Battery Run Time Calculator
Estimate how long a battery will power a device from battery capacity, voltage, load, usable capacity, and efficiency.
Battery run time starts with usable watt-hours
A battery run time calculator estimates how long a battery can power a load by converting capacity into watt-hours and dividing by load watts.
Core shortcut: Calculate battery run time by dividing battery capacity in watt-hours (Wh) by device power consumption in watts (W). Battery Run Time = Wh / W.
Reality check: Battery age, inverter efficiency, temperature, discharge rate, cable losses, and battery protection limits can reduce actual run time by 10% to 30% or more.
Estimated Battery Run Time
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Nominal Battery Energy
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Before usable-capacity and efficiency adjustments.
Usable Energy
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After usable capacity and system efficiency.
Load Power
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Equivalent load used for runtime.
Current and C-Rate
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Important: Battery run time is an estimate. High current draw, cold temperature, old cells, low cutoff voltage, and inverter or converter losses can shorten real-world runtime.
How to Use This Calculator
- Enter battery capacity: Use the rating on the battery label, such as Ah, mAh, Wh, or kWh.
- Enter battery voltage: Voltage is required when converting amp-hours or amps into watt-hours and watts.
- Enter the load: Use the device power in watts when available, or use current in amps if that is what the label provides.
- Adjust usable capacity: Use a lower percentage for lead-acid batteries, cold conditions, or a conservative reserve.
- Adjust system efficiency: Reduce efficiency when using an inverter, DC-DC converter, long cables, or other power electronics.
Battery Run Time Formula
Battery run time is calculated by converting battery capacity into watt-hours, adjusting for usable capacity and efficiency, then dividing by the load in watts.
Calculate battery run time by dividing battery capacity in watt-hours (Wh) by device power consumption in watts (W). Use the formula: Battery Run Time = Wh / W.
Battery Wh = Ah x V
Battery Run Time = Wh / W
Usable Wh = Battery Wh x usable capacity x efficiency
Run time hours = Usable Wh / Load W
Example: A 500 Wh battery powering a 100 W device runs for approximately 5 hours before real-world losses. A 12 V 100 Ah battery has 1,200 Wh of nominal energy; if 85% is usable and system efficiency is 90%, usable energy is 918 Wh, so a 60 W load would run for about 15.3 hours.
Sources: OpenStax: Electric Power and Energy and U.S. Energy Information Administration: Measuring Electricity.
Battery Capacity Unit Guide
| Unit | Meaning | Conversion | Common Use |
|---|---|---|---|
| Ah | Amp-hours | Wh = Ah x volts | Car, RV, marine, solar, and deep-cycle batteries |
| mAh | Milliamp-hours | Ah = mAh / 1,000 | Power banks, phones, and small rechargeable packs |
| Wh | Watt-hours | Run time = Wh / W | Laptop batteries, power stations, and energy labels |
| kWh | Kilowatt-hours | 1 kWh = 1,000 Wh | Home backup batteries and large energy storage systems |
Why Real Battery Run Time Can Be Lower
A calculator gives a clean estimate, but batteries are not ideal fuel tanks. Battery age, inverter efficiency, and temperature can reduce actual run time by 10% to 30%. The actual output also depends on cell chemistry, discharge current, state of health, cutoff voltage, wiring, and the device load profile.
High current draw
Large loads can reduce usable capacity, especially for lead-acid batteries and small battery packs.
Cold or aging cells
Low temperature and battery age can reduce capacity, raise internal resistance, and trigger earlier shutdown.
Inverter and cable losses
AC inverters, converters, connectors, and long cables use some energy before it reaches the device.
Practical Runtime Examples
These examples show how load size changes runtime quickly. Use your exact battery voltage, capacity, usable percentage, and device wattage for the best estimate.
Camping fridge
A 12 V 100 Ah battery at 85% usable capacity and 90% efficiency can run a 45 W average load for about 20 hours.
Laptop or router
A 300 Wh power station at 90% usable energy can run a 30 W device for about 9 hours before reserve losses.
Backup lights
A 500 Wh battery with 80% usable energy can run 40 W of LED lighting for about 10 hours.
Usable Capacity by Battery Type
The usable capacity setting is one of the biggest drivers of runtime. Use the table as a search-friendly starting point, then follow the manufacturer's depth-of-discharge limit for your exact battery model.
| Battery Type | Common Usable Range | Runtime Note |
|---|---|---|
| LiFePO4 lithium | 80% to 95% | Often supports deeper discharge, but the BMS may shut down at low voltage. |
| Lithium-ion power bank | 70% to 90% | USB and AC conversion losses can reduce delivered energy. |
| Deep-cycle lead-acid | 40% to 60% | Frequent deep discharge can shorten battery life, so many users keep a larger reserve. |
| Starter battery | 10% to 30% | Designed for short bursts of current, not long appliance runtime. |
Build a Load Inventory Before Estimating Backup Time
Battery backup estimates are more useful when every device load is listed separately. Add the wattage of each appliance, router, light, fan, pump, or charger that will run at the same time, then use the total as the load in the calculator.
Continuous loads
Routers, refrigerators, medical devices, and security systems may run for most of the backup window.
Intermittent loads
Compressors, pumps, and chargers cycle on and off, so use average wattage when you know it.
Surge loads
Motors and compressors can need a higher startup surge than their running wattage, even if average energy use is lower.
Quick method: Total load watts = device 1 watts + device 2 watts + device 3 watts. If a device lists amps, multiply amps by voltage to estimate watts.
Source: U.S. Department of Energy: Estimating Appliance and Home Electronic Energy Use.
Runtime Troubleshooting: Why the Estimate Was Too High
If a battery pack, UPS, solar battery, or power station shuts off earlier than the calculator predicted, compare the setup against these common causes before assuming the battery capacity label is wrong.
Temperature changed the available energy: Cold batteries often deliver less runtime, especially under high load.
The inverter had idle draw: Some inverters consume power even when the appliance load is small, which can matter overnight.
The battery reached cutoff voltage early: High discharge current, old cells, small cables, or a conservative battery management system can trigger shutdown before all nominal energy is used.
Source: National Renewable Energy Laboratory: Battery Lifespan.
Frequently Asked Questions
How does this battery runtime calculator estimate hours?
The calculator converts battery capacity to watt-hours, adjusts for usable capacity and system efficiency, then divides by the load wattage. The basic formula is runtime = usable watt-hour energy / device power. For example, 918 usable Wh divided by a 60 W load gives about 15.3 hours of estimated battery runtime.
How do I convert amp-hour capacity to watt-hour energy?
Multiply amp-hour capacity by battery voltage. A 100 Ah battery at 12 V has 1,200 Wh of nominal energy. A battery pack or power bank may list milliamp-hours instead; convert mAh to Ah first by dividing by 1,000. If the label already lists watt-hour or kWh capacity, you can use that value directly.
Why is usable capacity lower for lithium or lead-acid batteries?
Many batteries should not be fully discharged, and some energy is lost through wiring, voltage conversion, internal resistance, and discharge limits. Lead-acid batteries often need a more conservative usable-capacity setting than lithium batteries, while lithium battery management systems may shut off the pack before every watt-hour is available.
Should I enter watts, amps, or appliance amperage for the load?
Use watts if the device or appliance label provides wattage, because runtime is based on energy divided by power. Use amps when the label lists current or amperage instead. The calculator converts current to watts using voltage, which is why the battery voltage input matters.
Why does inverter efficiency reduce battery backup runtime?
An inverter converts DC battery power to AC power for an appliance or backup device, and that conversion is not perfectly efficient. If an inverter is 90% efficient, about 10% of the battery energy is lost as heat before reaching the AC load, reducing the final runtime estimate.
Can I use this for solar, UPS, power bank, or battery pack estimates?
Yes, for a planning estimate. Enter the solar battery, UPS, power station, power bank, or battery pack capacity in Wh or kWh when available, set the usable capacity, and use the average load in watts. Solar charging during use should be treated separately as added energy because it changes the net discharge rate.
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Disclaimer: This battery run time calculator provides planning estimates only. Follow battery manufacturer limits, charging instructions, ventilation guidance, fuse sizing, and electrical safety practices for real systems.
Last updated: May 11, 2026