Inverter surge rating from motor starts

Why the worst seconds of an outage size the inverter, not the average evening

Motor starting is a mechanical fact before it is an electrical one. At standstill a motor develops no back-voltage to oppose the supply, so current floods the windings until the rotor gathers speed — a demand lasting fractions of a second to a few seconds, largest for compressors and pumps that must start against pressure or a head of water. The multiplier condenses that episode into one number: peak demand as a multiple of running draw.

Inverters quote a surge rating precisely because this episode is survivable by design: semiconductors tolerate brief excursions their thermal limit forbids indefinitely. But the tolerance is seconds, and its protection is a trip — an undersized machine meeting a compressor start does not struggle valiantly, it disconnects, and the whole backed-up circuit goes dark at exactly the moment backup was the point.

The multiplier belongs to the equipment, not to folklore. Manufacturers publish the inrush or locked-rotor figure for their own machines, soft-start and variable-speed drives exist precisely to shrink it toward one, and the pack’s supplied default is declared illustrative for exactly that reason. The declared band runs from a multiplier of one — equipment that starts gently — to five, and stepping outside it is refused rather than sized.

Whether the surge or the steady figure buys the machine depends on the circuit. Lighting and electronics barely start harder than they run; a borehole pump can demand its multiple of everything else combined. The honest procedure sizes both — the census page for the evening, this page for its worst seconds — and shops to the larger verdict, while the declared reverse workflow answers the question backwards: given a machine’s quoted peak, the continuous draw whose starting it can absorb.

When this calculation is used

• Sizing backup for any circuit with a compressor on it — refrigeration, freezers, air conditioning, heat pumps — where starting demand dwarfs running demand.
• Specifying for well pumps, sump pumps and workshop machines, the classic loads that trip an inverter sized only on its steady census.
• Auditing a machine already owned — the declared reverse workflow returns the running draw whose starting moment its quoted peak absorbs.
• Judging whether a soft-starter is worth fitting: rerun with the reduced multiplier the device promises and compare the verdicts.

Worked example

Run the pack’s anchor duty: a 3 kW backed-up circuit whose dominant motor starts at the default multiplier of 2 — a hard-starting compressor absorbed mid-evening.

The figure that returns is the peak, in kilowatts, the inverter must deliver for the seconds the start lasts — the number to hold against the SURGE line of a datasheet, never its continuous line. A machine whose quoted peak clears it rides the start through; one sized only for the census page’s verdict meets the same instant as an overload trip. Whichever page returns the larger demand is the one that picks the hardware.

The pack’s other declared duties bracket the judgement: a 5 kW circuit starting at a multiplier of 3 shows how a pump-heavy site becomes surge-limited, while a gentle 1.5 kW circuit at 2.5 shows a small census still doubling and more at the instant of starting.

What each input represents

Assumptions and limits

• One starting event at a time: the multiplier is applied to the whole continuous draw, and two motors starting in the same instant is a coincidence the relation does not model.
• The surge is assumed brief enough to live inside the inverter’s quoted surge window; equipment that starts slowly under heavy load can outlast the window at a demand the peak alone does not describe.
• The multiplier is taken as constant, though real inrush varies with supply voltage, temperature and the pressure the motor starts against.
• The bank behind the inverter is assumed able to deliver the peak current the surge implies — a small or cold battery can sag first, and that limit belongs to the bank’s own datasheet, not to this relation.

What the guards protect against

• A multiplier below one is refused — the pack ships that refusal as a declared test vector. Equipment cannot demand less at the instant of starting than while running, and a fraction here almost always means the multiplier and an efficiency figure have swapped places.
• The multiplier is capped at five: beyond that band the entry is more likely a locked-rotor AMPERE figure copied where a ratio belongs, and the guard surfaces the confusion instead of sizing a machine around it.
• The continuous load must be positive and below a utility-scale cap, so an empty circuit or a watt-for-kilowatt slip is refused rather than turned into a peak no dwelling could ever demand.

Provenance

Inverter surge sizing, surge kW = continuous load kW × inrush multiplier

The continuous load is multiplied by the surge/inrush multiplier, returning the peak power rating the inverter must deliver during motor starting; the declared reverse workflow solves for the continuous load a fixed peak absorbs.

Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.