Battery & Load Inputs
Runtime Results
Total Capacity
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Wh
Usable Energy
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Wh
Delivered to Load
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Wh
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Estimated Runtime
CHARGE LEVEL INDICATOR (based on 8-hour reference)
0 h4 h8 h+
Total Ah (parallel banks)—
Effective Voltage (series)—
Usable Ah (after DoD)—
Discharge Rate (C-rate)—
C-rate Interpretation—
What is Battery Capacity (Ah) and Runtime?
Battery capacity in ampere-hours (Ah) tells you how much charge a battery holds, not directly how long it will power a load. To find runtime, you must account for three factors:
- Voltage: Energy (Wh) = Ah × Volts. A 100Ah 12V battery holds 1,200 Wh; a 100Ah 48V battery holds 4,800 Wh.
- Depth of discharge (DoD): The fraction of capacity you can safely use without degrading the battery. Lead-acid: 50%, AGM: 60%, Lithium LiFePO4: 80–90%.
- Inverter efficiency: AC inverters waste energy as heat — typically 85–92%. A 1000 Wh battery may only deliver 850–920 Wh to your AC load.
Runtime (hours) = (Ah × V × DoD% × Efficiency%) / Load (W). This calculator applies all three corrections automatically.
Lead-Acid vs AGM vs Lithium Battery Comparison
| Type | Typical DoD | Cycle Life | Weight | Relative Cost | Best Use |
|---|---|---|---|---|---|
| Lead-Acid (Flooded) | 50% | 300–500 cycles | Heaviest | Lowest | Large stationary banks, off-grid |
| AGM (Sealed) | 60% | 400–600 cycles | Heavy | Medium | UPS, marine, RV, solar |
| Lithium (LiFePO4) | 80–90% | 2000–5000 cycles | Lightest | Highest | Portable, EV, solar storage |
| Li-ion (NMC/NCA) | 80% | 500–1500 cycles | Light | High | Laptops, consumer electronics |
Common UPS Sizing Examples
| Load Scenario | Battery Bank | Runtime Example |
|---|---|---|
| Home office PC + monitor (300 W) | 12V 100Ah AGM × 1 | ≈ 2.4 h (60% DoD, 85% eff.) |
| Server room, small rack (1000 W) | 48V 100Ah Li × 1 | ≈ 3.3 h (80% DoD, 90% eff.) |
| Home essentials: fridge + lights (500 W) | 12V 200Ah AGM × 2 parallel | ≈ 4.9 h (60% DoD, 85% eff.) |
| Telecoms site, critical (200 W) | 48V 100Ah AGM × 1 | ≈ 10.2 h (60% DoD, 88% eff.) |
| CCTV + NVR system (150 W) | 12V 100Ah Li × 1 | ≈ 5.4 h (80% DoD, 90% eff.) |
Frequently Asked Questions
A 100Ah 12V battery stores 1,200 Wh of energy. Usable runtime depends on battery type and inverter efficiency. For lead-acid at 50% DoD with 85% inverter efficiency: usable = 1,200 × 0.50 × 0.85 = 510 Wh. A 200 W load runs for 510 / 200 = 2.55 hours. For AGM at 60% DoD: 612 Wh → 3.06 h. For lithium at 80% DoD: 816 Wh → 4.08 h on the same load. Use the calculator above to get the exact figure for your setup.
Depth of discharge is the percentage of a battery's rated capacity that is actually used before recharging. Discharging a lead-acid battery beyond 50% DoD significantly shortens its lifespan — regularly going to 100% DoD reduces a lead-acid battery to just 200–300 cycles. Staying within 50% DoD can achieve 500–700 cycles. Lithium LiFePO4 batteries tolerate 80–90% DoD with minimal cycle degradation, often achieving 2,000–5,000 cycles, making the higher upfront cost worthwhile for frequent-use systems.
UPS manufacturers typically rate runtime at a specific reference load under ideal conditions with a new, fully charged battery at 25°C. In practice, runtime is shorter for several reasons: battery capacity decreases with age (often 20–30% less after 3 years), cold temperatures reduce lead-acid capacity by 20–40%, high discharge rates reduce effective capacity (Peukert effect), and internal inverter losses are understated in marketing specs. Use this calculator with realistic efficiency (85%) and your battery's actual DoD for a more accurate real-world estimate.
Use the Size Battery Bank tab above. As a rough guide: Required Wh = Load (W) × Hours / Efficiency. For a 500 W load, 4 hours, 85% efficiency: Required Wh = 500 × 4 / 0.85 = 2,353 Wh. At 12V with lithium (80% DoD): Required Ah = 2,353 / 12 / 0.8 = 245 Ah. Options: one 250Ah battery, or three 100Ah batteries in parallel (giving 300Ah with margin). Stepping up to 24V halves the required Ah (≈ 123Ah) while keeping the same energy, reducing wiring losses.