One of the most costly mistakes in backup power system investment is choosing the wrong genset capacity — too small means overload and premature damage, too large means wasted purchase and fuel costs, plus wet stacking on lightly loaded diesel engines. This guide provides a systematic process for finding the right capacity.

Step 1: Inventory All Loads

Make a complete list of every piece of equipment to be powered along with its running wattage (check the nameplate or manual). Separate critical loads (must stay on during an emergency) from optional ones (can be switched off). Focus on critical loads to determine minimum capacity.

Step 2: Identify Inductive Loads

Mark every piece of equipment with an electric motor: AC units, pumps, refrigerators, washing machines, compressors, drills, blowers. Calculate the starting watts for each — use a factor of 2–3× for general motors, 3–7× for AC units and large compressors.

Step 3: Calculate Realistic Peak Load

Peak Load (W) = Σ Running Watts + Starting Watts of the largest motor that might switch on simultaneously

This is the figure the genset must be able to handle under worst-case conditions — for example, a sudden outage while all equipment is in use.

Step 4: Add a 20–25% Safety Margin

A genset should never run continuously above 80% capacity. Beyond that point, engine temperature rises sharply, fuel consumption increases disproportionately, and component lifespan drops significantly. The ideal operating zone is 60–75% of rated capacity.

Minimum capacity (kW) = Peak Load × 1.25

Step 5: Anticipate Load Growth

Add an extra 10–15% capacity for load growth over the next 3–5 years. Standard genset capacities available in the Indonesian market: 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 100, 125, 150, 200, 250, 300, 400, 500 kVA.

Complete Genset Sizing Formula

kVA = (Total Running Watts + Max Starting Surge) × 1.25 ÷ PF (0.8)
Then choose the nearest standard capacity that's higher.

Signs of an Undersized Genset

• Frequent tripping under full load
• Dropping output voltage (voltage sag)
• Excessive engine heat
• Very high fuel consumption relative to output

Signs of an Oversized Genset

• Always running below 30% load
• Thick black smoke (wet stacking in diesel engines)
• Fuel cost disproportionate to output
• Poor investment ROI

For a fast and accurate calculation process, use the Jerbindo load simulator — enter your equipment list and the system will automatically calculate all parameters.