Generator Sizing: Design Margin and Loading Factor
The standby generator sizing tool uses a design margin and a manufacturer loading factor to convert your calculated load into a recommended generator size. Both defaults are based on common residential standby practice and manufacturer guidance, and both are user-adjustable.
Note: This page provides general educational information only. Final generator selection and loading must comply with the manufacturer’s instructions and applicable codes and must be approved by the authority having jurisdiction (AHJ).
Design margin (default 15%)
The design margin adds headroom for motor starting inrush, seasonal peaks, and future load growth. A typical range for residential standby is 10–25%. The tool uses 15% as a conservative default; you can adjust it between 5% and 30% in the Generator options section. Manufacturer installation guides often recommend similar headroom. Standards such as NFPA 110 (Standard for Emergency and Standby Power Systems) reference sizing to accommodate starting loads and operating at a fraction of nameplate.
Manufacturer loading factor (optional)
The loading factor ensures the recommended generator size does not exceed the fraction of nameplate capacity that the manufacturer allows for standby operation. A factor of 0.8 means “do not exceed 80% of nameplate”—so a 24 kW unit would be limited to 19.2 kW for sizing purposes. Many residential standby manuals specify a maximum loading percentage (e.g. 80%), and NFPA 110 defines standby ratings that imply operating at a portion of nameplate. The tool leaves this blank by default so it does not stack with the design margin—enter a value only when you must stay below a specific fraction of nameplate. Always follow your generator manufacturer’s published limits for your model and application.
Motor starting (essential-loads path)
When you mark a row as a motor, set a start multiplier (for example 4× running current during locked-rotor start). The tool adds extra starting kVA from (multiplier − 1) × running watts ÷ power factor, using the largest motor surge on the running load (coincident start of every motor is not assumed in the calculator). If you leave multipliers at 1 (or only check Motor without raising the multiplier), you can still use the optional 20% on running kW shortcut. Always confirm against the generator and motor manufacturer data.
Optional max voltage dip %: when set, the tool checks whether the motor starting step (the surge above the running load) would sag the bus past your chosen limit, and raises the recommended size if needed. It uses a simplified transient model—required kVA ≥ transient reactance × starting step ÷ dip limit— assuming a generator transient reactance of about 0.25 per unit (typical units are roughly 0.15–0.30). The check only raises the size when it exceeds the margin-based requirement, and it has no effect when there are no motor/inrush loads. This is not a literal terminal-voltage calculation; actual dip depends on the specific machine, AVR, and motor, so confirm with manufacturer voltage-dip curves. A shared note in the tool reminds you that dip checks are simplified.
Quantity and demand %
Each load row can include a quantity (identical units) and a demand % (0–200%) so the effective running contribution is: watts × quantity × demand % ÷ 100. Motor starting extra kVA uses that same effective running watts for the row.
Environmental derate %
Optional environmental derate (e.g. altitude or high ambient) increases required kVA after margin, loading factor, and any dip check: required kVA ÷ (1 − derate). It is separate from the manufacturer loading factor, which limits how much of nameplate you use in service.