Inputs
Formula used
The calculator estimates usable battery energy and divides by average load. Optional Peukert correction estimates runtime under high discharge current.
- System voltage: Vsys = Vbatt × Series
- Total capacity: Ahtotal = Ahbatt × Parallel
- Nominal energy: Whnom = Vsys × Ahtotal
- Total efficiency: η = (1 − Losses) × (Inverter η if AC)
- Usable energy: Whuse = Whnom × DoD × Health × Temp × η
- Runtime: Hours = Whuse ÷ Loadavg
- Peukert (optional): t = tref × (Iref/I)k × DoD × Health × Temp
How to use this calculator
- Enter battery voltage, capacity, and the series/parallel configuration.
- Set a realistic depth of discharge and derating factors.
- Add your load and duty cycle to estimate average power.
- If your load is AC, keep the inverter option enabled.
- Enable Peukert when discharge current is high or uncertain.
- Submit and use the export buttons to share results.
Example data table
| Scenario | Bank (V × Ah) | DoD | Avg load (W) | Usable energy (kWh) | Runtime (h) |
|---|---|---|---|---|---|
| Home essentials | 24 V × 400 Ah | 80% | 600 | 7.06 | 11.76 |
| Small office | 48 V × 200 Ah | 70% | 900 | 5.72 | 6.36 |
| Telecom backup | 12 V × 300 Ah | 60% | 120 | 1.97 | 16.42 |
Practical notes
- Use average power, not peak surge, when estimating autonomy.
- Cold temperatures and aging reduce effective capacity; use derating to stay conservative.
- For mixed loads, calculate a weighted average load or use duty cycle.
- If you need precise engineering, validate with manufacturer discharge curves.
Autonomy drivers and load profiling
Autonomy starts with average watts, not nameplate peaks. If a 900 W appliance runs 40% of the time, the modeled average is 360 W. With 7 kWh usable energy, that profile yields about 19 hours. Tight load audits commonly reduce oversizing by 10 to 25% versus guessing. For mixed circuits, group loads into critical, intermittent, and optional tiers, then model each tier separately to avoid overlap during audits.
Bank configuration and usable energy
Series wiring increases voltage; parallel wiring increases amp hours. Four 12 V, 200 Ah batteries as 2S2P become 24 V and 400 Ah, or 9.6 kWh nominal. Applying 80% discharge, 95% health, and 95% temperature derating produces about 6.9 kWh before conversion losses. Lithium banks may permit 90% discharge, yet many budgets use 80% plus 5% contingency.
Losses and derating assumptions
Conversion losses often dominate small systems. A 92% inverter and 3% extra losses combine to 89% effective delivery. On a 600 W average AC load, the bank supplies roughly 670 W. Improving inverter efficiency from 88% to 94% can add about 7% runtime, holding everything else constant. Reducing wiring loss by 2% has a similar effect at high loads.
Peukert correction under high current
Lead acid capacity falls as discharge current rises. Using a 20 hour rating and k equals 1.15, a bank that delivers 20 A may deliver fewer amp hours at 80 to 120 A. The correction is most useful when current exceeds 0.3C, such as 120 A on a 400 Ah bank. For lithium chemistries, k is often near 1.05, so the adjustment is smaller.
Cost metrics for planning decisions
Finance outputs translate autonomy into comparable unit costs. If a bank costs $2,000 and provides 6.5 kWh usable, that is about $308 per usable kWh. With 2,000 cycles and a 10 hour runtime, cost per backup hour trends near $0.10. At 50 backups per year, cycle life supports about 40 years of events, before calendar aging limits apply.
FAQs
1) What does “average load” mean?
It is the load’s time weighted power. If a 500 W device runs half the time, its average is 250 W. Use duty cycle to capture cycling equipment, then sum averages across devices for a realistic autonomy estimate.
2) When should I enable Peukert correction?
Enable it for lead acid banks when discharge current is high, especially above about 0.3C. It estimates the capacity loss at heavier currents. For many lithium banks, the effect is smaller but can still add conservatism.
3) How do series and parallel change results?
Series increases system voltage, which reduces current for the same power. Parallel increases total amp hours, which increases stored energy. The calculator combines both to compute nominal watt hours and then applies your derating and loss settings.
4) Why does inverter efficiency matter?
AC loads require conversion. If efficiency is 90%, the bank must supply about 11% more power than the load uses. Higher conversion losses raise battery current, which can also worsen Peukert effects on some chemistries.
5) What depth of discharge should I use?
Use the value tied to your warranty or cycle life target. Many planners use 50 to 80% for lead acid and 70 to 90% for lithium. Lower depth of discharge reduces usable energy but often extends service life.
6) How should I read cost per backup hour?
It spreads the bank cost across expected cycles and the runtime per event. It helps compare scenarios, chemistries, and sizing options on a common basis. Real projects should also account for installation, maintenance, and calendar aging.