Generator sizing
kVA = (Surged load ÷ Power factor) × (1 + Margin%) ÷ 1000
The starting/surge allowance accounts for equipment — especially AC compressors and pumps — that draws far more current at startup than during normal running. A flat surge percentage is a starting point; an actual load list with the real starting currents of your largest motors gives a much more precise answer.
Battery bank sizing
Ah required = Wh ÷ (Battery voltage × Inverter efficiency)
This gives the raw capacity needed to deliver your load for the requested backup time. Add 20-30% on top if you don't want to regularly deep-discharge the battery bank — repeated deep discharge shortens battery life significantly, especially for lead-acid batteries.
Frequently asked questions
What starting surge allowance should I use?
25% is a reasonable general default, but equipment with large motors — air conditioners, well pumps, refrigeration compressors — can briefly draw 3-7× their running current at startup. If your load includes big motors, build an actual load list with their real starting currents rather than relying on a flat percentage.
Why does power factor matter for generator sizing?
Generators are rated in kVA (apparent power), while your connected load is usually in watts (real power). Power factor is the ratio between them — a lower power factor (more inductive load, like motors) means you need a proportionally larger kVA-rated generator to deliver the same real wattage.
Should I size a battery bank to fully discharge every cycle?
No — regularly discharging a battery bank to near-zero shortens its usable life considerably, especially for lead-acid chemistry. Add 20-30% extra capacity beyond the raw calculated Ah so day-to-day cycling stays in a shallower, healthier discharge range.