How Long Will a Battery Last?
Whether you are sizing a backup system, planning an RV trip, or designing a solar installation, estimating battery runtime is one of the most practical engineering questions you can ask. The answer depends on four variables: capacity, voltage, load, and efficiency.
Why Simple Division Is Not Enough
The most common mistake is dividing capacity (Ah) directly by load current (A) to get runtime. This works only if your system is 100% efficient, you discharge the battery to 0%, and the battery delivers its full rated capacity at that discharge rate. In practice, none of these conditions hold.
Real-world runtime is always shorter than the ideal calculation because energy is lost in the inverter, wiring, and battery management system. The battery chemistry also determines how much of the rated capacity is actually available, especially at higher discharge rates.
Runtime Formulas
DoD (Depth of Discharge) represents the percentage of total capacity you are willing to use. Efficiency accounts for inverter conversion losses, cable resistance, and BMS overhead.
Step 1: Determine Usable Energy
Usable energy is the amount of energy your battery can actually deliver before hitting its low-voltage cutoff or DoD limit. Multiply the rated capacity (Ah) by the nominal voltage to get total energy in watt-hours, then multiply by the DoD percentage.
For example, a 200 Ah LFP battery at 12.8V has a total energy of 2,560 Wh. At 80% DoD, the usable energy is 2,048 Wh. A lead-acid battery of the same rating at 50% DoD would only provide 1,280 Wh of usable energy.
Step 2: Account for System Efficiency
The load does not draw power directly from the battery. Energy passes through an inverter (for AC loads), wiring, and potentially a DC-DC converter. Each stage consumes a percentage of the energy as heat.
| Component | Typical Efficiency |
|---|---|
| Pure sine wave inverter | 85–92% |
| DC wiring (short run) | 97–99% |
| DC wiring (long run) | 90–96% |
| MPPT charge controller | 95–98% |
| Total system (typical) | 85–92% |
A system with 90% efficiency means the battery must supply 11% more energy than the load actually consumes. Ignoring this leads to a significant overestimate of runtime.
Worked Example
Given:
- Battery: 100 Ah at 12.8V (LFP 4S pack)
- DoD limit: 80%
- System efficiency: 90%
- Continuous load: 200 W (through an inverter)
Step 1: Usable energy:
Step 2: Effective load seen by battery:
Step 3: Runtime:
Quick Reference: Common Battery Runtimes
The table below shows approximate runtimes for common 12V LFP batteries powering typical loads at 90% system efficiency and 80% DoD.
| Battery | 100W Load | 250W Load | 500W Load |
|---|---|---|---|
| 50 Ah 12.8V | 2.3 hrs | 54 min | 27 min |
| 100 Ah 12.8V | 4.6 hrs | 1.8 hrs | 54 min |
| 200 Ah 12.8V | 9.1 hrs | 3.6 hrs | 1.8 hrs |
| 300 Ah 12.8V | 13.7 hrs | 5.5 hrs | 2.7 hrs |
Factors That Reduce Runtime
Temperature
Cold environments reduce available capacity. LFP loses roughly 10% at 0°C and up to 30% at -20°C. Lead-acid is even more sensitive, losing up to 50% capacity at -20°C.
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. Lithium batteries are far less affected.
Battery Age
As batteries age, capacity fades. A battery at 80% State of Health (SOH) delivers only 80% of its original capacity. After 2,000 cycles, most LFP batteries are at 80% SOH.
Voltage Sag Under Load
Heavy discharge causes terminal voltage to drop. If the inverter's low-voltage cutoff triggers early, the battery is disconnected before it is fully depleted, wasting remaining capacity.
Try It
Use the Runtime Calculator to estimate discharge duration for your specific system configuration.
Open Runtime CalculatorRelated
Need to size the battery first? Use the Battery Sizing Calculator to determine required capacity.
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How to Calculate Battery Runtime
The detailed engineering method for calculating discharge runtime step by step.
Read Guide →How Long Will a 200Ah Battery Last?
Specific runtime examples for 200Ah batteries across common load scenarios.
Read Guide →Frequently Asked Questions
How do I calculate how long my battery will last?
Divide the usable energy (capacity × voltage × DoD) by the effective load (load watts ÷ efficiency). For example, a 100 Ah 12.8V LFP battery at 80% DoD with 90% system efficiency powering a 200W load lasts approximately 4.6 hours.
Does battery chemistry affect how long a battery lasts?
Yes. Lead-acid batteries suffer the Peukert effect, delivering less energy at high discharge rates. Lithium batteries maintain more consistent capacity across discharge rates. LFP chemistry also tolerates deeper discharge, giving more usable energy per cycle.
Why does my battery drain faster in cold weather?
Cold temperatures increase internal resistance and slow the electrochemical reaction. At 0°C, lithium batteries lose roughly 10% of rated capacity. At -20°C, usable capacity can drop 30% or more, directly reducing runtime proportionally.
What reduces battery runtime the most?
The biggest factors are high load power, low system efficiency (poor inverter or long cable runs), deep discharge limits on lead-acid batteries, and cold temperatures. Addressing efficiency losses often yields the largest runtime improvement.