How to Calculate Battery Runtime
Estimating how long a battery will power a given load requires more than dividing capacity by current. This guide walks through the complete calculation including efficiency, depth of discharge, and voltage.
The Basic Principle
Battery runtime is the ratio of usable energy to the power demanded by the load. Usable energy depends on the battery's rated capacity, its nominal voltage, and how much of it you are willing to discharge (DoD limit).
The load power seen by the battery is higher than the load itself because the inverter and wiring consume a fraction of the energy. This is why system efficiency must be factored in.
Runtime Calculation
The efficiency factor accounts for inverter conversion losses, cable resistance, and battery management system overhead.
Worked Example
Given:
- Battery: 100 Ah at 12.8V (LFP 4S pack)
- DoD limit: 80%
- System efficiency: 90% (inverter + wiring)
- Continuous load: 250 W
Step 1: Calculate usable energy:
Step 2: Calculate effective load seen by battery:
Step 3: Divide usable energy by effective load:
Factors That Reduce Runtime
Temperature
At 0°C, lithium capacity drops roughly 10%. At -20°C, it can drop 30% or more. Lead-acid is even more sensitive to cold.
Peukert Effect
Lead-acid batteries deliver less total energy at high discharge rates. A 100 Ah lead-acid battery at 1C may only deliver 60-70 Ah of usable capacity.
Battery Age
As batteries age, internal resistance increases and capacity fades. A 5-year-old battery may deliver 80% of its original capacity, directly reducing runtime.
Voltage Sag
Under heavy load, battery terminal voltage drops. If the inverter's low-voltage cutoff triggers early, the battery is disconnected before its rated capacity is fully depleted.
Try It
Use the Runtime Calculator to estimate discharge duration for your specific system.
Open Runtime CalculatorRelated
Need to size the battery first? Use the Battery Sizing Calculator to determine the required capacity.
Open Sizing CalculatorFrequently Asked Questions
Why does the runtime calculator ask for efficiency?
Inverters, DC-DC converters, and cable resistance all consume power between the battery and the load. A 90% efficient inverter means 10% of your battery energy is lost as heat before it reaches the load. Ignoring efficiency overestimates runtime.
How does temperature affect runtime?
Cold temperatures increase internal resistance and reduce the electrochemical reaction rate. At 0°C, a lithium battery may deliver 90% of its rated capacity. At -20°C, usable capacity can drop to 70% or less, directly reducing runtime proportionally.
What is Depth of Discharge (DoD)?
DoD is the percentage of total battery capacity that has been discharged. Discharging to 80% DoD means 80% of the capacity was used. Most lithium batteries should not be regularly discharged beyond 80-90% DoD to preserve cycle life.
Why is runtime different from capacity divided by load?
Simple division (capacity / load) assumes 100% efficiency, 100% DoD, and ideal conditions. Real-world runtime accounts for inverter losses, cable resistance, temperature effects, and the fact that you typically don't discharge to 0%.