How to Size a Diesel Generator: Complete Step-by-Step Guide

To size a diesel generator, calculate the total running wattage of all loads it must power simultaneously, add the largest single-motor starting surge (typically 3× its running wattage), apply a 20–25% capacity buffer, then derate for altitude and ambient temperature. The result is the minimum generator kVA rating you need. For example: a facility with 40 kW of running loads, a 15 kW motor as the largest single starter (requiring a 45 kW surge), and operations at 1,500m altitude needs a generator rated for at least 68–75 kVA after all adjustments. Undersizing causes overload trips and engine damage; oversizing wastes fuel and causes wet-stacking in diesel engines. This guide walks through every step of the sizing process with worked examples, load tables, and correction factors.

Step 1 — Identify and List All Electrical Loads

The foundation of generator sizing is a complete load inventory. Missing even one large load — a compressor, an elevator motor, or a central air conditioning unit — can invalidate the entire sizing calculation. Organize loads into three categories based on their electrical behavior:

• Resistive loads — incandescent lighting, electric heaters, toasters, water heaters; these draw steady current with a power factor of 1.0 and no starting surge; running watts = nameplate watts
• Inductive loads (motors) — air conditioners, pumps, compressors, fans, power tools; these draw 3–7× their running current at startup for 0.5–3 seconds; this starting surge is the primary driver of generator sizing in most applications
• Electronic / non-linear loads — computers, VFDs (variable frequency drives), UPS systems, LED drivers, battery chargers; these draw non-sinusoidal current that introduces harmonic distortion; require generator alternators rated for harmonic service (typically THD <5% at full load)

For each load, record the nameplate running watts (or kW), voltage, and phase (single-phase or three-phase). If nameplate data is unavailable, use the amperage rating and calculate: Watts = Volts × Amps × Power Factor (use 0.85–0.90 for most motors if power factor is not stated).

Step 2 — Calculate Total Running Load and Motor Starting Requirements

Total Running Load

Sum all running watts for every load that will operate simultaneously. Do not include loads that are never used at the same time — a standby generator powering a building after a utility outage does not need to serve both the chilled water plant and the heating system simultaneously if they operate in different seasons. However, be conservative: include loads that could theoretically overlap even if unusual.

Motor Starting Current: The Critical Surge Demand

When an electric motor starts, it draws a locked-rotor current (LRC) that is typically 3 to 7 times its full-load running current. For generator sizing, this surge is expressed as starting watts — the instantaneous power demand at motor start. The most commonly used multipliers by motor type are:

• Direct-on-line (DOL) start motors — starting watts = 3× running watts (conservative commonly used value; actual LRC may be up to 7× for large motors)
• Capacitor-start motors — starting watts = 1.5–2× running watts; the start capacitor reduces inrush current significantly
• Motors with soft starters or VFDs — starting watts ≈ running watts; soft starters and variable frequency drives ramp voltage or frequency gradually, limiting inrush to 110–150% of running current; this dramatically reduces generator sizing requirements for motor-heavy facilities

The generator must handle the scenario where the largest motor starts while all other running loads are already drawing power. The critical calculation is: Generator sizing load = (Total running watts of all loads) + (Starting surge of the largest single motor − its running watts). This represents the peak instantaneous demand at the moment the largest motor starts.

Worked Example: Office Building Standby Generator

Consider an office building requiring standby power for:

• Lighting and receptacles: 12,000 W (12 kW)
• Server room UPS: 8,000 W (8 kW)
• Elevator motor (DOL start): 15,000 W running (15 kW), starting surge = 3 × 15,000 = 45,000 W
• HVAC fan motors: 10,000 W running (10 kW), starting surge = 3 × 10,000 = 30,000 W
• Fire pump motor (DOL start): 7,500 W running (7.5 kW), starting surge = 3 × 7,500 = 22,500 W

Total running load: 12 + 8 + 15 + 10 + 7.5 = 52.5 kW
Largest motor starting surge: Elevator motor at 45 kW starting − 15 kW running = 30 kW additional surge demand
Peak instantaneous demand: 52.5 + 30 = 82.5 kW

Step 3 — Convert to kVA and Apply Power Factor

Generator capacity is rated in kVA (kilovolt-amperes) — apparent power — rather than kW (kilowatts) — real power. The relationship is:

kVA = kW ÷ Power Factor

Most diesel generators are rated at a power factor of 0.8 lagging — this is the standard assumption unless otherwise specified. A generator rated 100 kVA at 0.8 power factor delivers 80 kW of real power. This means you must divide your kW requirement by 0.8 to find the required kVA rating.

Continuing the worked example:

• Peak instantaneous demand: 82.5 kW
• Required kVA: 82.5 ÷ 0.8 = 103 kVA

If your load is predominantly resistive (heaters, lighting) with very few motors, the actual power factor may be closer to 0.9–1.0, and dividing by 0.8 is overly conservative. If your load is predominantly inductive motors, the actual power factor may be 0.7 or lower, and a 0.8 assumption may under-size the generator. For precision sizing, measure or calculate the weighted average power factor across all loads.

Step 4 — Apply the Capacity Buffer (Headroom Factor)

Running a diesel generator at 100% of rated capacity continuously causes excessive thermal stress, accelerates wear, and leaves no margin for load additions or calculation errors. Industry practice is to operate diesel generators at 70–80% of rated capacity at full running load, leaving 20–30% headroom.

Apply the headroom factor by dividing the calculated kVA requirement by the target loading fraction:

• At 80% loading: Required generator kVA = Calculated kVA ÷ 0.80
• At 75% loading: Required generator kVA = Calculated kVA ÷ 0.75

Continuing the example at 80% loading: 103 kVA ÷ 0.80 = 129 kVA minimum rated generator. The nearest standard generator size above this is typically a 150 kVA unit.

A note on minimum loading: diesel engines also have a minimum load requirement of 30–40% of rated capacity. Running a diesel generator below this threshold for extended periods causes wet-stacking — incomplete combustion deposits unburned fuel and carbon in the exhaust system and cylinders, increasing maintenance costs and reducing engine life. If your expected running load is frequently below 30% of the generator's rating, the unit is oversized and you should select a smaller generator or implement load banking (connecting artificial resistive load to maintain minimum engine loading).

Step 5 — Derate for Altitude and Ambient Temperature

Diesel generator power output is rated at standard conditions: sea level (0m altitude), 25°C (77°F) ambient temperature, and 30% relative humidity per ISO 8528-1 or SAE J1349. Operating above sea level or in high ambient temperatures reduces the air density reaching the engine, reducing combustion efficiency and power output. The generator must be derated — its effective output is less than the nameplate rating, so the nameplate rating must be higher than calculated.

Altitude Derating

The standard derating rule for naturally aspirated diesel engines is approximately 3–4% power loss per 300m (1,000 ft) above sea level. Turbocharged engines derate less — typically 1–2% per 300m — because the turbocharger compensates for reduced air density up to its design limit, after which derating increases sharply. Always use the manufacturer's specific derating curves; the values below are representative:

Temperature Derating

Above the standard 25°C rating temperature, generators derate at approximately 1% per 5.5°C (10°F) above 25°C for most turbocharged engines. In a tropical environment with a 45°C peak ambient temperature (20°C above standard), expect an additional 3–4% power reduction. Combined altitude and temperature derating is multiplicative — both factors apply simultaneously.

To find the required nameplate kVA after derating: Required nameplate kVA = Required effective kVA ÷ (Altitude factor × Temperature factor)

Example: A 129 kVA effective requirement at 1,500m altitude (factor 0.94) and 40°C ambient (factor 0.97) requires: 129 ÷ (0.94 × 0.97) = 129 ÷ 0.912 = 141 kVA nameplate minimum, so select the next standard size: 150 kVA.

Common Load Types and Their Sizing Multipliers

Prime Power vs. Standby Rating: Choosing the Right Rating Class

Diesel generators are sold with multiple rating classifications that define how hard and how long the engine can sustain a given output. Using a generator beyond its intended rating class causes premature engine failure. The four main ISO 8528 rating classes are:

• Standby (ESP — Emergency Standby Power) — maximum output for emergency use during utility outage only; no overload permitted; typical use limited to 200 hours per year; this is the highest kVA rating on the nameplate but is not appropriate for prime power or frequent use applications
• Prime Power (PRP — Prime Rated Power) — continuous operation for unlimited hours where no utility supply exists; 10% overload permitted for 1 hour in 12; rated at approximately 80–90% of the same engine's standby rating; correct for off-grid sites, construction power, mining operations
• Continuous Power (COP) — base load operation at constant power for unlimited hours with no overload permitted; approximately 70–80% of standby rating; used in island power generation and base load applications
• Limited Time Running Power (LTP) — operation for defined limited durations in non-emergency applications; typically 500 hours per year maximum

A generator marketed as "100 kVA Standby / 90 kVA Prime" has two different power limits depending on how it is used. For a hospital backup generator used only during power outages, the 100 kVA standby rating applies. For a mining camp generator running continuously as the only power source, the 90 kVA prime rating governs — and the sizing calculation must use 90 kVA as the reference, not 100 kVA.

Three-Phase vs. Single-Phase Generators and Load Balancing

Generators above approximately 15–20 kVA are almost always three-phase (3Φ) because three-phase power provides more efficient power delivery and is required for three-phase motors. When sizing a three-phase generator for a mixed load (some three-phase motors plus single-phase loads), phase balance becomes a critical consideration.

Three-phase generators are rated for balanced loads — equal power on each phase. If single-phase loads are distributed unevenly across the three phases, the most heavily loaded phase limits total generator output and can cause voltage imbalance that harms motors and electronics. Most generator manufacturers specify that single-phase load imbalance between any two phases should not exceed 25% of the generator's rated current per phase.

When preparing your load list for a three-phase generator, assign each single-phase load to a specific phase and verify that no phase carries more than approximately 1/3 of total load + 12.5% of total kVA. In practice, distribute loads as evenly as possible and verify balance with an electrician during installation.

Sizing for Non-Linear Loads: UPS Systems and VFDs

Non-linear loads — UPS systems, variable frequency drives, switch-mode power supplies, and battery chargers — draw non-sinusoidal current that introduces harmonic distortion into the generator's voltage output. This harmonic content causes additional heating in the alternator windings and can interfere with the generator's automatic voltage regulator (AVR), causing voltage instability.

The industry guideline for sizing generators feeding predominantly non-linear loads:

• UPS systems — size the generator at 1.5 to 2× the UPS kVA rating; a 50 kVA UPS requires a 75–100 kVA generator minimum; this accounts for harmonic derating, UPS input power factor, and battery recharge demand during the first minutes after generator start
• Variable frequency drives (VFDs) — VFDs reduce motor starting surge but introduce harmonics; size the generator at 1.25× the kVA required by all VFD loads; specify a generator with a "12-pulse" or low-THD alternator if VFD loads exceed 50% of total generator load
• Data center / server loads — modern server power supplies have power factors of 0.95–0.99 with moderate harmonic content; size at 1.25–1.5× total IT load to account for power distribution unit (PDU) losses and cooling equipment

Complete Sizing Example: Industrial Workshop

A manufacturing workshop in a mountainous region at 1,200m altitude with a peak ambient temperature of 38°C requires a prime power generator for the following loads:

Sizing calculation:

1. Total running load: 53.6 kW
2. Largest motor surge addition: Air compressor surge (45 kW) − running (15 kW) = +30 kW
3. Peak instantaneous demand: 53.6 + 30 = 83.6 kW
4. Convert to kVA at PF 0.8: 83.6 ÷ 0.8 = 104.5 kVA
5. Apply 80% loading headroom: 104.5 ÷ 0.8 = 130.6 kVA
6. Altitude derating at 1,200m (turbocharged, factor ≈ 0.953): 130.6 ÷ 0.953 = 137 kVA
7. Temperature derating at 38°C (factor ≈ 0.975): 137 ÷ 0.975 = 140.5 kVA
8. Select standard generator size: 150 kVA Prime rated

Common Sizing Mistakes and How to Avoid Them

• Ignoring motor starting surge — the most frequent cause of undersizing; a generator that handles running loads easily may trip immediately when a large motor starts; always calculate peak demand including the largest motor start-up
• Confusing kW and kVA — a supplier quoting "100 kW generator" at 0.8 power factor is offering 125 kVA; verify whether the quoted figure is kW or kVA to avoid undersizing by 25%
• Using standby rating for prime power applications — a generator running continuously off-grid must be sized to its prime power rating, not the (higher) standby rating; using the standby figure for continuous duty leads to engine overloading and premature failure
• Oversizing to "be safe" without checking minimum load — a 500 kVA generator installed for a 50 kW load runs at 10% capacity, causing severe wet-stacking; minimum operating load should be 30–40% of rated capacity
• Omitting altitude and temperature derating — a 100 kVA generator at 2,000m altitude may deliver only 91 kVA; failing to account for this can result in chronic overloading at high-elevation sites
• Not accounting for future load growth — a generator sized exactly for today's loads has no room for expansion; add a realistic growth projection (typically 10–20% additional capacity for facilities expecting expansion within 5 years)