To calculate genset kVA: add up the running watts of every load, divide by the power factor (typically 0.8) to get kVA, then add a 20-25% safety margin plus motor surge. The basic formula is kVA = Watt ÷ (PF × 1000). Always confirm against a real load list before locking in your size.
Pick a genset that's too small and it trips the moment a motor starts. Pick one that's too big and you pay more for the rental and diesel than you need. Getting the sizing right from the start saves a lot.
Watt vs kVA — why the difference matters
Watt (kW) is the real power that actually does the work, while kVA is the apparent power the genset has to deliver, including the part that gets "lost".
Gensets are rated in kVA because that accounts for power factor. Purely resistive loads (incandescent lamps, heaters) have a PF close to 1.0, so kW and kVA are almost identical. But loads with motors (compressors, pumps, air conditioners) draw reactive power, so the kVA has to be higher than the kW.
That's why if you size using watts alone without power factor, you'll undersize the genset. It's the number-one reason a genset trips even though it "looked sufficient" on paper.
- Watt (kW) = the real power that does the work
- kVA = the apparent power the genset must supply
- kVA is always ≥ kW because power factor is < 1
- Genset nameplates use kVA, not kW
| Term | Meaning | When it's used |
|---|---|---|
| kW | Real power | Load / equipment rating |
| kVA | Apparent power | Genset rating |
| PF | Ratio of kW ÷ kVA | Converting between the two |
The Watt to kVA conversion formula
The basic formula for calculating genset kVA is kVA = Watt ÷ (Power Factor × 1000), using a default PF of 0.8 for most mixed loads.
For a 3-phase system, the full formula is kVA = (√3 × Volts × Amps) ÷ 1000. Many equipment nameplates already state the kVA or ampere value directly, so you can skip working it out from watts.
If you only have the ampere value, use kVA = (Volts × Amps) ÷ 1000 for single-phase. Make sure the voltage matches your local supply (240V single-phase or 415V 3-phase in Malaysia).
- Single-phase: kVA = (Volts × Amps) ÷ 1000
- 3-phase: kVA = (√3 × Volts × Amps) ÷ 1000
- From watts: kVA = Watt ÷ (PF × 1000)
- Default PF of 0.8 if you're unsure
| Known value | kVA formula |
|---|---|
| Watt + PF | Watt ÷ (PF × 1000) |
| Amps (single-phase) | (V × A) ÷ 1000 |
| Amps (3-phase) | (√3 × V × A) ÷ 1000 |
Power factor & motor surge — don't ignore them
Power factor and starting surge are the two factors that most often trip people up when sizing a genset.
Power factor (PF) measures how "efficiently" a load uses power. Inductive loads like motors, pumps and compressors typically run a PF of 0.7-0.85. Use 0.8 as a safe estimate if you don't have the actual figure.
Starting/surge is the momentary spike in current when a motor switches on — it can reach 3 to 6 times the running load. The genset has to absorb that peak, not just the normal operating load. This is exactly why a 10kVA genset can trip even when the steady load is only 6kVA.
- Typical motor/pump PF: 0.7-0.85 (use 0.8 when in doubt)
- Motor starting surge: 3-6× running load
- Resistive loads (lamps, heaters): PF ~1.0, no large surge
- A VSD/soft starter reduces the starting spike
| Load type | Estimated PF | Starting surge |
|---|---|---|
| Lamps / heaters | ~1.0 | Almost none |
| Air conditioner / fridge | 0.8 | Moderate-high |
| Motor / pump | 0.8 | High (3-6×) |
Worked example + safety margin
Let's run through one example: a worksite with 2,000W of lighting, a 3,000W pump and 2,000W of tools — a total running load of 7,000W.
Step 1: Convert to kVA → 7,000 ÷ (0.8 × 1000) = 8.75 kVA. Step 2: Add a 20-25% safety margin for surge and extra load → 8.75 × 1.25 = ~10.9 kVA. So a 12-15kVA genset is a comfortable estimate for this load.
This margin matters so the genset doesn't run at 100% — continuous operation at full load shortens engine life and raises the risk of tripping. This is only an estimate; confirm it against a complete load list before locking in.
- Total running watts → divide by PF → get base kVA
- Add a 20-25% margin for surge + buffer
- Aim for an operating load of ~70-80% of genset capacity
- Estimate only — confirm with a full load list
| Step | Calculation | Result |
|---|---|---|
| Running load | 2000+3000+2000 | 7,000W |
| Convert to kVA | 7000 ÷ (0.8×1000) | 8.75 kVA |
| + 25% margin | 8.75 × 1.25 | ~10.9 kVA |
Genset sizing reference table (estimates)
This table is a rough estimate for quick reference — not a substitute for an actual load calculation.
The real genset size depends on the combination of loads, the number of motors, and the starting profile. Two sites with the same total watts can need different kVA if one has several large motors.
PowerRent Malaysia stocks gensets from 20kVA to 500kVA+, so almost any requirement can be matched. Send us your equipment list and we'll help match the right size.
- Available range: 20kVA–500kVA+
- Rental from RM80/day, DOSH certified
- More motors = push the margin higher
- Always confirm with a load list, not guesswork
| Used for | Estimated kVA |
|---|---|
| Home / small event | 10-20 kVA |
| Shop / small office | 20-40 kVA |
| Medium construction site | 40-100 kVA |
| Factory / industrial | 100-500kVA+ |
- Basic formula: kVA = Watt ÷ (Power Factor × 1000), default PF of 0.8.
- kVA is always higher than kW because power factor is less than 1.
- Motors draw a 3-6× surge at startup — the genset must be able to absorb that peak.
- Add a 20-25% margin so the genset runs at 70-80%, not full load.
- The sizing table is only an estimate — confirm with a real load list before locking in.
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Calculate genset kVA from watt load, power factor & motor surge. Includes the formula, worked examples & a quick reference table. WhatsApp us for a quote.
WhatsApp NowFAQ
What's the formula to convert watts to kVA?
The basic formula is kVA = Watt ÷ (Power Factor × 1000). The default power factor is 0.8 for mixed loads. For 3-phase, use kVA = (√3 × Volts × Amps) ÷ 1000.
Why are gensets rated in kVA and not kW?
Because kVA accounts for power factor, so it shows the real power the genset has to supply, including reactive power. kW alone isn't enough for motor loads, so the kVA rating is more accurate for sizing.
How much safety margin should I use for genset sizing?
A safe estimate is to add 20-25% on top of the calculated kVA load. This leaves room for starting surge and keeps the genset from running at 100%, which shortens engine life.
Why does a genset trip even when the size looks sufficient?
Usually because the motor starting surge — which can reach 3-6 times the running load — wasn't accounted for. Even when the operating load is low, the spike at startup can exceed the genset's capacity.
Which power factor should I use if I'm not sure?
Use 0.8 as a safe estimate for most mixed and motor-driven loads. Purely resistive loads such as lamps and heaters can use a PF close to 1.0.
How do I make sure the genset size is right?
Build a complete load list of all equipment with their watts and motor types, then calculate using the formula with a margin. WhatsApp your list to us and we'll help match the right size.