Calculator inputs
Capacity trend graph
The graph maps required battery capacity against runtime using your entered operating limits and design margins.
Example data table
| Scenario | Mode | Load | Voltage | Hours | Efficiency | DoD | Approx. Result |
|---|---|---|---|---|---|---|---|
| Home inverter backup | Power | 600 W | 12 V | 4 h | 90% | 50% | 681.48 Ah |
| Small solar control cabinet | Power | 120 W | 24 V | 8 h | 95% | 80% | 67.04 Ah |
| Mobility electronics pack | Current | 18 A | 48 V | 3 h | 92% | 80% | 86.48 Ah |
| RV service battery | Power | 300 W | 12 V | 6 h | 88% | 60% | 417.45 Ah |
Formula used
Power mode: Capacity (Ah) = (Load Power × Backup Hours × Aging Factor × Expansion Factor) ÷ (System Voltage × Efficiency × DoD × Temperature Factor)
Current mode: Capacity (Ah) = (Load Current × Backup Hours × Aging Factor × Expansion Factor) ÷ (Efficiency × DoD × Temperature Factor)
Efficiency, DoD, and temperature derating are entered as percentages and converted into decimal factors. Aging reserve and future expansion are added as multipliers above one.
How to use this calculator
- Choose whether you know the total load in watts or current in amps.
- Enter system voltage and how long the battery must support the load.
- Add efficiency, depth of discharge, temperature derating, aging reserve, and expansion margin.
- Optionally enter single battery voltage and capacity to estimate series and parallel configuration.
- Press calculate to view results above the form, then export the summary as CSV or PDF.
Frequently asked questions
1. What does battery capacity mean?
Battery capacity is the amount of charge or energy a battery bank can store. Engineers often express it in amp-hours, while system energy is usually described in watt-hours.
2. Why does depth of discharge matter?
Depth of discharge limits how much stored energy you plan to use. Lower discharge improves cycle life, while higher discharge reduces the bank size needed for the same load.
3. Should I size from watts or amps?
Use watts when you know the appliance power. Use amps when you already know current draw on the battery side. The calculator converts both approaches into required bank capacity.
4. Why include efficiency losses?
Losses in inverters, converters, wiring, and charging equipment reduce deliverable energy. Including efficiency keeps the design realistic and avoids undersizing the battery bank.
5. What is temperature derating?
Cold or hot operating conditions can reduce usable battery performance. Temperature derating applies a correction factor so the calculated capacity still meets the required runtime.
6. Why add aging and expansion margins?
Batteries lose performance with age, and many systems grow over time. These margins build reserve into the design, making future operation more reliable.
7. Can this help with series and parallel battery counts?
Yes. Enter a single battery voltage and amp-hour rating, and the tool estimates how many batteries you need in series and how many parallel strings are required.
8. Is the rounded recommended capacity mandatory?
No. It is a practical recommendation that rounds the exact result to a common build size. You can still choose a different commercial battery combination.