How to Calculate Runtime of a Battery
Calculating battery runtime requires understanding how energy flows from the battery through the system to the load. This guide walks through the complete engineering method, from capacity rating to final runtime estimate, accounting for the real-world factors that naive calculations miss.
Why Naive Calculations Fail
The most common approach — dividing Ah by load current — produces wildly optimistic results. A 100Ah battery powering a 10A load appears to last 10 hours. In reality, the system loses energy through the inverter, wiring, and battery management system, and you do not discharge to 0%.
The accurate method accounts for five factors: total energy storage, usable capacity (DoD), system efficiency, load power, and voltage considerations. Each factor reduces the theoretical maximum by a measurable amount.
Complete Runtime Formula
System efficiency includes inverter conversion losses, DC wiring losses, BMS consumption, and any DC-DC converter overhead. A typical value is 85–92%.
Worked Example
Given:
- Battery: 200 Ah at 12.8V (LFP 4S pack)
- DoD limit: 80%
- Load: 300W AC (through pure sine inverter)
- System efficiency: 90%
Step 1: Total energy:
Step 2: Usable energy at 80% DoD:
Step 3: Effective load:
Step 4: Runtime:
The naive calculation (200Ah ÷ 25A = 8 hours) overestimates by 30%.
Step-by-Step Method
Identify Battery Specifications
Record the rated capacity (Ah), nominal voltage (V), and chemistry. LFP nominal voltage is 3.2V per cell (12.8V for 4S). NMC is 3.6–3.7V per cell.
Determine Usable DoD
LFP batteries can safely discharge to 80–90% DoD. Lead-acid should be limited to 50% for cycle life. Set your DoD based on chemistry and cycle life requirements.
Calculate Effective Load
Divide the actual load wattage by system efficiency. For AC loads through an inverter, account for inverter losses (8–15%). For DC loads, account for wiring losses (1–5%).
Divide Usable Energy by Effective Load
The result is your runtime in hours. For loads that cycle on and off (like a fridge), use the average power draw over a 24-hour period rather than the compressor's peak wattage.
Common Mistakes
Ignoring Efficiency
Skipping the efficiency step overestimates runtime by 10–15%. An inverter alone consumes 8–12% of the power, and this is the single largest source of error.
Using Full Capacity
Discharging lead-acid below 50% DoD causes permanent damage. Even lithium batteries degrade faster at 100% DoD. Always apply the chemistry's recommended DoD limit.
Ignoring Temperature
Cold batteries lose 10–30% of rated capacity. If your battery is in an unheated compartment, apply a temperature derating factor of 0.85–0.90 for winter conditions.
Peak vs. Average Power
Motors and compressors draw 3–5× their rated power during startup. Using peak power instead of average power gives a pessimistic runtime estimate. Use average consumption over time.
Try It
Use the Runtime Calculator to estimate discharge duration for your specific system.
Open Runtime CalculatorRelated
Browse Solar, RV, and Home Backup calculators for application-specific sizing.
Solar CalculatorsRelated Articles
Battery Runtime Formula
Reference sheet for all runtime formulas including voltage, efficiency, and temperature corrections.
Read Guide →12V vs 24V vs 48V Runtime
How system voltage affects runtime, efficiency, and cable sizing decisions.
Read Guide →Frequently Asked Questions
What is the simplest way to calculate battery runtime?
Multiply capacity (Ah) by voltage (V) to get total watt-hours, apply your DoD limit, then divide by the effective load (load watts ÷ efficiency). For a 100Ah 12.8V LFP at 80% DoD with a 100W load and 90% efficiency, runtime is (100 × 12.8 × 0.80) ÷ (100 ÷ 0.90) = 9.2 hours.
Why can't I just divide Ah by amps to get runtime?
Simple division ignores efficiency losses, DoD limits, and the Peukert effect. Real systems lose 8–15% through the inverter and wiring, and you typically discharge only to 80% DoD to preserve battery life. This makes actual runtime 30–40% shorter than the naive calculation.
Does runtime change with different battery voltages?
For the same load power, a higher-voltage battery draws less current, reducing cable losses and improving efficiency. A 48V system loses less energy in wiring than a 12V system for the same wattage, so runtime is slightly longer at higher voltages.
How accurate are battery runtime calculations?
Well-executed calculations are typically within 10–15% of measured values. The main sources of error are temperature variations, battery age (SOH), and the Peukert effect in lead-acid batteries. Lithium batteries are more predictable because they maintain voltage more consistently during discharge.