Calculators · Machinery

Generator size, kVA and fuel consumption calculator

A generator must cover both the steady load of everything running and the brief surge when an electric motor starts, which can draw several times its running power. Generators are rated in kVA, while loads are usually listed in kW. These calculators size a generator for your loads, convert between kW, kVA and current, and estimate fuel consumption and runtime at a given load.

Generator size for your loads

Adds the extra starting power of the largest motor to the total running load, applies a safety margin and converts to kVA. Start the largest motor first in practice, so only one surge occurs at a time.

Results
Peak load while starting—kWRunning load plus the largest motor’s starting surge.
Recommended rating—kWPeak load with the safety margin.
Recommended rating—kVAThe same rating in kVA at the given power factor.

Formula
peak = running + largest motor × (multiplier − 1)
rating = peak × (1 + margin)
kVA = kW / power factor

kW, kVA and current

Converts real power in kW into apparent power in kVA and the current each phase must carry. Current determines cable and breaker sizes, so it matters as much as the power rating.

Results
Apparent power—kVAkW divided by power factor.
Current per phase—ACurrent in each line conductor.

Formula
kVA = kW / PF
I = kVA × 1,000 / (√3 × V)  (three-phase)
I = kVA × 1,000 / V  (single-phase)

Fuel consumption and runtime

Estimates diesel use from the load and the engine’s brake-specific fuel consumption, and how long a tank will last. Generators are least efficient at light load, so fuel per kWh rises when a large set runs a small load.

Results
Fuel consumption—L/hDiesel used per hour at this load.
Runtime on one tank—hoursTank volume divided by consumption.

Formula
fuel [kg/h] = kW × load × g/kWh / 1,000
L/h = kg/h / 0.835

Worked example: a site generator

  1. Running loads total 8 kW; the largest motor runs at 2 kW and draws three times that to start.
  2. Peak load: 8 + 2 × 2 = 12 kW. With a 20% margin, choose at least 14.4 kW — 18 kVA at 0.8 power factor.
  3. A 20 kW three-phase load at 400 V and 0.8 power factor draws 36 A per phase.
  4. A 100 kW set at 75% load burns about 20.7 L/h, so a 200 L tank lasts about 9.7 hours.

Estimated diesel use (L/h), assuming 290, 245, 230 and 225 g/kWh at 25, 50, 75 and 100% load

Generator25% load50% load75% load100% load
20 kWabout 1.7about 2.9about 4.1about 5.4
60 kWabout 5.2about 8.8about 12.4about 16.2
100 kWabout 8.7about 14.7about 20.7about 26.9

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About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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