Calculator Inputs
Example Data Table
| Device | Watts | Qty | Util % | Surge Mult | Notes |
|---|---|---|---|---|---|
| Refrigerator | 200 | 1 | 60 | 3.0 | Compressor start surge expected |
| LED Lights | 60 | 6 | 80 | 1.0 | Low surge, steady usage |
| Wi‑Fi Router | 15 | 1 | 100 | 1.0 | Continuous critical device |
Formula Used
- RunningW per device = watts × quantity.
- SurgeW per device = watts × quantity × surgeMultiplier.
- EnergyWh per device = RunningW × (util% ÷ 100) × runtimeHours.
- AdjustedRunningW = ΣRunningW × simultaneityFactor.
- AdjustedSurgeW = ΣSurgeW × simultaneityFactor.
- PeakW = AdjustedRunningW + (AdjustedSurgeW − AdjustedRunningW) × surgeCoincidence.
- RecommendedW = PeakW × (1 + safetyMargin).
- kVA = watts ÷ powerFactor ÷ 1000.
- BankStoredWh = (EnergyWh × (1 + safetyMargin)) ÷ inverterEfficiency ÷ usableDepth.
- RequiredAh = BankStoredWh ÷ batteryBankVoltage.
How to Use This Calculator
- List devices you want to keep powered during an outage.
- Enter running watts, quantity, and utilization percent.
- Set surge multiplier for motors or compressors.
- Adjust simultaneity and surge coincidence if needed.
- Choose safety margin, power factor, and battery settings.
- Click Calculate Backup Load to view results.
- Use the CSV or PDF buttons to export the report.
Load Inputs That Matter
Accurate backup sizing starts with device watts, quantity, and runtime. Utilization percent converts nameplate load into expected average demand. A refrigerator at 200 W with 60% utilization averages 120 W over the chosen hours. Simultaneity applies a portfolio discount when not every device runs together. Set 80% to reflect cycling loads and shared outlets.
Interpreting Running Versus Surge
Motors and compressors briefly draw higher current at startup. The surge multiplier models that behavior, often 2–5× for pumps and HVAC blowers. The surge coincidence factor controls how many devices are likely to start at once. PeakW is calculated as RunningW plus the surge extra times coincidence, then expanded by a safety margin.
From Watts To kVA Capacity
Generators and inverters are commonly rated in kVA, not watts. The calculator converts watts to kVA using power factor: kVA = W ÷ PF ÷ 1000. With PF 0.9, a 1,800 W continuous requirement becomes 2.00 kVA. Use the recommended continuous kVA for steady operation and the recommended peak kVA to cover startups.
Battery Bank Planning Metrics
Energy is tracked in watt-hours. The model adjusts energy by simultaneity and safety margin, then accounts for inverter efficiency and usable battery depth. StoredWh = (EnergyWh × margin) ÷ efficiency ÷ usableDepth. RequiredAh follows: Ah = StoredWh ÷ bankVoltage. Higher voltage banks reduce current and cable losses, improving system stability.
Reporting And Review Workflow
After calculation, export CSV for procurement comparisons or share a PDF for approvals. Review device rows for outliers such as high surge multipliers or unrealistic utilization. Re-run scenarios with different runtimes to evaluate budget impacts. The chart highlights which devices dominate surge and energy, supporting prioritization when backup capacity must be staged.
For critical circuits, validate totals against breaker ratings and receptacle limits. Consider derating generators for altitude and temperature, often 3–5% per 1,000 feet above sea level. If using lithium batteries, confirm recommended depth aligns with warranty. Document assumptions so future expansions stay consistent during audits or upgrades.
FAQs
1) What does the simultaneity factor change?
It scales totals when devices do not run together. For example, 80% simultaneity assumes only four-fifths of the listed running load is active at the same time.
2) How should I choose a surge multiplier?
Use 1.0 for electronics and lighting. Use 2–5 for motors, compressors, and pumps. If you know a device’s starting watts, divide by running watts to estimate the multiplier.
3) Why are results shown in kVA?
Many backup systems are rated in apparent power. The calculator converts watts to kVA using power factor, helping you compare your requirement to inverter or generator nameplate ratings.
4) Does this tool size wiring and breakers?
No. It estimates power and energy needs for backup planning. Cable, breaker, and transfer-switch sizing should follow local electrical codes and professional verification.
5) What does the battery count estimate assume?
It assumes 12V batteries with the Ah rating you enter. The calculator builds a series string to reach the selected bank voltage, then adds parallel strings to meet the required amp-hours.
6) Why does energy look higher than expected?
Energy includes runtime, utilization, safety margin, inverter efficiency losses, and usable depth limits. Reducing runtime, lowering margin, or correcting utilization often brings the estimate closer to real-world operation.