a 100ah last in hours
Finally, you can figure out the battery backup time by using the formula:
Battery Runtime (hours) = (Battery Capacity × Voltage × DoD) ÷ Total Load Power (W)
For example, a 12V 100Ah battery running a 200W device at 80% DoD would last 4 hours 48 minutes. ((12V × 100Ah × 0.8) ÷ 200W).
Similarly, a 24V 100Ah battery powering a 400W load would also last 4 hours 48 minutes.((24V × 100Ah × 0.8) ÷ 400W)
These estimates assume ideal conditions. However, if you factor in inverter efficiency and load surge power, the actual battery runtime will be slightly lower.
Estimated runtime of a 100Ah battery (12V/24V)
Below are estimated runtimes for a 100Ah battery at different voltages and power loads.
| Load Power | 12V Lithium (90% DoD) | 12V Lead-Acid (50% DoD) | 24V Lithium (90% DoD) | 24V Lead-Acid (50% DoD) |
|---|---|---|---|---|
| 100W | 10.8 hours | 6 hours | 21.6 hours | 12 hours |
| 200W | 5.4 hours | 3 hours | 10.8 hours | 6 hours |
| 300W | 3.6 hours | 2 hours | 7.2 hours | 4 hours |
| 500W | 2.2 hours | 1.2 hours | 4.3 hours | 2.4 hours |
Battery Runtime Formula with Efficiency Considerations
Please note that the basic calculator does not account for inverter efficiency, power surges, or energy losses, so the actual battery backup time will be slightly lower.
For more accurate calculations when using an inverter, adjust the formula to account for efficiency losses:
Accurate Runtime (hours) = (Battery Capacity × Voltage × DoD × Inverter Efficiency) ÷ Total Load Power
Where inverter efficiency is typically 85-95% (use 0.85-0.95 in calculations).
Conclusion
Several key factors affect how long a battery will last:
- Battery Voltage： Higher voltage batteries (e.g., 12V or 24V) store more energy and last longer than lower voltage ones for the same Ah capacity.
- Depth of Discharge (DoD)： Discharging a battery too much shortens its lifespan. Lithium batteries can safely use 80-90% of their capacity, while lead-acid batteries are typically limited to 50%.
- Load Power Consumption： The higher the wattage of connected devices, the faster the battery drains.
- Inverter Efficiency： Inverters are not 100% efficient; some energy is lost during the conversion from DC to AC, reducing battery runtime.
- Power Surges： Appliances with high startup power requirements (e.g., refrigerators or pumps) can temporarily draw more energy, affecting overall battery backup time.
To get the most out of your battery, consider these factors when planning your system. Using a runtime calculator, choosing the right battery type, and adding a safety margin can help ensure reliable power and extend battery lifespan.
