Find the right diesel generator (DG set) capacity in kVA, full-load current, and estimated fuel consumption from your connected load, power factor, and voltage.
Pick single or three phase, then enter your load, power factor, voltage, and safety margin.
Nearest standard DG set size for your load
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 standard commercially 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, 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 full-load current at 415 V is I = (120 × 1000) ÷ (1.732 × 415) ≈ 167.0 A, and estimated fuel consumption is roughly 80 × 0.27 ≈ 21.6 L/hr.
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.
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, even momentarily, will overload the alternator regardless of the kVA figure on the nameplate.
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 of the main reasons the standard 15–25% safety margin exists, 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.
| DG Size (kVA) | Typical Application | Approx. Full-Load kW (0.8 PF) |
|---|---|---|
| 5–10 | Home / small shop backup | 4–8 kW |
| 15–30 | Small office / retail outlet | 12–24 kW |
| 62.5–125 | Small commercial building / clinic | 50–100 kW |
| 160–320 | Mid-size industrial unit | 128–256 kW |
| 500–1000+ | Large factory / data centre / hospital | 400–800 kW+ |
Figures are indicative only. Always confirm exact standard DG set sizes and duty ratings with the manufacturer's catalogue.
Content last reviewed: July 2026
A generator must be able to supply both the real power (kW) that does useful work and the reactive power drawn by inductive loads like motors. kVA represents this total apparent power the generator's windings and alternator must handle, so nameplates are always rated in kVA rather than kW.
A margin of 15–25% is typical for most facilities, covering motor starting surges and future load growth. Installations with large motors starting direct-on-line, or expected expansion, should lean toward the higher end of that range.
The 0.27 L/hr per kW figure is a widely used field rule of thumb for diesel gensets running near full load. Actual consumption depends on engine efficiency, altitude, ambient temperature, and how lightly or heavily loaded the generator runs, so always cross-check against the manufacturer's fuel consumption chart for critical sizing.
Yes — every formula used is the same standard formula taught in electrical engineering coursework and referenced in field handbooks. That said, for safety-critical or code-compliance decisions, always verify results against your local electrical code and have them reviewed by a licensed engineer.
For a typical home backup load of 3–5 kW at 0.8 power factor with a 20% margin, a 5–7.5 kVA single-phase DG set is usually sufficient. For a small office or shop with 15–20 kW connected load, a 25–30 kVA three-phase set is a common starting point, though the exact figure depends on motor starting loads such as air conditioners or pumps. Enter your own numbers above for a precise recommendation.
Standby-rated generators are sized for occasional backup use during grid outages and typically run at a lower average load factor, while prime-rated generators are designed for continuous or regular daily operation at a sustained load with a smaller overload allowance. Prime power applications generally need a larger safety margin and a genset explicitly rated for continuous duty by the manufacturer.