Generator Sizing Calculator
Size a standby or prime diesel generator properly — not just for the running load, but for the motors it must start. List the loads with their starting method (DOL, star-delta, soft starter, VFD) and starting sequence, add a future allowance, and the calculator returns the minimum generator kVA, the recommended commercial size, the running percentage on that set, and an approximate diesel consumption and monthly fuel cost.
Pairs with the Load Schedule Calculator for establishing the essential load, and the Cable Sizing and Voltage Drop Calculator for the generator feeder.
| Description | Starting method | Running kW | PF | Start × run kVA | Start step |
|---|
Generator selection CALC-04 · REV 2026-07
| Total running load | – |
|---|---|
| Running load with future allowance | – |
| Peak starting demand (worst step) | – |
| Minimum kVA — running criterion | – |
| Minimum kVA — starting criterion | – |
| Minimum generator size (governing) | – |
| Recommended commercial size | – |
| Running load on recommended set | – |
| Fuel consumption at running load | – |
| Estimated fuel per day / month | – |
| Estimated monthly fuel cost | – |
| Step | Already running (kVA) | Starting (kVA) | Transient total (kVA) |
|---|
Guidance tool only. Starting multipliers, alternator dip capability and fuel burn vary between manufacturers — confirm the final selection against the genset technical data sheet (prime vs standby rating, alternator starting kVA at the accepted dip, and the engine fuel curve) with a registered engineer.
How this calculator works
- Running load. Each row contributes its running kW, converted to kVA through its power factor. A future allowance (typically 20%) is added before sizing.
- Starting demand. Motors draw far more than their running kVA at start: typically 6× for direct-on-line, about 2.2× for star-delta, 2.5–3× for soft starters and about 1.5× on a VFD. Each row carries an editable multiplier, so nameplate or starter-datasheet values can be entered directly.
- Load sequence. Loads are assigned to start steps 1–5. For each step the transient demand is everything already running plus the starting kVA of that step. The worst step governs, and the full step table is shown so you can see which one — re-sequencing the largest motor to a later step often shrinks the set.
- Sizing criteria. The set must satisfy both: running load with future allowance at no more than the chosen continuous loading (typically 80% of the prime rating), and peak starting kVA within the alternator starting capability (typically 2.5× prime kVA for a 20–25% voltage dip). The larger of the two governs, and the calculator says which.
- Commercial size and fuel. The minimum kVA is matched to the next standard prime rating (10–2000 kVA). Fuel burn uses the widely used linear diesel approximation L/h = 0.246 × load kW + 0.084 × rated kW, then your running hours and diesel price give litres and shillings per month.
Assumptions and limits
- Running below about 30% of the set for long periods risks wet stacking — the calculator flags this and suggests options.
- Prime ratings are assumed; standby ratings are typically about 10% higher but carry running-hour limits. Confirm against the manufacturer data sheet.
- Fuel figures are an engine-agnostic approximation — use the actual engine fuel curve for contractual guarantees.
- The tool does not design the changeover (ATS), synchronising, fault level or neutral earthing arrangements — all part of a full design.
Need a full standby power design, ATS schematic or tender specification? See our consultancy services and the design document library, or learn the method in a training program.