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SHEET TWF·T-02 REV 2026-09-02 METHOD M.2026-09-02.1

Welding generator size calculator

Sums the welding and auxiliary loads you enter in kVA, models the worst single sequential start from entered multipliers and adds your planning margin — a planning estimate from nameplate data, not an electrical installation design.

Classification Planning estimate

InputsT-02
F-01 Generator siting

A combustion generator indoors, in an enclosed or restricted space or in a trench stops this estimate.

F-02 Rating basis

Load figures must come from the nameplate or manufacturer data before a record can be issued.

F-03 Load 1
F-03

Optional — names this row on the issued record.

F-03 Rating entry

Enter apparent power directly, or active power with its power factor.

F-03 Nameplate
kVA

Nameplate apparent power, greater than 0.

F-03 Nameplate

Manufacturer or user-supplied figure of at least 1, recorded as entered data and never inferred. Leave empty for a running-only load.

F-04
%

Added on top of the worst modelled figure; 20 is the worked-example margin.

JavaScript is off in this browser, so no live record can be issued. The worked example below walks the full method with real figures, and the blank card on this page prints as a worksheet.

R-GEN-nnn

Welding generator size calculator

Awaiting inputs — enter values and issue a record. Empty slots below print as a blank worksheet.

Field slots

F-01Generator siting
F-02Rating basis
F-03Loads
F-04Planning margin
RECORD VOID — inputs changed since issue. Recalculate to issue a new record.

Worksheet · https://theweldfab.com/tools/welding-generator-size-calculator/ · METHOD M.2026-09-02.1 · REV 2026-09-02

Worked example

STATIC — not a live result

A 1.6 kW welding set at power factor 0.9 with an entered 2.5× starting multiplier, plus a 0.8 kW grinder at power factor 0.8 already running, with a 20% planning margin: running total ≈ 2.78 kVA, worst start peak ≈ 5.44 kVA, planned figure ≈ 6.53 kVA.

R-GEN-EX

Welding generator size calculator

Planning estimate

Plan around 6.53 kVA

Entered assumptions

F-01Generator sitingOpen air, outdoors
F-02Rating basisNameplate / manufacturer data
F-03.1Load 1 · Welding set1.6 kW @ PF 0.9 · start ×2.5
F-03.2Load 2 · Grinder0.8 kW @ PF 0.8
F-04Planning margin20 %

Planning figures

R-01Running total2.78 kVA
R-02Peak — start of Welding set5.44 kVA
R-03Planned figure (+20% margin)6.53 kVA

Planning support only — verify against nameplate/manufacturer data.

RECORD VOID — inputs changed since issue. Recalculate to issue a new record.

Method

M.2026-09-02.1

Each load enters apparent power directly, or active power with a power factor over 0 and at most 1, in which case kVA = kW ÷ PF. An optional starting multiplier of at least 1 may be entered per load; it is treated as entered data, never inferred.

  • running kVA = Σ load kVA
  • start peak = starting load kVA × multiplier + Σ other running kVA — modelled one sequential start at a time, for every load with a multiplier
  • peak kVA = max(running total, every start peak)
  • planned kVA = peak × (1 + margin ÷ 100)

Worked figures: 1.6 kW ÷ 0.9 ≈ 1.78 kVA and 0.8 kW ÷ 0.8 = 1 kVA give a running total of 2.78 kVA; starting the welding set at 2.5× while the grinder runs gives a 5.44 kVA peak; a 20% margin lands the planned figure at 6.53 kVA.

You can model up to eight loads in a live record.

Limits & stop conditions

T-02
  • STOP — a combustion generator indoors, in an enclosed or restricted space, or in a trench ends the estimate; re-plan the siting in open air per the manufacturer’s instructions, or have a competent person assess the installation before any generator runs[S1].
  • HOLD — an empty load list, missing or non-numeric values, unresolved siting, or a rating basis other than nameplate/manufacturer data keeps the record unissued.
  • This site does not select a generator for you; use the result only as a planning figure to compare against manufacturer data and the actual installation requirements.
NEXT CHECKS

Equipment options

CHECK BEFORE HIRE

When the checks above are closed out, compare generator hire options from National Tool Hire. Use the planned figure only to shortlist equipment, then verify running and starting demand, voltage, phase, outlets, duty and outdoor siting against manufacturer data. This calculator does not approve a unit or installation.

When the checks above are closed out, compare generator hire options from Mammoth Hire. Compare equipment only after running demand, starting peak, voltage, phase, outlets, duty and safe siting have been checked against manufacturer data. This calculator does not select or approve the unit.

Source register

  1. S1 Construction hazardous substances: carbon monoxide Health and Safety Executive (HSE) Regulator guidance accessed
Tool record Awaiting values