You've carefully calculated all your running watts — a total of 5 kW. You buy a 6 kVA genset just to be safe. Then, when the AC turns on, the genset trips instantly. What happened? The answer: starting watts.

What Are Starting Watts?

Equipment with electric motors requires a large momentary power surge the instant it's switched on. This phenomenon is called inrush current, or Locked Rotor Amps (LRA). During the first 0.1–2 seconds, a motor can draw 2–7 times more current than its normal running current. A genset that can't handle this surge will trip instantly or suffer a severe voltage drop.

Surge Factors by Equipment Type

• AC units / refrigeration compressors: 3–7× running watts — the most common cause of genset trips
• Large industrial motors: 3–6× running watts
• Water pumps: 2–3× running watts
• Refrigerators / freezers: 2–3× running watts
• LED / resistive lighting: ~1× — no meaningful starting surge

How to Correctly Calculate Peak Load

Peak Load = Σ Running Watts of all loads + Starting Watts of the largest motor

Example: 10 lights (500 W) + a 750 W pump + 900 W AC running + AC starting surge (3×900 = 2,700 W) = 4,850 W peak. A 4 kW genset is clearly insufficient even though total running watts is only 2,150 W.

Technical Solutions

• Soft Starter: Reduces motor starting current by 40–60%. Suitable for pumps and compressors.
• VFD (Variable Frequency Drive): Ramps up motor speed gradually, nearly eliminating inrush current.
• Staggered Startup: Don't switch on all motors at once — leave a 30–60 second gap between each motor.
• Oversize the genset: Choose a capacity 20–30% larger than total running watts alone.

Use the Jerbindo load simulator, which automatically accounts for the starting-watts factor for every piece of equipment you enter.