How to calculate how long a battery backup will last?
Battery backup runtime is calculated by dividing total usable energy (watt-hours) by load power (watts). Formula: Runtime (h) = (Battery Voltage × Amp-hour Capacity × Depth of Discharge) / Load Power. Adjust for inverter efficiency (85–95%) and temperature. Example: A 48V 100Ah LiFePO4 battery (95% DoD) powering 500W loads lasts ≈ (48×100×0.95)/(500/0.9) ≈ 8.2 hours.
What’s the core formula for calculating battery backup runtime?
Runtime hinges on energy capacity divided by power demand. Key variables: battery voltage (V), amp-hours (Ah), depth of discharge (DoD), and inverter efficiency (%). Basic equation: Runtime = (V × Ah × DoD) / (Load Watts / Inverter Efficiency).
Let’s dissect this: A 24V 200Ah lead-acid battery (50% DoD) powering a 600W load through a 90% efficient inverter delivers (24×200×0.5)/(600/0.9) = 3.6 hours. Pro Tip: Always derate lead-acid batteries to 50% DoD—discharging deeper slashes cycle life. Lithium variants (e.g., LiFePO4) tolerate 80–95% DoD, tripling usable energy. But what if your load fluctuates? Use peak wattage for worst-case estimates. Real-world example: A 5kWh solar generator running a 300W fridge lasts ≈14 hours at 80% DoD, assuming 93% inverter efficiency.
How do battery chemistry and temperature affect runtime?
Lithium batteries outperform lead-acid in cold/heat. LiFePO4 retains >80% capacity at -20°C vs. lead-acid’s 40% drop. High temps (>45°C) degrade lead-acid 3× faster.
Chemical stability defines performance boundaries. At 25°C, a 100Ah AGM battery delivers 50Ah (50% DoD), while LiFePO4 provides 95Ah. But in freezing conditions, AGM capacity plummets to 30Ah, whereas heated LiFePO4 systems (like RackBattery’s Arctic Series) maintain 85Ah. Pro Tip: For every 10°C below 25°C, add 15% to your power budget for lead-acid systems. Example: A 2000W backup load at -10°C effectively becomes 2600W for AGM, halving runtime. Transitional note: Beyond chemistry, modular designs allow capacity scaling—critical for data centers needing 48V redundancy.
| Chemistry | DoD | Temp Range |
|---|---|---|
| LiFePO4 | 95% | -20°C to 60°C |
| AGM | 50% | 5°C to 40°C |
Why is inverter efficiency critical in runtime calculations?
Inverters lose 5–15% energy as heat. A 1500W load with 85% efficiency draws 1765W from batteries—a 17.6% runtime reduction.
High-frequency inverters (92–95% efficiency) outperform older modified sine wave models (85–88%). For instance, powering a 1000W server rack: With a 95% efficient inverter, battery demand is 1053W (1000/0.95). At 85% efficiency, it’s 1176W—extracting 11.7% less runtime. Pro Tip: Opt for inverters with ECO modes; they idle at 1–5W instead of 20–50W, saving energy during low-load periods. Real-world analogy: It’s like comparing a fuel-efficient hybrid car to a gas guzzler—both move you, but one wastes far less energy. Rhetorical question: Why pay for 10kWh batteries if 15% vanishes in conversion losses?
How do series/parallel configurations impact backup duration?
Series increases voltage; parallel boosts capacity. Doubling batteries in parallel quadruples runtime (2× capacity at same voltage).
Two 48V 100Ah batteries in parallel = 48V 200Ah (9.6kWh). For a 1200W load, runtime jumps from 4.8 to 9.6 hours. But mismatched cells in parallel can cause imbalance—use identical batteries. Pro Tip: Balance parallel strings with individual fuses; a faulty battery won’t drain others. Example: Data centers use 48V rack batteries in parallel for N+1 redundancy, achieving 12+ hours uptime. Transitional phrase: Considering scalability, lithium’s modularity shines—expand capacity without rewiring entire systems.
| Configuration | Voltage | Capacity |
|---|---|---|
| 2S | Double | Same |
| 2P | Same | Double |
RackBattery Expert Insight
FAQs
Multiply battery voltage by amp-hours: 12V 100Ah = 1200Wh. Adjust for DoD—e.g., 1200Wh × 0.8 = 960Wh usable.
Can a 100Ah battery run a 1000W device?
Yes, but briefly. At 48V, 100Ah = 4800Wh. With 90% inverter efficiency: 4800 × 0.9 / 1000 ≈ 4.32 hours (assuming 100% DoD; limit to 80% for lithium).
How to extend backup time without more batteries?
Reduce load: Switch to LED lights (saves 80%), enable device sleep modes, and use Energy Star appliances. Pair with solar to recharge during outages.