How to Calculate the Right DG Set Size for Your Site

Step 1 — List every load in watts

Walk the site (or the event plan) and list everything that will draw power at the same time: lights, fans, air conditioners, pumps, motors, sound systems, catering equipment. Use the nameplate rating where you can find it; typical values otherwise.

Step 2 — Separate motors from the rest

Motors (AC compressors, pumps, refrigerators) draw a large starting surge — often 2.5 to 3 times their running watts for a few seconds. Lighting and electronics don't. Mark every motor load.

• Lighting, fans, electronics: count at face value
• Motors: count running watts, then add a starting allowance
• Note the single largest motor — it dominates the surge calculation

Step 3 — Add running load plus a surge allowance

Add up all running watts. Then add roughly 40% of the largest motor's starting surge on top (motors rarely all start in the same second). This gives your peak watt demand.

Step 4 — Convert to kVA and add headroom

Divide peak watts by 800 (that's a 0.8 power factor, the standard assumption for DG sizing) and then by 1,000 to get kVA. Add 20% headroom — running a set at 100% continuously shortens its life.

Worked example

A site runs 20 tube lights (720W), 10 fans (750W), a 1.5T AC (1,800W running, motor), a 1HP pump (746W running, motor) and site tools (~2,000W). Running total: ~6,000W. Largest motor surge: 1,800 × 2.5 = 4,500W; 40% of that is 1,800W. Peak ≈ 7,800W → 9.75 kVA → plus 20% headroom ≈ 11.7 kVA → pick a 15 kVA set.

When to call a professional

This method covers most rentals well. Bring in a qualified electrical professional when: loads include large motors (above ~20 HP), you need multiple sets in parallel, the site has sensitive medical or industrial equipment, or the hire runs for months — fuel and maintenance economics then matter as much as sizing.