1. Enter Capacity
Enter the battery capacity in amp-hours, milliamp-hours, or watt-hours. Add the rated voltage when using Ah or mAh.
Battery Backup Estimator
Battery Runtime Calculator
Estimate how long a battery can power your device using its capacity, voltage, load, usable discharge level, and system efficiency.
Enter Battery Details
Provide your battery and device specifications.
Simple calculation process
How the Battery Runtime Calculator Works
The calculator converts battery capacity into watt-hours and adjusts it for usable discharge capacity and real-world system losses.
2. Add Device Load
Enter the amount of power your equipment consumes in watts or the current draw in amps.
3. Get Runtime
The tool applies efficiency and depth-of-discharge settings to estimate the expected operating time.
What Is a Battery Runtime Calculator?
A Battery Runtime Calculator estimates how long a battery can operate a particular device or electrical system before reaching its selected discharge limit.
It can be useful when planning backup power for routers, security cameras, computers, portable electronics, solar systems, camping equipment, medical devices, inverters, and uninterruptible power supplies.
Information Required for an Accurate Estimate
- Battery capacity in Ah, mAh, or Wh
- Nominal battery voltage
- Total device load in watts or amps
- Number of batteries in the system
- System or inverter efficiency
- Usable battery depth of discharge
Why Actual Runtime Can Be Different
A battery does not always deliver its full rated capacity. Runtime can change because of battery chemistry, temperature, battery age, discharge speed, inverter losses, cable resistance, and changing device power consumption.
Common battery settings
Suggested Depth of Discharge Values
Different battery chemistries have different recommended usable capacity ranges and operating characteristics.
| Battery Type | Typical Usable Capacity | Common Efficiency Setting | General Application |
|---|---|---|---|
| Lithium-ion | 80% to 90% | 90% to 95% | Portable electronics and backup systems |
| LiFePO4 | 85% to 95% | 92% to 98% | Solar storage, RVs, and off-grid systems |
| AGM lead-acid | 50% to 70% | 80% to 90% | UPS, vehicles, and emergency backup |
| Flooded lead-acid | 40% to 50% | 75% to 85% | Solar banks and deep-cycle applications |
Important: These are general planning values. Always
follow the battery manufacturer's recommended discharge limits, charging
instructions, and safety guidelines.
Frequently asked questions
Battery Runtime Calculator FAQs
Find answers to common questions about battery capacity, backup time, system losses, and runtime calculations.
How do I calculate battery runtime?
Convert the battery capacity into watt-hours, multiply it by the usable discharge percentage and system efficiency, and divide the result by the device load in watts.
How long will a 100 Ah battery run a 100-watt device?
A 12 V, 100 Ah battery stores about 1,200 Wh. At 80% depth of discharge and 85% efficiency, it may run a 100-watt device for approximately 8.16 hours.
What is the difference between Ah and Wh?
Amp-hours measure electrical charge, while watt-hours measure stored energy. Watt-hours are calculated by multiplying amp-hours by the battery voltage.
What efficiency value should I use?
For systems using an inverter, a value between 80% and 90% is often suitable for estimation. Direct DC systems may achieve higher efficiency, depending on the equipment and wiring.
Does connecting more batteries increase runtime?
Yes. Increasing the total available battery energy generally increases runtime. The exact effect depends on whether the batteries are connected in series, parallel, or a combined configuration.
Why does my battery run for less time than calculated?
Battery age, cold temperatures, high discharge rates, inverter losses, poor wiring, load surges, and inaccurate capacity ratings can reduce real-world runtime.
Can this calculator be used for a UPS or inverter?
Yes. Enter the total battery capacity, voltage, connected load, and an appropriate efficiency value. Include all equipment powered by the UPS or inverter in the total load.