| Scenario | Pack (V × Ah) | DoD | Eff. | Load | Usable energy (kWh) | Est. runtime (h) |
|---|---|---|---|---|---|---|
| Backup for home router | 12 V × 100 Ah | 80% | 92% | 30 W | ≈ 0.88 | ≈ 29.3 |
| Small inverter setup | 24 V × 200 Ah | 80% | 90% | 500 W | ≈ 3.46 | ≈ 6.9 |
| 48 V rack battery | 48 V × 100 Ah | 90% | 94% | 1500 W | ≈ 4.06 | ≈ 2.7 |
- Pack voltage = NominalVoltage × SeriesCount
- Pack capacity (Ah) = BatteryAh × ParallelCount
- Total energy (Wh) = PackVoltage × PackAh
- Temp factor (simplified) derates capacity away from 25°C
- Effective Ah = PackAh × PeukertAdj × TempFactor
- Usable energy (AC, Wh) = (PackVoltage × EffectiveAh) × DoD × Efficiency × (1 − Losses)
- Current limit (A) = min(ContinuousC × PackAh, HardCurrentCap)
- Max continuous power (W) = PackVoltage × CurrentLimit × Efficiency × (1 − Losses)
- Runtime (h) = UsableEnergyWh ÷ LoadW
- Enter your battery voltage and amp-hours, then set series and parallel counts.
- Choose a chemistry and adjust depth of discharge for your longevity target.
- Set inverter efficiency and estimated wiring/system losses.
- Add your continuous and peak load wattage from device labels or a power meter.
- Review max power limits and runtime; fix any warning items.
- Optionally add cost and cycle life to estimate cost per delivered kWh.
- Export CSV for records or PDF for sharing and reporting.
Usable energy vs nameplate rating
Nameplate watt‑hours equal pack voltage × amp‑hours. This calculator converts that to usable AC energy using depth of discharge, inverter efficiency, and wiring losses. For example, a 24 V × 200 Ah bank stores 4,800 Wh. At 80% DoD, 90% inverter efficiency, and 2% losses, usable energy is about 3.46 kWh.
Continuous and peak power limits
Power is constrained by allowable discharge current. Continuous limit uses C‑rate × pack Ah (and an optional hard current cap), then converts DC watts to AC watts with efficiency and losses. A 48 V, 100 Ah pack at 1C can supply ~4,512 W AC at 94% efficiency and 2% losses, while a 2C peak rating doubles short‑term capability.
Temperature and high‑current derating
Cold conditions reduce available capacity. The model derates below 25°C by ~0.5% per degree (floored at 70%). Lead‑acid options also apply a Peukert adjustment: higher discharge currents reduce effective Ah. At heavy loads, Peukert can cut runtime materially, even when the nameplate capacity looks adequate.
Runtime curve for load planning
Runtime is computed as usable energy (Wh) divided by your continuous load (W). Because runtime is inversely proportional to load, doubling load roughly halves runtime. The included Plotly chart visualizes this curve up to the estimated continuous power limit and marks your chosen load point for quick scenario checks.
Cost per delivered kWh
If you enter battery cost and expected cycle life, the calculator estimates cost per delivered kWh as Cost ÷ (Usable kWh × Cycles). A $2,500 battery delivering 3.5 kWh per cycle for 2,000 cycles yields about $0.36 per kWh. Comparing that to your grid price helps approximate simple breakeven cycles. For critical loads, add a 15–25% headroom margin, and confirm inverter surge rating, cable gauge, and BMS limits before finalizing your design safely.
1) What does “usable energy” represent?
Usable energy is the AC energy you can realistically deliver after applying depth of discharge, inverter efficiency, and estimated system losses. It is lower than nameplate watt‑hours.
2) Why can my load exceed runtime expectations?
High loads increase current draw, which can trigger voltage sag, heating, and protective limits. Lead‑acid batteries may also lose effective capacity at high current due to the Peukert effect.
3) How should I choose depth of discharge?
Lower DoD usually improves cycle life. Lead‑acid users often target 50–60% DoD, while many lithium packs can operate at 80–90% depending on the warranty and operating temperature.
4) What is the difference between continuous and peak power?
Continuous power is what the battery can sustain without overheating or hitting limits. Peak power is a short surge capability for motor starts or compressor inrush and should match inverter surge ratings.
5) Can I rely on the temperature factor?
Treat it as a planning estimate. Real performance depends on chemistry, internal resistance, enclosure insulation, and battery age. Always validate cold‑weather capability with the battery datasheet.
6) How is cost per delivered kWh calculated?
The calculator divides battery cost by total lifetime energy delivered: usable kWh per cycle multiplied by expected cycle life. It helps compare storage economics to your grid price, excluding installation and maintenance.