Diesel Generator Sizing Guide: How Many kW Do You Really Need?

POWER INSIGHTS | August 2026 | ZTA Power Technical Team | 6 min read

Diesel Generator Sizing Guide: How Many kW Do You Really Need?

Quick Answer

Key Takeaways

✔ kW = real power your equipment actually uses; kVA = apparent power the generator must supply (kW ÷ power factor)

✔ Motor starting surge can demand 3–6× running power — this is the #1 reason generators trip on startup

✔ Undersizing causes voltage dips, stalling, and equipment damage; oversizing wastes fuel and causes wet stacking

✔ Altitude and ambient temperature derate output above 1,000 m or 40°C

✔ Always specify prime power if running >4 hours/day; standby power is for emergency use only

✔ Pair your genset with an ATS (Automatic Transfer Switch) for seamless grid-to-generator switchover

Understanding kW, kVA, and Power Factor

The single biggest source of confusion in diesel generator sizing is the difference between kW (kilowatts) and kVA (kilovolt-amperes). kW measures the real power that actually does work — turning motors, heating elements, lighting rooms. kVA measures apparent power, which includes both real power and reactive power (kVAR) that flows back and forth in inductive loads like motors and transformers without being consumed.

The relationship is simple: kVA = kW ÷ Power Factor. Most industrial diesel generator sets are rated at 0.8 power factor, meaning a 1,000 kVA generator delivers 800 kW of usable power. If your facility needs 600 kW continuously, you need at least 750 kVA (600 ÷ 0.8) — and that is before adding any safety margin.

The 5-Step Sizing Method

Step 1 — List every load. Record running kW for each device. For motors, note both running kW and starting kVA. A 75 kW motor may need 225–375 kVA to start (3–5× multiplier).

Step 2 — Apply demand factor. Not all equipment runs simultaneously. Apply a demand factor (typically 0.7–0.85) to reduce the total. A factory with 800 kW connected load at 0.8 demand = 640 kW effective.

Step 3 — Add motor starting surge. Sequence motor starts so the largest motor starts first when the generator is lightly loaded. Check that the momentary kVA does not exceed the generator’s motor starting kVA rating.

Step 4 — Apply derating factors. Above 1,000 m altitude or 40°C ambient, output drops roughly 3–5% per 500 m and 1% per 5°C above rated. A 1,000 kVA genset at 1,500 m and 45°C may only deliver ~870 kVA.

Step 5 — Add 20–25% headroom. This reserve absorbs future load additions, voltage recovery transients, and prevents the engine from running at 100% load continuously — which shortens engine life and increases fuel burn.

Sizing Example: 500 kW Industrial Facility

Common Sizing Mistakes (And What They Cost You)

Undersizing — The generator trips, voltage dips, motors stall, and sensitive electronics shut down. In one case, a hospital backup generator sized at exactly peak load failed during a real outage because two MRI machines started simultaneously. The voltage sag triggered the UPS, which drained in 8 minutes. Lesson: never size to match your peak — size to exceed it.

Oversizing — A silent diesel generator running below 30% load for extended periods develops wet stacking — unburned fuel accumulates in the exhaust, carbon builds up on injectors, and oil dilution accelerates. A 2,000 kVA genset running a 400 kW load wastes roughly 15–20% more fuel per kW-hour than one running at 60–75% load. Engine life drops by 30–40%.

Ignoring motor starting kVA — This is the most frequent error. Induction motors draw 3–6× their running current for the first 5–15 seconds. If your generator cannot deliver that surge, the motor will not start — or worse, the voltage dip will trip every other device on the bus.

Forgetting altitude and temperature derating — A mining site at 2,500 m with 35°C ambient may see 12–18% less output than the nameplate rating. If you sized by the book at sea level, your genset will underperform on day one at site.

Real-World Case: Right-Sizing a Factory in Ethiopia

A textile factory in Addis Ababa (elevation 2,355 m) requested a 1,500 kVA diesel generator set based on their electrical engineer’s calculation at sea level. ZTA Power’s technical team reviewed the load schedule and identified three issues:

1. Altitude derating: At 2,355 m, the 1,500 kVA unit would deliver only ~1,260 kVA (16% loss).

2. Motor starting sequence: Three 110 kW compressors were set to auto-start simultaneously — a 990 kVA surge on a 1,260 kVA available capacity. We recommended staged starting with 10-second intervals.

3. No headroom: The 1,500 kVA selection left zero margin. We recommended upgrading to 1,800 kVA, which after derating delivered ~1,510 kVA at altitude — comfortably covering the 1,200 kVA peak with 25% reserve.

Result: Zero voltage dips, no tripping, and the Cummins-powered generator set with ATS panel has run reliably for 18 months on prime power. Fuel consumption at 65% load: 185 g/kWh — well within the engine’s sweet spot.

Prime Power vs. Standby Power: Which Rating Do You Need?

Rule of thumb: If your generator runs more than 4 hours per day on average, specify prime power rating. Standby-rated units cost less upfront but are not built for continuous duty.

Frequently Asked Questions

Q: Can I use a standby-rated generator for daily operation?

A: Not recommended. Standby generators are designed for occasional outages (≤200 hours/year). Running them daily shortens engine life and voids warranty. Choose a prime power diesel generator set for continuous applications.

Q: What happens if my generator is too big?

A: Running below 30% load causes wet stacking — unburned fuel and carbon accumulate in the exhaust, diluting engine oil and reducing efficiency. A genset at 30% load can consume 15–20% more fuel per delivered kW than one at 65% load.

Q: Do I really need an ATS?

A: If you want automatic switchover without human intervention, yes. An Automatic Transfer Switch (ATS) detects grid failure, signals the generator to start, and transfers load within 10–30 seconds. Without it, someone must manually start the genset and throw the switch — which is unacceptable for hospitals, data centers, or any facility that cannot tolerate downtime.

Q: How much derating should I apply for high altitude?

A: Approximately 3–5% per 500 m above 1,000 m, plus 1% per 5°C above 40°C. Always confirm with the engine manufacturer’s derating chart for your specific model.

Q: Should I choose an open or enclosed (silent) generator?

A: Open sets are fine for remote sites with no noise regulations. For urban or residential areas, a silent diesel generator with a soundproof canopy (65–75 dB(A) at 1 m) is essential. The canopy also protects against weather and vandalism.

Need Help Sizing Your Generator?

Send us your load list and site conditions — ZTA Power’s engineers will recommend the right diesel generator set within 24 hours.

✉ nancy@ztapower.com