How to Calculate Generator kVA Size: A Step-by-Step Guide for NE India Workshops and Sites

Why Generator Sizing Is Critical in NE India

Across Northeast India — from Assam's industrial estates to Arunachal's remote hydropower project camps — generators are the backbone of reliable power. Grid power in the region is frequently unreliable: outages in rural Assam can run for 4–8 hours; hill-district sites may have no grid connection at all. This means your generator is not occasional backup — it is your primary power infrastructure.

Undersizing is the most common and most expensive mistake. A chronically overloaded generator runs hot, suffers premature alternator wear, burns more fuel, and fails at the worst possible time — mid-pour on a concrete slab, or during a night-shift welding run. Oversizing wastes fuel and, at very low loads (below 30% rated output), causes wet stacking on diesel engines (unburnt fuel deposits in the exhaust system).

Step-by-Step Generator kVA Sizing

1. List every load that will run simultaneously. Write down every electrical device you will run from the generator at the same time. Include welding machines, motors (pumps, compressors, concrete mixers), power tools (angle grinders, drills), lighting, battery chargers, computers, and any HVAC. Note the rated wattage or kW of each from the nameplate.
2. Convert all loads to watts (W) or kW. Sum them up for the total connected load in kW. Example: 1× 250A MMA welder (8,000 W) + 1× 2HP submersible pump (1,500 W) + 4× 100W lights (400 W) + 1× 4-inch angle grinder (900 W) = 10,800 W = 10.8 kW.
3. Apply a motor-starting factor. For every electric motor in the list, multiply its rated running kW by 3 (to account for starting surge). Take the largest single motor starting kVA as your starting demand. This must not exceed the generator's starting capacity. For the example above, the largest motor is the 1,500W pump; its starting demand is ~4,500W — well within a 15 kVA generator's capacity.
4. Apply a power factor. Most generators have a rated power factor of 0.8. Divide your total kW load by 0.8 to get the minimum kVA requirement. Example: 10.8 kW ÷ 0.8 = 13.5 kVA required.
5. Add a 25% headroom allowance. Never run a generator at 100% of its rated output continuously. Apply a 25% margin: 13.5 kVA × 1.25 = 16.9 kVA. Round up to the next standard size: 20 kVA.
6. Apply altitude derating if above 1,000 m. Diesel generators lose approximately 3% of output per 300 m above 1,000 m ASL due to lower air density. At 2,000 m (common in Arunachal projects near Tawang, Mechuka or Ziro), apply a 10% derating: 20 kVA × 0.9 = 18 kVA effective — so your 20 kVA generator delivers 18 kVA at altitude, and your 16.9 kVA requirement is still met. At 3,000 m, apply 20% derating and step up one size.
7. Consider duty cycle for welding loads. Welding machines have a duty cycle (e.g. 60% at 250A — means 6 minutes welding per 10-minute period). On a generator, size for the peak draw during the welding period, not the average. In practice, a site running two MMA welders simultaneously should size the generator as if both are running at full load — arc strike timing is unpredictable.
8. Choose between petrol and diesel. For loads above 5 kVA or run-times above 4 hours, diesel is more economical. For lighter loads, portable use, or situations where petrol is the available fuel, the Alpha A2200 petrol generator is a proven option for site lighting and small tool loads. See our diesel vs petrol generator guide for a full comparison.

Quick kVA Sizing Reference

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