Why Generator Sizing Matters
Choosing the right generator size is the single most important decision when investing in backup power. An undersized unit trips under load and leaves you without power at the worst moment. An oversized unit wastes fuel, runs inefficiently, and costs more than necessary on rental or purchase.
At Aiwa Powers, we have sized generators for hundreds of clients across Karachi, Hyderabad, and Sukkur since 2006. Here is the method our engineers use.
Step 1: List All Electrical Loads
Start by cataloguing every piece of equipment that will run on the generator. For each item, note:
- Running wattage — steady power consumed during normal operation
- Starting (surge) wattage — the spike needed to start motors (can be 2–7× running watts)
Common loads in Pakistani commercial settings include air conditioners, motors (pumps, compressors), lighting, computers, HVAC systems, and refrigeration.
Step 2: Calculate Total Running Load
Add the running wattages of all devices you expect to operate simultaneously. Not everything runs at once—a 5,000 sq ft factory does not have every machine and every light on at the same moment. Use a realistic 70–80% concurrency factor.
Example: A mid-size Karachi office with 20 PCs, 4 ACs (1.5-ton), lighting, and servers has a running load of approximately 18–22 kW, requiring a 25 kVA diesel generator rental.
Step 3: Account for Starting Surge
The largest motor in your facility dictates the surge demand. If you have a 10 HP water pump, its starting surge alone can be 15–20 kW. Your generator must handle this surge without tripping.
Add the starting wattage of your single largest motor to your total running load to get your peak demand.
Step 4: Apply the 20% Safety Margin
Generators should not run continuously above 80% of rated capacity. Divide your peak demand by 0.8 to get the minimum generator size.
Formula: Min Generator kVA = (Total Running Load + Largest Motor Start Load) ÷ 0.8 ÷ Power Factor (0.8 for diesel)
Aiwa Powers kVA Range Guide
| Business Type | Recommended Size |
|---|---|
| Small shop, kiosk, small office | 10–15 kVA |
| Mid-size office (20–30 people) | 25 kVA |
| Warehouse, factory floor | 50–100 kVA |
| Hospital, large commercial | 150–300 kVA |
| Industrial plant, telecom tower | 300–500 kVA |
Still Unsure? We Will Size It for You
Our engineers at Aiwa Powers provide free load assessments for clients across Pakistan. We visit your facility, measure actual loads, and recommend exactly the right diesel generator—whether for rental or purchase. Contact us today to schedule your free assessment.