How To Calculate Battery Run Time, Step By Step
By The Calcumatix Team Reviewed by Calcumatix Editorial Review 3 min read
Quick Answer
Battery run time equals battery capacity in amp-hours multiplied by voltage, divided by the load in watts. If you have a 100Ah 12V battery and a 150W continuous load, first find total energy: 100 multiplied by 12 equals 1,200 watt-hours. Then divide by the load: 1,200 divided by 150 equals 8 hours. That is the theoretical maximum run time before efficiency losses and discharge limits.
A battery’s capacity tells you how much energy it holds, but its run time tells you how long that energy will actually power your devices. Run time is simply the total usable energy inside the battery divided by the power drawn by the load attached to it. Calculating this correctly requires converting amp-hours to watt-hours so the battery and the load are measured in the same unit. This guide walks through the formula to determine how long a battery will last under a continuous load, with a step-by-step example.
How Do You Calculate Battery Run Time Step By Step?
The formula connects what the battery holds (capacity) to what the device pulls (load). Because battery capacity is usually rated in amp-hours (Ah) and device loads are rated in watts (W), the formula uses the battery voltage to convert amp-hours into watt-hours first.
The battery run time formula: Runtime (hours) = (Capacity Ah times Voltage) divided by Load Watts
Step by step:
- Find the battery capacity in amp-hours (Ah). If rated in milliamp-hours (mAh), divide by 1,000 first.
- Find the nominal battery voltage (V), such as 12V or 24V.
- Multiply the capacity by the voltage to find total energy in watt-hours (Wh).
- Find the continuous power draw of your device in watts (W).
- Divide the total watt-hours by the load watts.
- The result is the theoretical run time in hours.
Worked example: Inputs: Battery capacity 50Ah, voltage 12V, device load 60W. Step 1 (total watt-hours): 50 × 12 = 600 Step 2 (run time hours): 600 / 60 = 10
Run time: 10 hours (rounded to the nearest hour)
This produces a theoretical maximum. In practice, no battery delivers 100% of its rated capacity to the load due to chemical and electrical limitations. Use the Battery Runtime Calculator to model this calculation automatically.
What Factors Reduce Actual Battery Run Time?
The basic formula assumes perfect efficiency and complete discharge. Actual run time is always lower than the theoretical calculation because of four physical limits.
Depth of discharge (DoD). Draining a lead-acid battery to 0% permanently damages it. Most lead-acid systems are limited to 50% DoD to preserve lifespan, which immediately cuts the theoretical run time in half. Lithium iron phosphate (LiFePO4) batteries can safely discharge to 80% or 90% DoD, offering much longer usable run times from the same rated amp-hour capacity.
Inverter efficiency. If your battery is DC but your load requires AC power through an inverter, the inversion process loses energy as heat. Most inverters operate at 85% to 90% efficiency. That means a 100W AC load actually pulls about 111W from the DC battery, draining it faster.
The Peukert effect. In lead-acid batteries, drawing power very quickly reduces the total energy the battery can deliver. A 100Ah battery might deliver 5 amps for 20 hours, but if you pull 50 amps, it will not last 2 hours. High-current loads shrink the effective capacity. Lithium batteries are largely immune to this effect.
Temperature. Cold temperatures slow down internal chemical reactions, temporarily reducing the available capacity. A battery operated at freezing temperatures will run out faster than one operated at standard room temperature.
How Do You Calculate Run Time Using Only Amps?
If your load is rated in amps instead of watts, the math is simpler because you do not need to convert to watt-hours. This only works if the load amps and battery capacity amp-hours are at the same voltage.
Formula using amps: Runtime (hours) = Capacity Ah divided by Load Amps
If you have a 100Ah 12V battery and a DC appliance drawing 5 amps at 12V, the run time is 100 divided by 5, which equals 20 hours. Do not use this shortcut if an inverter is involved, because the AC load amps and the DC battery amps are at different voltages. See the engineering calculators hub for related sizing tools.
Sources and References
Frequently asked questions
Why did my battery run out faster than the formula predicted?
The formula calculates theoretical maximum capacity. It does not account for the energy lost as heat in the wiring, the efficiency loss if you are using an inverter, or the battery's age. An older battery loses capacity over time, meaning a battery rated for 100Ah when new might only hold 80Ah after two years of heavy use.
How do I find the load watts of my device?
Look at the compliance sticker or power supply brick attached to the device. It will list the maximum input power in watts (W). If it only lists volts and amps, multiply them together (Volts times Amps equals Watts). Note that the listed wattage is a maximum rating; many devices, like laptops or refrigerators, pull far less than their maximum rating during normal continuous use.
What is the difference between watt-hours and amp-hours?
Amp-hours (Ah) measure charge. Watt-hours (Wh) measure total energy. Amp-hours do not tell you how much work a battery can do unless you also know the voltage. A 100Ah 12V battery holds 1,200Wh. A 100Ah 24V battery holds 2,400Wh. Because watt-hours include voltage, they are the best unit for comparing the true run time capacity of different batteries.
Can I run two devices at the same time?
Yes. To calculate the run time for multiple devices, simply add their wattages together to find the total load. If you plug a 40W laptop and a 20W fan into the same battery, the total load is 60W. Use 60W as the load in the formula.
Does a higher voltage battery run longer?
Yes, assuming the amp-hour rating and the load stay the same. A 100Ah 24V battery holds twice as much energy (2,400Wh) as a 100Ah 12V battery (1,200Wh). Against a fixed 100W load, the 24V battery will run exactly twice as long as the 12V battery.