DG Size Calculator

DG set sizing rule of thumb: kVA = kW ÷ 0.8, then add a planning margin for future load growth and normal operating variation — for example, a steady 80 kW load needs roughly a 100–125 kVA generator, not the bare 100 kVA calculated figure. Motor starting performance is a separate consideration from this margin and needs its own starting-kVA or voltage-drop assessment, especially for large motors starting direct-on-line — see the section below. Sizing a diesel generator (DG set) correctly is a balance between reliability and cost: an undersized generator will stall, overheat, or trip under motor-starting surges and peak demand, while an oversized one runs inefficiently at light load, wastes fuel, and costs more upfront. This calculator estimates the right DG set capacity for a given electrical load by converting the real power demand (kW) into the apparent power (kVA) a generator must be rated for, then matching that figure against a reference series of commonly available DG set sizes. It also estimates the full-load current the DG set will supply and the approximate diesel fuel consumption per hour, both useful figures when planning fuel storage and the outgoing cable or breaker from the generator.

The required DG capacity is found from DG Size (kVA) = Load (kW) ÷ PF — or Load (kW) ÷ 0.8 when the load power factor is higher than 0.8, as explained below — since a generator's nameplate is always rated in kVA while the actual connected load is usually expressed in kW; dividing by the power factor accounts for the reactive power the generator must also supply. A safety margin is then added on top of this calculated value, since real installations rarely run at a perfectly steady, ideal load — motor starting surges, future load additions, and day-to-day demand fluctuations all call for some spare capacity.

The resulting current draw is calculated using I = (kVA × 1000) ÷ (√3 × V) for a three-phase supply, or I = (kVA × 1000) ÷ V for single-phase, which determines the cable and breaker sizing on the generator's output. Fuel consumption is approximated using a commonly used diesel-generator rule of thumb of roughly 0.27 litres per hour for every kW of load, though actual consumption varies by engine efficiency and load factor.

Worked example: A facility has a three-phase connected load of 80 kW at a power factor of 0.8, supplied at 415 V, with a 20% safety margin. The base DG capacity is kVA = 80 ÷ 0.8 = 100 kVA. Adding the 20% margin: 100 × 1.20 = 120 kVA, which would typically be rounded up to the nearest standard DG size of 125 kVA. The load current at 415 V is I = (100 × 1000) ÷ (1.732 × 415) ≈ 139.1 A, the rated current of the 125 kVA set is (125 × 1000) ÷ (1.732 × 415) ≈ 173.9 A — the figure used for the generator output cable and breaker — and estimated fuel consumption is roughly 80 × 0.27 ≈ 21.6 L/hr near full load.

125 kVA DG load calculation (reverse direction): if you already have a 125 kVA DG set and want to check how much load it can safely carry, work backward using the same convention: recommended continuous load ≈ DG capacity × min(load PF, 0.8) ÷ (1 + margin%). At PF 0.8 and a 20% margin, that's 125 × 0.8 ÷ 1.20 ≈ 83.3 kW — treat this as a planning estimate, not the generator's actual prime/standby rating, which is set by the manufacturer's datasheet.

This tool is especially useful for facility engineers and contractors selecting a backup or prime power DG set, sizing the generator's output cable and protection, or estimating fuel budget and tank capacity for continuous or standby operation.

kW vs kVA: Why Generators Are Never Rated in kW Alone

Real power (kW) is the power that actually does useful work — turning a motor shaft, lighting a bulb, heating an element. Apparent power (kVA) is the total power the generator's alternator and windings must be capable of delivering, including the reactive component absorbed and released by inductive loads such as motors, transformers, and fluorescent or induction lighting ballasts. The ratio between the two is the power factor: kW = kVA × PF. Because the generator's physical winding size and heat dissipation are governed by current (and therefore by kVA, not kW), manufacturers always publish a kVA nameplate rating. A DG set rated 100 kVA at 0.8 PF can deliver 80 kW of real power — trying to draw 100 kW of real load from it overloads the engine, whose kW output is matched to kVA × 0.8, regardless of the kVA figure on the nameplate.

Engine kW vs Alternator kVA: Why a Load PF Above 0.8 Does Not Shrink the Set

A generator set has two limits. The alternator is limited by current, so it is rated in kVA. The engine is limited by mechanical power, and most sets are built with an engine sized for the alternator’s kVA at 0.8 power factor — the usual rating basis in ISO 8528 and manufacturer data. If the load power factor is lower than 0.8, the alternator’s kVA runs out first, so kVA = kW ÷ PF governs. If the load power factor is higher than 0.8 — resistive heating, corrected loads, many modern drives — the engine’s kW runs out first, and the set must still be at least kW ÷ 0.8 in kVA. This calculator therefore sizes on kVA = kW ÷ min(PF, 0.8). A 100 kW load at unity power factor needs a 125 kVA set, not a 100 kVA one. A leading power factor from lightly loaded capacitor banks or some UPS input filters is a separate problem: it can destabilise the alternator’s voltage regulator and should be reviewed with the manufacturer.

Checking Motor Starting and Transient Load

When a large motor starts, the generator sees a sudden block of low-power-factor kVA. Unlike a utility transformer, the alternator’s internal reactance is high, so the voltage dip is larger for the same starting kVA. The calculator screens it with dip % ≈ 100 × S_start ÷ (S_start + S_gen ÷ X″d), where S_start ≈ motor kW ÷ 0.8 × starting-current multiple and X″d is the alternator subtransient reactance in per unit, which sets the initial dip before the voltage regulator responds. It ignores the voltage regulator’s recovery, the load already connected and the engine’s speed dip, so treat it as a screen. Dips up to about 15% are commonly tolerated; ISO 8528-5 performance classes allow roughly 15–20% transient voltage deviation on load application for the general-purpose classes, and contactors can drop out beyond about 25–30%. The engine also has a load-acceptance limit — a turbocharged diesel may accept only part of its rating in one step — so large loads should be sequenced. Where the dip is too large, reduced-voltage starting (star-delta, soft starter or VFD), load sequencing or a larger alternator (sometimes on the same engine) is the usual answer, confirmed against the manufacturer’s motor-starting kVA data.

Non-linear Loads, Duty and Site Conditions

Non-linear loads. UPS systems, VFDs and rectifiers draw distorted current that heats the alternator and, through its source impedance, distorts the voltage. Sets supplying mainly such loads usually need an oversized alternator or a low-reactance design; UPS and drive manufacturers publish generator-sizing ratios for their equipment. Duty. ISO 8528-1 distinguishes emergency standby power (ESP), for outages only with a limited number of hours per year, from prime power (PRP), for an unlimited number of hours at variable load, where the average load over 24 hours should not exceed 70% of the prime rating unless the manufacturer agrees otherwise. A set’s prime rating is lower than its standby rating, so compare like with like. Site conditions. Engine output falls with altitude and high air temperature, and each data sheet states the conditions its rating applies to; above them, apply the manufacturer’s derating chart.

Why Motor Starting Loads Need Extra Margin

Induction motors, pumps, compressors, and air-conditioning compressors typically draw 5 to 7 times their full-load current for a fraction of a second at start-up (direct-on-line starting). A generator sized only for the steady-state running load can experience a large, sudden voltage dip or stall entirely when such a motor starts, especially if the starting motor represents a large fraction of the total connected load. This is one reason a planning margin is commonly applied on top of the calculated running kVA, and why installations with large single motors — borewell pumps, chillers, or big compressors — often need either a larger safety margin, a soft starter/VFD on the motor, or a generator explicitly sized for the starting kVA rather than just the running kVA.

How to Use This DG Size Calculator

1. Select the phase type — single phase for smaller domestic or shop loads, three phase for industrial and commercial installations.
2. Enter the connected load in kW and the expected power factor (typically 0.8 for a mixed motor and lighting load; check your electricity bill or panel meter for a more accurate figure).
3. Enter the supply voltage (commonly 230V single-phase or 400/415V three-phase, or use the quick presets for your region), a planning margin for future load growth, and whether the set is for standby or prime duty.
4. Fill in the motor starting & site checks — largest motor, its starting current, the alternator X″d, non-linear load share, ambient and altitude — to screen the starting voltage dip and derating conditions.
5. Click Calculate DG Size to see the reference DG capacity in kVA, the load and rated output current, the estimated diesel consumption and the screening checks.

Common Mistakes When Sizing a Generator

• Sizing purely on kW without converting to kVA. Buying a "100 kW" generator when the actual requirement is 100 kVA at 0.8 PF (i.e. only 80 kW usable) is a frequent and costly sizing error.
• Ignoring motor starting surges. A generator that comfortably handles the running load can still stall or trip when a large motor starts, if no allowance is made for inrush current.
• Skipping the safety margin. Sizing exactly to the calculated load leaves no headroom for future expansion, seasonal peak loads, or day-to-day demand variation.
• Using an unrealistic power factor. Assuming PF = 1 for a load that is actually 0.75–0.85 (typical for motor-heavy loads) understates the kVA; use your actual metered power factor where possible. And a genuinely high load PF does not allow a set smaller than kW ÷ 0.8, because the engine is rated for kVA × 0.8.
• Confusing standby and prime power ratings. A generator rated for occasional standby duty is not necessarily suitable for continuous, daily prime power operation at the same load — check the manufacturer's duty rating.

Standard DG Size Chart (kVA)

The table below lists the same standard DG (diesel generator) sizes in kVA that this calculator's reference series uses to round your calculated capacity up to a realistic, commonly available rating. Treat this as a general reference chart, not a guarantee of availability — actual standard DG sizes and step increments vary by manufacturer and region, so confirm the exact catalogue sizes with your local supplier before finalizing a purchase.

Illustrative DG Capacity Ranges and Common Applications

These are illustrative capacity ranges for context, not an exact match to this calculator's reference series — actual available DG set sizes vary by manufacturer and region. Always confirm exact standard sizes and duty ratings with the manufacturer's catalogue.

The kVA sizing formula itself (kVA = kW ÷ PF) is universal — only the supply voltage, and which reference kVA sizes are locally common, change by region. This calculator supports common low-voltage generator output voltages used across India, the UK and Europe, the USA and Canada, the Middle East, Australia, and South Africa, plus any custom voltage.

For a given kW load and power factor, the DG kVA demand itself doesn't depend on voltage — voltage mainly determines the resulting line current. Which DG set sizes are readily available, and their standard step sizes, varies by manufacturer and region; treat the reference rating from this calculator as a planning starting point, then confirm actual standard availability with your local DG supplier before specifying.