DG Set Sizing Calculator
DG Set Sizing Calculator — quick estimate
Estimate the diesel generator rating (kVA) your building needs from its connected load, demand factor and backup scope. Adjust the inputs for an instant figure — then get an engineered electrical design from our team.
Your load
Estimates running load = connected load × demand factor, then adds margin and converts to kVA at your power factor. Motor starting / harmonic loads may need a larger set.
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Our electrical engineers will firm up the load schedule, DG rating, synchronising and AMF panel scope for your site. We'll attach the estimate above automatically.
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Indicative estimates only — not a binding design. Actual DG rating depends on the detailed load schedule, motor starting, harmonics, altitude and ambient derating.
How this calculator works — and where it stops
What it calculates
The diesel-generator rating in kVA for a site's backup load, the resulting loading on the set, the motor-starting check, and whether one set or a synchronised pair is the sensible configuration.
Inputs
- Connected load in kW
- Demand factor — the share of connected load that runs together
- Scope — the whole load or essential loads only
- Design margin
- Power factor
- Largest motor and its starting method — DOL, star-delta or VFD
Method
- Running load = connected load × demand factor × scope factor (1.0 for the whole load, 0.6 for essential loads); design load = running load × (1 + margin); kVA = design load ÷ power factor.
- Selects the next standard rating from 25 kVA to 2,000 kVA (or a 250 kVA step above that), reports the loading percentage, and suggests two sets in N+1 or synchronised operation above 600 kVA.
- Motor starting: the largest motor's kW × a starting multiplier (6 for DOL, 2.5 for star-delta, 1.3 for VFD) is checked against 1.5 × the set's rating, the short-term capacity most gensets tolerate.
Assumptions
- Demand and scope factors are the user's judgement; the tool does not derive them from a load list.
- The 1.5× surge tolerance is a conservative generalisation — the OEM datasheet governs.
- A single largest motor starts alone; simultaneous starting is not modelled.
Limitations
- No harmonic derating for drive-heavy loads, no altitude or temperature derating, and no fuel, exhaust or acoustic design.
- AMF and synchronising panel design, changeover logic and CEIG or pollution-board requirements are outside the tool.
- Load growth is only what the margin allows for.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Connected load: 500 kW
- Demand factor: 0.6
- Scope: whole load
- Power factor: 0.8
- Design margin: 15%
- Largest motor: 55 kW, star-delta start
Working
- Running load = 500 × 0.6 × 1.0 = 300 kW
- Design load = 300 × 1.15 = 345 kW; kVA = 345 ÷ 0.8 = 431.3 kVA
- Next standard rating above 431.3 kVA is 500 kVA; loading at design load is 86%
- Motor check: 55 kW ÷ 0.8 = 68.8 kVA running, × 2.5 (star-delta) = 171.9 kVA starting; step load = (431.3 − 68.8) + 171.9 = 534 kVA against 1.5 × 500 = 750 kVA short-term capacity
Result. 500 kVA set, 86% loaded at design load; the 55 kW star-delta start is within the set’s short-term capacity. Confirm the surge figure against the OEM datasheet.
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 load before the load list is final.
When a professional design must replace it
Before purchase: the set is sized from a load list with starting characteristics and harmonic content, and confirmed against the manufacturer's derating and surge data.
Need an exact BOQ, rate analysis or measurement sheet?
This is a quick engineering estimate. For tendering, billing or audit, our QS & estimation team prepares an item-wise BOQ, rate analysis and a verified measurement sheet to IS / CPWD norms — backed by 15+ years and ISO 9001:2015 quality processes.
Got your number — what next?
A calculator gives you a first figure. These take it further.