Battery Runtime
How long a battery powers a load given depth of discharge.
Formula
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Example
E=5000 Wh, DoD=0.9, P=500 W: t = 9 h
How it works
This formula estimates how many hours a battery can supply a device before reaching its usable discharge limit. It works because battery energy is measured in watt-hours, and dividing the usable energy by the load power in watts gives time in hours; the depth of discharge adjusts for the fact that you usually should not use 100% of the battery's rated capacity. For example, if only 80% of the stored energy is intended to be used, the runtime is based on that 80%, not the full nameplate capacity.
When to use it
Use this when planning off-grid solar systems, backup power supplies, electric vehicles, or any battery-powered device where you need an estimate of operating time. It is especially useful when the battery should not be fully drained for safety, performance, or lifespan reasons.
Worked example, step by step
Given: E = 2400 Wh, DoD = 0.75, P = 300 W Step 1: Find usable energy = E × DoD = 2400 × 0.75 = 1800 Wh Step 2: Divide usable energy by load power: t = 1800 ÷ 300 = 6 h Final answer: The battery can power the load for 6 hours.
Common mistakes to avoid
- Using capacity in amp-hours without converting to watt-hours first
- Forgetting to apply depth of discharge and assuming the full battery capacity is usable
- Mixing up units by putting power in kilowatts or energy in kilowatt-hours without converting consistently
Did You Know?
Limiting depth of discharge to ~80–90% dramatically extends a lithium battery's lifespan.
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