Battery Runtime & Capacity Calculator
Use this free battery calculator to estimate how long a battery will run your devices, or how many amp-hours you need for a target runtime.
Typical values: Lead-acid 50%, AGM 50-60%, LiFePO4 80-95%.
How to Use a Battery Calculator
A battery calculator helps you answer two practical questions:
- Runtime: How long will my battery power a device?
- Capacity: How large should my battery be for a target number of hours?
Whether you are planning an RV setup, a solar backup system, marine electronics, or a home UPS, these calculations keep you from undersizing or overspending.
Key Battery Formulas
1) Convert battery capacity to energy (Wh)
Watt-hours (Wh) = Volts (V) × Amp-hours (Ah)
Example: A 12V 100Ah battery has nominal energy of 1,200Wh.
2) Adjust for usable energy
Real systems don’t use 100% of nominal energy. You usually apply:
- Depth of Discharge (DoD): limits how deeply you drain the battery
- System efficiency: inverter and wiring losses
Usable Wh = V × Ah × (DoD/100) × (Efficiency/100)
3) Compute runtime
Runtime (hours) = Usable Wh ÷ Load Power (W)
4) Compute required capacity
Required Ah = (Load W × Desired Hours) ÷ (V × DoD × Efficiency) (DoD and Efficiency as decimals)
Practical Example
Suppose you have a 12V 100Ah lithium battery, a 60W load, 90% system efficiency, and 80% usable DoD:
- Nominal energy = 12 × 100 = 1200Wh
- Usable energy = 1200 × 0.8 × 0.9 = 864Wh
- Runtime = 864 ÷ 60 = 14.4 hours
That means your system can run for roughly 14 hours and 24 minutes under steady conditions.
Battery Chemistry Matters
Lead-acid (Flooded, AGM, Gel)
- Lower upfront cost
- Heavier and bulkier
- Best life when kept around 50% DoD or less
Lithium Iron Phosphate (LiFePO4)
- Higher upfront cost, lower lifetime cost in many cases
- Lighter weight
- Often supports 80-95% DoD and long cycle life
Common Sizing Mistakes to Avoid
- Ignoring inverter losses: AC systems always lose some power in conversion.
- Using nameplate load only: many devices have startup surges.
- No safety margin: temperature, battery age, and cable losses reduce performance.
- Mixing old and new batteries: this can reduce overall bank performance.
Tips for Better Battery Planning
1) Add a safety buffer
Add 15-30% extra capacity for real-world variance.
2) Use realistic duty cycle
If a device cycles on and off, use average wattage over time instead of peak draw.
3) Plan for aging
Batteries lose effective capacity over years. Sizing slightly larger helps maintain performance over the long term.
4) Watch temperature
Cold weather can reduce available capacity significantly, especially in lead-acid systems.
Quick Reference Checklist
- Know your battery voltage (12V, 24V, 48V, etc.)
- Use total load in watts
- Set realistic efficiency (typically 85-95%)
- Choose DoD based on chemistry
- Add reserve capacity
Final Thoughts
A good battery calculator turns guessing into planning. By combining voltage, amp-hours, load power, efficiency, and depth of discharge, you can quickly estimate runtime and required battery size for solar systems, backup power, RV living, and off-grid projects. Use the calculator above to test different scenarios and build a setup that is both reliable and cost-effective.