DG Preliminary Load Calculator — Steady-State kVA for DG Sizing
Start a DG plan from the load that has to run during an outage — not from the transformer rating or the sum of nameplates. Enter that essential running load, its power factor and the planning allowance you choose. List the big motors and the tool keeps them, with every other transient, for the generator maker's check.
What this tool estimates
The preliminary steady-state capacity in kVA that your essential running load needs at its power factor plus the allowance you choose — checked against the engine's kW as well when you give the set's rated power factor.
What it cannot decide
- The generator rating or model to buy — the manufacturer selects that against motor starting, load steps and your site conditions.
- Whether the set can start your largest motors or accept your load steps: those are listed for the manufacturer, not calculated.
- Derating for altitude, air-intake temperature or cooling water at your site, or the rating basis (standby, prime, continuous).
- Emission and noise norms, the Electrical Inspector's inspection or any other authority's decision.
Result
Your load's power factor is higher than the set's rated power factor, so the engine's kW runs out before the alternator's kVA. The figure above is the planning kW divided by the rated power factor, not the kVA your load draws.
The engine's kW has not been checked. Add the set's rated power factor from its datasheet: if your load's power factor is higher than that, the kW basis gives a larger figure.
| Step | Value |
|---|
| Item | What to check |
|---|
| Power factor | Allowance | Planning kVA |
|---|
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
How this calculator works — and where it stops
What it calculates
A preliminary steady-state planning capacity for a diesel generator, in kVA: the essential running load at its power factor plus a planning allowance you choose — and, when you give the set's rated power factor, the kVA that the engine's kW limit implies. Motor starts and every other transient are listed for the generator manufacturer, not calculated.
Inputs
- Essential running load in kW (or kVA) — the coincident running load of what must run on the set, from an essential-load schedule or from main-switchboard readings taken with non-essential loads off. Not the transformer rating, not the sum of nameplates.
- Running power factor of that load (more than 0, at most 1). A load entered in kVA is converted to kW with it.
- Planning allowance, % — your explicit choice for uncertainty and near-term growth, from 0 to 100. No value is recommended.
- Generator rated power factor (optional) — the power factor the set's kVA rating is stated at, from its datasheet.
- Motor starts (optional) — each large motor's rating in kW or hp, its starting method and, if its maker gives it, its starting kVA. Listed for the manufacturer; never added to the figure.
Method
- Running apparent power (kVA) = essential running kW ÷ running power factor. A load entered in kVA is used as entered, and its kW = kVA × PF.
- kVA basis = running kVA × (1 + planning allowance).
- kW basis, only when a rated PF is given = running kW × (1 + allowance) ÷ the set's rated power factor — the kVA rating at which the engine's kW limit still covers the planning kW.
- Preliminary steady-state planning capacity = the larger of the two bases. Without a rated PF it is the kVA basis, and the page says the kW side was not checked.
- Motor starts, load steps, special loads, site derating, the rating basis and emission and noise norms are returned as a list for the generator manufacturer. None of them is converted into a multiplier or added to the figure.
- Sensitivity: the planning capacity is recomputed at the power factor ±0.05 and the allowance ±10 points, with the running kW held at your figure.
Assumptions
- The essential running load is already coincident — what actually runs together during an outage — so no demand or diversity factor is applied on top of it.
- The power factor applies to the whole essential load. Drives and other electronic loads can make the true power factor lower than a displacement-PF meter shows.
- A set's kVA rating is taken as stated at the rated power factor you enter, so its engine delivers at most rating × that power factor in kW. BEE's guide book notes that a.c. generators are designed for 0.8 PF lagging as specified by standards; use the figure on your datasheet.
- The allowance is your choice; the tool recommends none.
Limitations
- Steady state only: motor starting, load acceptance and the voltage and frequency dips during load steps are not calculated — they are the manufacturer's check.
- No derating for altitude, air-intake temperature or cooling-water temperature, and no fuel, exhaust, ventilation or acoustic design.
- Does not pick a standard rating, a number of sets, redundancy or a synchronising scheme — those are design decisions.
- Unbalanced single-phase loads and regenerative loads are not modelled.
- Does not assess emission or noise norms, the changeover or AMF panel, or the Electrical Inspector's process.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Essential running load 160 kW
- Running power factor 0.8
- Planning allowance 20%
- Generator rated power factor 0.8 (from a datasheet)
- Largest motor 30 kW, star-delta start, starting kVA not given
Working
- Running apparent power = 160 ÷ 0.8 = 200 kVA.
- kVA basis = 200 × (1 + 20%) = 240 kVA — the allowance adds 40 kVA.
- kW basis = 160 × 1.2 = 192 kW ÷ 0.8 rated PF = 240 kVA.
- Planning capacity = the larger basis = 240 kVA. The two bases agree because the load's power factor equals the set's rated power factor.
- The 30 kW star-delta motor goes to the manufacturer's list with its starting kVA marked “not given”; the 240 kVA does not change.
Result. 240 kVA preliminary steady-state planning capacity (192 kW). Not a generator selection: the manufacturer still checks the motor start, the load steps, site derating and the rating basis.
Sensitivity — what moves the answer
- Power factor moves the kVA basis: at PF 0.75 the same 160 kW needs 256 kVA, and at PF 0.9 the kVA basis falls to 213.3 kVA — but a set rated at 0.8 PF still needs 240 kVA on the kW basis, which is why the page asks for the rated PF.
- The allowance passes straight through: each 10 points adds 20 kVA here — 220 kVA at 10%, 260 kVA at 30% — and 0% leaves the running 200 kVA.
- An error in the running load passes through in proportion: every 10 kW left out at PF 0.8 with a 20% allowance removes 15 kVA from the figure.
How engineers use the result
To size the DG room, foundation and cable entry at concept stage, and to check a quoted set against the steady-state load before the load list is final — then to give the manufacturer a clear list of the transients and site conditions they must confirm.
When a professional design must replace it
Before a set is specified or bought: the manufacturer confirms motor starting and load steps against the voltage and frequency dips your loads tolerate, derates the set for altitude and intake and cooling-water temperature, matches the rating basis to how long it will run, and checks special loads; the installation then goes through the emission norms and the Electrical Inspector's process that apply to it.
Sources
- BEE Guide Book 3 (Energy Efficiency in Electrical Utilities), Chapter 9: DG Set System — a.c. generators designed for 0.8 PF lagging; altitude, intake and cooling-water derating tables; transient limits on both kW and kVA and load sequencing; application checks for rectifier, welding and furnace loads; essential load logged at the main bus-bar
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets — Part 1: Application, ratings and performance — where the rating classes on a datasheet are defined; the tool applies none of them
- CPCB: generator set notifications — emission limits for new engines for gensets up to 800 kW gross mechanical power — G.S.R. 804(E), 3 Nov 2022, amended by G.S.R. 436(E), 14 Jun 2023 — and separate limits above 800 kW; confirm the text in force
- CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 (notified 8 Jun 2023) — regulation 34: the capacity above which a generating unit is inspected by the Electrical Inspector before commissioning is set by the Appropriate Government's notification — check your state's
- Apparent power: kVA = kW ÷ power factor — definition
Questions people ask before using it
What information do I need before using the DG preliminary load calculator?
The loads that must run on the DG during an outage and their coincident running kW — from an essential-load schedule or, on an existing site, from readings at the main switchboard. BEE's guide book suggests logging current, voltage and power factor at the main bus-bar every half hour for two to three days of normal operation, with non-essential loads switched off. Add that load's power factor, the planning allowance you want to carry and, if you have it, the generator's rated power factor from its datasheet. For the manufacturer, list the largest motors with their kW or hp, how they start and, if the motor or starter maker gives it, the starting kVA.
Which site conditions could change the DG size?
Altitude, air-intake temperature and cooling-water temperature all derate engine output, and BEE publishes correction tables for them. Motor starting and the order in which loads come on limit a set on both kW and kVA. Rectifier and thyristor loads such as drives and UPS, welding and furnace loads need an application check by the engine and alternator makers. A load whose power factor is above the set's rated power factor is limited by the engine's kW before the alternator's kVA. And the rating the set is bought on — standby, prime or continuous — has to match how long it will actually run.
What should I send an engineer for a project-specific review?
The single-line diagram showing what the DG will feed and where the changeover sits; the essential-load schedule with running kW, power factor and duty; the largest motors with their starting method; any drives, UPS, welding or furnace loads; recent demand records or logged readings; the site's altitude and the proposed DG room or enclosure location; and, for an existing set, its datasheet and log book.
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