Battery Runtime Calculator
Estimate how long a battery can power a device using its capacity, electrical load, depth of discharge, starting charge and system efficiency. Compare watt-hour, kilowatt-hour and amp-hour ratings without confusing capacity with power.
Battery & Load Inputs
Estimated Runtime
Constant-load estimate, not a guaranteed runtime.
Actual performance depends on battery chemistry, age, temperature, discharge rate and equipment behavior. Battery management systems can cut power earlier than estimated.
Battery Runtime Calculator: Formula and Examples
The Battery Runtime Calculator estimates the operating time of electrical devices using stored battery energy and connected power demand. This Battery Runtime Calculator supports Wh, kWh and Ah inputs, plus reserve and efficiency adjustments. Battery runtime is the estimated duration for which stored energy can support an electrical load. The simplest calculation divides energy in watt-hours by power in watts. For a more useful estimate, account for available capacity and system losses.
Worked example: 1,200 Wh battery and 100 W load
Suppose a battery has 1,200 Wh nominal capacity, starts at 100% charge, must retain a 20% reserve, has an 80% maximum depth-of-discharge setting and delivers energy through a 90%-efficient system. Available capacity is 80%, so usable output energy is 1,200 × 0.80 × 0.90 = 864 Wh. At a constant 100 W load, runtime is 864 ÷ 100 = 8.64 hours, or about 8 hours 38 minutes.
Convert Ah to Wh
Amp-hours alone do not identify battery energy without voltage. For an approximate nominal capacity, multiply amp-hours by nominal volts. A 12 V, 100 Ah battery stores about 1,200 Wh nominally. Real output varies with battery chemistry and discharge conditions.
What is depth of discharge?
Depth of discharge (DoD) is the fraction of the nominal battery capacity used. Battery manufacturers may specify recommended limits. A 100% DoD assumption is not suitable for every chemistry or installation. Use the manufacturer guidance for the specific battery.
Why does efficiency matter?
Inverters and DC converters consume energy. If the battery delivers 1,000 Wh but conversion efficiency is 90%, only about 900 Wh reaches the connected load under this simplified model. Additional idle losses, surges and changing efficiency can affect real runtime.
Series and parallel batteries
Combining battery capacity is valid only when batteries are connected in a compatible configuration, with suitable protection and matched equipment. Never connect batteries solely based on a calculator result. Follow manufacturer instructions and electrical safety requirements.
Constant loads vs changing loads
The estimate assumes constant electrical power. Refrigerators, air conditioners, motors and pumps may cycle or have large starting surges. Use measured average consumption for energy estimates, while separately verifying peak load and inverter capability.
Battery Runtime Calculator Accuracy and Limitations
Use the battery manufacturer’s rated energy and usable capacity, measured equipment consumption, a realistic conversion efficiency and the required reserve. Compare estimates with observed runtime. Temperature, age and high discharge rates can reduce available capacity. The Battery Runtime Calculator is a planning aid, not a certified battery performance test.
Battery Safety and Reference Information
For broader information on energy storage technology and battery research, consult the U.S. Department of Energy energy storage resources. Follow the battery manufacturer’s operating, installation and charging instructions for the actual equipment.
Frequently Asked Questions
How long will a 100 Ah battery run a 100 W appliance?
At 12 V, a 100 Ah battery has about 1,200 Wh nominal energy. Runtime depends on allowed discharge and conversion efficiency. At 80% usable capacity and 90% efficiency, the estimate is about 8.64 hours.
Can I use a kWh battery rating?
Yes. Select kWh and enter the battery’s rated energy. The calculator converts kWh into Wh before calculating runtime.
Does battery runtime depend on voltage?
Voltage is required when converting an amp-hour rating into energy. If capacity is already in Wh or kWh, voltage is not needed for this energy-based runtime estimate.
Why is actual battery runtime shorter?
Battery aging, temperature, conversion losses, high discharge currents, equipment surges and protective shutdowns may shorten actual runtime.
Can I estimate runtime for multiple batteries?
Yes, for compatible identical batteries whose capacities can safely be combined. The calculator multiplies nominal energy by the number entered.
What if the load changes during operation?
Use measured average wattage as a first approximation, but check surge power and manufacturer requirements separately.
Does the Battery Runtime Calculator replace a manufacturer runtime chart?
No. It is an educational approximation. Manufacturer discharge curves and tested runtime charts may better represent the specific battery and load.