TAHA Engineering Toolsby TAHA Knowledge Lab

Energy & Renewables

Generator Sizing Calculator

Running kW and kVA of a list of loads, the worst motor-starting demand, demand factor and design margin, and an estimated generator rating in kVA and kW.

Inputs

Two bars: running load with margin, and running load plus the largest motor starting step; the generator rating must cover the larger.running+ marginrunning + startlargeststarting steprating ≥ largestkVA = kW ÷ PFgen kW = kVA × 0.8
The rating must cover the running load with its design margin (in kVA, and in kW at the generator power factor), and the running load plus the largest starting step — one motor starting, or the group you mark as starting together. Diagram not to scale.

Input power while running, in W or kW (for a motor: shaft kW ÷ motor efficiency). Rating plate or data sheet.

1 for heaters and incandescent lamps; motors and electronics are lower (data sheet).

Share of the connected running load that runs at the same time. 1 (100%) = everything runs together (cautious).

Extra capacity added to the running load for future loads and uncertainty.

Most generator sets are rated at 0.8: rated kW = rated kVA × 0.8.

Result

Estimated generator rating (kVA)
45.3362
Corresponding rated real power (kVA × generator PF)
36.269
What sets the size
Motor starting: running load + largest starting step
Running real power (after demand factor)
11.8
Running apparent power (after demand factor)
12.9833
Overall load power factor
0.9089
Connected running power (all loads, before demand factor)
11.8
Worst starting demand (running load + largest starting step)
45.3362
Largest starting step
32.3529 kVA — Water pump (one unit, ×6)
Design margin added
2.59665
Design margin added (real power)
2.36
Load summary
1. Lighting: 1 × 2 kW, PF 1 → 2 kW, 2 kVA 2. Water pump: 1 × 5.5 kW, PF 0.85 → 5.5 kW, 6.47059 kVA, start ×6 (alone) 3. Air conditioner: 2 × 1.5 kW, PF 0.9 → 3 kW, 3.33333 kVA, start ×3 (alone) 4. Computers: 4 × 0.25 kW, PF 0.95 → 1 kW, 1.05263 kVA 5. Fridge: 1 × 0.3 kW, PF 0.8 → 0.3 kW, 0.375 kVA, start ×5 (alone)
Working
Connected: P = 11.8 kW, Q = 5.41524 kvar; × demand factor 1 → P = 11.8 kW, Q = 5.41524 kvar Running S = √(P² + Q²) = 12.9833 kVA (PF 0.908863) With margin 20%: 12.9833 kVA × 1.2 = 15.5799 kVA; and 11.8 kW × 1.2 ÷ 0.8 = 17.7 kVA Starting: 12.9833 kVA + largest step 32.3529 kVA = 45.3362 kVA Estimated rating = largest of the three = 45.3362 kVA = 36.269 kW at PF 0.8

Estimated generator size: about 45.3362 kVA (36.269 kW). Running load 11.8 kW / 12.9833 kVA; worst starting demand 45.3362 kVA.

  • A first estimate, not a generator or load study. The final choice depends on the generator’s motor-starting capability and the voltage dip you can accept (manufacturer data), and on voltage, phase, frequency, altitude, temperature (derating), transient response, non-linear loads and the duty rating (standby or prime).
  • Starting is checked one event at a time, with every other load already running (cautious): each motor starts on its own unless you mark loads that start together. The starting kVA is added directly to the running kVA (a cautious upper bound). The design margin is added to the running load, not to the short starting surge.

Educational estimate — check the assumptions below.

About this calculator

A generator must supply three things: the real power (kW) of the loads, the apparent power (kVA) their power factor requires, and the short starting surge when a motor starts. Generator sets are rated in kVA at a power factor of usually 0.8, so their kW rating is 0.8 × kVA.

List your loads with their running power, power factor and, for motors, the starting multiplier. Mark loads that start at the same moment; all others are assumed to start one at a time, which is how most installations are arranged. The calculator adds the loads as real and reactive power, applies a demand factor and design margin, checks the worst starting event, and gives an estimated rating. For the background on kW, kVA and power factor see the Power Factor article.

Formula

Per load: Pi = ni × Punit    Si = Pi ÷ PFi    Qi = Pi × tan(cos⁻¹ PFi)

Running: P = d × ΣPi    Q = d × ΣQi    S = √(P² + Q²)    kVA = kW ÷ PF

Starting step: (k − 1) × S of what starts (one unit, or the whole “together” group)    Sstart = S + largest step

Rating (kVA) = max[ S × (1 + m), P × (1 + m) ÷ PFgen, Sstart ]    rated kW = rating × PFgen

n = quantity, PF = power factor, d = demand factor, k = starting multiplier (starting kVA ÷ running kVA), m = design margin, PFgen = generator rated power factor (usually 0.8).

Worked example

  • One resistive load of 20 kW at PF 0.8, margin 20%, generator PF 0.8: running S = 20 ÷ 0.8 = 25 kVA; with margin 25 × 1.2 = 30 kVA; real-power check 20 × 1.2 ÷ 0.8 = 30 kVA; no starting surge → 30 kVA = 24 kW (margin 5 kVA / 4 kW).
  • Default list (lighting, pump with ×6 start, two air conditioners, computers, fridge): running 11.8 kW / 12.98 kVA (PF 0.909); the pump start adds (6 − 1) × 6.471 = 32.35 kVA, giving 45.34 kVA. Motor starting sets the size: about 45.3 kVA (36.3 kW). A soft starter on the pump (k ≈ 2.5) would reduce this a lot.

Assumptions and limitations

  • Running loads are added as real and reactive power (vectors), not by adding kVA.
  • During a start every other load is assumed to be running already; only the largest starting event is checked. Loads marked “together” start in one event; all others start one unit at a time.
  • Starting kVA = multiplier × running kVA, added directly to the running kVA (a cautious upper bound, because starting kVA has a low power factor).
  • The rating must also cover the starting demand as a continuous kVA — a simple, cautious rule. Many generators can supply more kVA for a few seconds; check the manufacturer’s motor-starting kVA and voltage-dip data.
  • Not included: voltage, phase balance, frequency, altitude and temperature derating, harmonics of non-linear loads, standby/prime duty rating, and future load growth beyond the margin.
  • An estimate only; it does not replace a professional generator or load study.

References

  • ISO 8528-1 — rating of reciprocating internal-combustion-engine driven generating sets (kVA rating at power factor 0.8, prime and standby ratings).
  • IEEE Std 446 (Orange Book) — emergency and standby power systems, motor starting on generators.