Battery Runtime Calculator
What the Battery Runtime Calculator does
The Battery Runtime Calculator estimates how long a battery can supply a constant electrical load. It combines nominal battery voltage and rated capacity with your assumptions about usable capacity and delivery efficiency, then divides the available energy by the load’s power draw.
The result, labeled Estimated Runtime, is shown in hours to two decimal places. For example, a result of 9.00 hours means the calculator estimates nine hours of operation under the values entered.
This can be useful when planning power for automotive accessories, camping equipment, backup lighting, or other devices connected to a battery. It is an energy estimate, so the result depends on how closely your inputs reflect the battery, equipment, and operating conditions.
The default values are assumptions, not recommendations or guarantees. Replace them with figures appropriate for your battery and setup. In particular, the amount of capacity you can reasonably use depends on the battery and how you intend to operate it.
How to use the Battery Runtime Calculator
Enter the following five values to calculate an estimated runtime:
- Nominal Battery Voltage (V): Enter the battery’s nominal voltage in volts. The default is 12 V, and the minimum is 0.1 V.
- Rated Battery Capacity (Ah): Enter the rated capacity in amp-hours. Check the battery label or manufacturer’s specifications for a suitable value. The default is 100 Ah, and the minimum is 0 Ah.
- Usable Capacity (%): Enter the percentage of rated capacity you plan to make available to the load. The default is 50%, and the allowed range is 0% to 100%.
- Delivery or Conversion Efficiency (%): Enter an estimate of how much usable battery energy reaches the load after relevant delivery or conversion losses. The default is 90%, and the allowed range is 1% to 100%.
- Constant Load Power (W): Enter the load’s power draw in watts. The default is 60 W, and the minimum is 0.1 W.
Read the Estimated Runtime in hours. If you are powering several devices at the same time, add their assumed wattages and enter the combined constant load. Use a representative figure for the period you want to estimate; equipment that cycles on and off may not draw its listed power continuously.
If you change a value, consider whether the other assumptions still fit. For example, adding an inverter may affect the efficiency you enter, while adding another device increases the total load.
How the Battery Runtime Calculator formula works
The calculator first estimates the battery’s nominal energy in watt-hours by multiplying voltage by rated amp-hour capacity. It then applies the usable capacity and efficiency percentages before dividing by the constant load in watts.
Estimated Runtime (hours) = Voltage × Capacity (Ah) × (Usable Capacity ÷ 100) × (Efficiency ÷ 100) ÷ Load (W)
Using the default inputs, the calculation is:
- Nominal voltage: 12 V
- Rated capacity: 100 Ah
- Usable capacity: 50%
- Delivery or conversion efficiency: 90%
- Constant load: 60 W
First, 12 V × 100 Ah gives 1,200 Wh of nominal energy. Applying 50% usable capacity gives 600 Wh. Applying 90% efficiency leaves an estimated 540 Wh delivered to the load. Finally, 540 Wh ÷ 60 W gives an Estimated Runtime of 9.00 hours.
These figures illustrate the calculation; they do not establish how long a particular battery will run. If the usable capacity is set to 0%, or the rated capacity is 0 Ah, the formula returns zero hours.
Use cases for the Battery Runtime Calculator
A battery runtime calculator can help you compare setups before connecting equipment or buying a battery. Useful examples include:
- Vehicle accessories: Estimate how long a constant accessory load might run from a battery while the engine is off. Consider the energy you need to retain for your vehicle separately.
- Camping and travel: Estimate the runtime of lights, fans, or other equipment connected to an auxiliary battery.
- Backup power: Compare estimated runtimes for a constant load under different battery capacities or efficiency assumptions.
- Load planning: See how reducing the combined wattage changes the estimated time available.
- Battery comparisons: Compare nominal voltage and rated capacity figures while keeping the other assumptions consistent.
For example, if every other input stays the same, doubling the constant load from 60 W to 120 W cuts the calculated runtime from 9.00 hours to 4.50 hours. Likewise, reducing that load to 30 W gives a calculated 18.00 hours. These are mathematical comparisons based on the same assumptions; actual battery performance can vary with the load and conditions.
Other factors to consider when calculating Estimated Runtime
Rated capacity is measured under specified conditions. A battery’s available capacity can differ from its label rating depending on temperature, age, condition, and discharge rate. Consult the battery manufacturer’s specifications when choosing inputs.
Usable capacity is a planning assumption. Entering 100% tells the formula to use the entire rated capacity, but that does not establish whether doing so is suitable for your battery. The calculator does not determine a chemistry-specific safe discharge level or predict exact battery lifespan.
Efficiency depends on the equipment. Inverters, converters, wiring, and other components can introduce losses. Use an efficiency estimate that reflects the path between the battery and the load. Avoid counting the same loss twice when choosing the wattage and efficiency inputs.
Power draw may change over time. A refrigerator, pump, or fan may cycle, and some devices draw different amounts of power in different operating modes. This calculator uses one constant wattage; it does not simulate changing loads or brief starting surges.
Voltage changes during discharge. The formula uses nominal voltage as an energy estimate. It does not infer state of charge from a voltage reading or model voltage drop throughout the discharge. It also does not model Peukert behavior, which can affect available capacity at different discharge rates.
For a vehicle’s starting battery, retain an appropriate reserve and follow the battery and vehicle manufacturers’ guidance. This calculator does not model engine starting or tell you whether enough power remains to start an engine.
Battery Runtime Calculator FAQ
How do I convert battery amp-hours to watt-hours?
Multiply the battery’s nominal voltage by its rated amp-hour capacity. A 12 V, 100 Ah battery has an estimated nominal energy of 1,200 Wh before applying usable capacity and efficiency assumptions. Actual delivered energy may differ.
How long will a 12 V, 100 Ah battery run a 60 W load?
With the calculator’s illustrative defaults of 50% usable capacity and 90% efficiency, the result is 9.00 hours. A different usable capacity, efficiency, battery condition, or load can change the estimate.
What usable capacity percentage should I enter?
Use a value appropriate for your battery and intended operation, based on manufacturer guidance and your need for reserve energy. The default 50% is an editable assumption; the calculator does not select a safe discharge level for a particular battery chemistry.
Does the calculator account for inverter losses?
You can account for an estimated conversion loss through the Delivery or Conversion Efficiency input. The calculator does not identify or measure inverter losses automatically, so enter a figure appropriate for your equipment and setup.
Can I use the result to tell whether my car will start?
No. The result estimates energy available to a constant load; it does not calculate the power needed to start an engine or assess the battery’s starting condition. Allow for your starting requirements separately and follow the relevant manufacturer’s guidance.