Find starting watts in the equipment manufacturer’s generator guidance or electrical specifications, then use suitable inrush measurements when those figures are unavailable. Keep total startup demand separate from running demand so a motor’s normal watts are counted only once in the generator calculation.
Open the Generator Starting Wattage Calculator.
Identify the starting quantity you actually need
“Starting watts” is often used loosely for several different electrical quantities. A manufacturer might publish total starting watts, additional surge watts, locked-rotor amperes, starting kVA, or a recommended generator size. Those figures are related, but they are not interchangeable.
Total starting watts describe the device’s real-power demand during a stated start. Additional starting watts describe the amount above its already-counted running load. Starting kVA describes apparent power, which matters to generator current capacity even when the corresponding real watts are lower.
Write the quantity and unit beside every number you collect. An entry of “start: 3,000” is incomplete. “Total starting demand: 3,000 W, manufacturer’s estimate” is useful and can be compared with another source.
Also record the starting method. A motor connected directly to its supply can behave differently from a motor operated through a variable-frequency drive or a compatible soft starter. A recommendation for one configuration should not be transferred automatically to another.
Search the equipment documentation in a useful order
Begin with the exact model’s manual and specification sheet. Look for a generator compatibility section, input electrical data, startup demand, and any restrictions on source quality or capacity. A generator recommendation from the equipment manufacturer may already account for characteristics that a simple watts calculation cannot capture.
Next, inspect the accessible rating plate without removing covers or exposing electrical parts. Record the model, voltage, phase, running current, and any locked-rotor or starting information. Photographing the entire plate helps a supplier interpret abbreviations correctly.
If documentation gives only running input, ask the manufacturer for the missing starting data. Provide the model and intended supply voltage rather than a generic description such as “a small pump.” Different versions of apparently similar equipment can use different motors and controls.
Generic starting multipliers belong last in this sequence. They are useful for screening possible demand, but they cannot resolve an installation where voltage dip, acceleration time, or a control dropout determines whether the equipment starts.
Translate nameplate current without calling everything watts
For single-phase equipment, apparent power is VA = volts × amps. Real power is W = volts × amps × power factor. For a balanced three-phase load, apparent power is VA = √3 × line-to-line volts × line amps.
Suppose a 240 V single-phase motor has an illustrative locked-rotor current of 45 A. Its locked-rotor apparent demand is 240 × 45 = 10,800 VA, or 10.8 kVA. Calling that “10,800 starting watts” assumes a power factor of one, which is not established by the current figure.
Locked-rotor data also represent a particular condition rather than a complete startup trace. The motor must accelerate, the generator voltage may move, and the current changes through the event. Ask a supplier to evaluate the starting kVA and duration rather than choosing a generator by relabeling the current calculation.
Cummins explains that motor starting involves changing real and reactive demands and generator recovery behavior in its motor-starting technical paper. This is why one maximum-current number cannot fully describe compatibility.
When is an inrush measurement useful?
A suitable measurement can help fill a documentation gap or investigate a difficult start. It must capture the event with a defined method, not merely show a stable current a second later.
Fluke describes triggered inrush capture and the measurement window used by applicable instruments in its inrush-current guide. Ask the technician to record the instrument, capture function, measurement interval, equipment condition, and supply voltage with the result.
An ordinary plug-in energy meter can be useful for accumulated energy and running watts if it is appropriately rated for the appliance. Its display maximum is not automatically a verified startup peak. A meter may average samples, update slowly, or lack a trigger suited to a brief event.
Current capture also does not necessarily provide real watts. To characterize actual starting power and power factor, a technician may need an appropriate power analyzer. Measurements on fixed wiring or exposed conductors belong with a qualified person using suitable equipment and procedures; there is no need for a homeowner to open an energized panel to prepare a load worksheet.
Repeatable starting conditions matter. A compressor restarting against pressure can differ from a relaxed start, and a loaded machine can differ from an unloaded one. Record which condition was tested.
A worked example with three identical devices
Assume three identical devices each run at 800 W. Manufacturer information provides an illustrative total starting demand of 2,400 W per device. The additional startup demand is therefore 2,400 − 800 = 1,600 W each.
With all three included in the running total, running load is 3 × 800 = 2,400 W. If one starts while the other two run, the estimated event is 2,400 + 1,600 = 4,000 W.
| Scenario | Calculation before allowance | Estimated demand |
|---|---|---|
| All three running | 3 × 800 | 2,400 W |
| One starts, two run | 2,400 + 1,600 | 4,000 W |
| Two start, one runs | 2,400 + 2 × 1,600 | 5,600 W |
| All three start | 2,400 + 3 × 1,600 | 7,200 W |
With a 20% planning margin, the one-device start becomes 4,800 W, while the three-device simultaneous start becomes 8,640 W. The margin is explicit, so a reviewer can distinguish equipment demand from the extra allowance.
These results assume the quoted starting figures are real watts and that the devices behave as specified. If the available data are starting kVA instead, retain the kVA and evaluate generator current capacity separately rather than substituting them into a watt total.
Use the calculator’s multiplier as an assumption
The calculator implements this relationship:
Estimated starting load = [running watts × quantity + running watts × (surge factor − 1) × simultaneous starts] × margin
For the example, enter 800 running watts, three devices, a 3× surge factor, one simultaneous start, and 20% margin. The result is 4,800 W. The factor of three describes total device starting watts relative to running watts; subtracting one prevents double-counting the running component.
The tool offers several broad surge categories. Their names do not establish the starting multiplier of your specific compressor or pump. Replace a category-based assumption with manufacturer or appropriate measured information before relying on a close capacity comparison.
Keep simultaneous starts at or below the number of devices. The inputs permit separate values, so the user must preserve that relationship. Entering four simultaneous starts for a quantity of three creates a mathematical result for an impossible schedule.
This tool also assumes identical running watts and surge factors for all devices in that calculation. A workshop containing a saw, extractor, compressor, and heater needs a separate load schedule. Averaging unlike motor loads can hide the single event that determines generator selection.
Distinguish startup planning from energy planning
A starting event can dominate generator capacity without contributing much energy over an hour. Conversely, a heater may have little special startup requirement but contribute heavily to fuel consumption because it remains on.
Keep those analyses separate. For capacity, examine watts, VA, current, start duration, and permitted voltage and frequency changes. For fuel or battery planning, examine energy over the operating period.
Do not use average household electricity consumption to infer starting watts. A daily kWh figure spreads use across time and omits the momentary overlap you need to examine. Likewise, a utility meter’s interval demand may average over a much longer period than a motor takes to accelerate.
What should you verify before accepting the result?
Check the generator’s continuous output and its documented short-duration capability on the intended fuel. Confirm voltage, phase, receptacle ratings, and the effect of other connected loads. Obtain a supplier review when starting performance depends on a large motor or a small margin.
Then check the operating sequence after an outage. Devices that normally start at different times may all request power when supply returns. A dependable sequencing control can change that scenario; a hopeful assumption that compressors will avoid each other cannot.
If a motor fails to accelerate, repeatedly cycling it is not a useful sizing test. Have the equipment, supply, and starting arrangement assessed. Retain the final documentation and any measurement report with the load worksheet, including the circumstances under which the start was verified.
Ask what a published starting number includes
A supplier may quote a recommended generator rating rather than the appliance’s own measured start demand. That recommendation can include reserve, assumptions about other loads, or compatibility testing. Do not automatically add another full generic allowance as though the quoted number were raw motor input.
Likewise, ask whether a starting figure describes the complete appliance or only its compressor or motor. A packaged machine may contain controls, heaters, fans, or pumps that operate during the same event. Preserve the boundary supplied by the manufacturer and add only components outside it.
Keep the original wording beside your interpretation. If two documents disagree, the equipment manufacturer can often resolve the difference more reliably when given both figures and their stated conditions.
References
- Fluke: how and why to measure inrush current.
- Cummins: specifying and validating motor-starting capability.
- Cummins T-030 generator application manual.