How to Size a Load Bank for a Generator

To size a load bank for a generator, match the load bank to the generator's nameplate power and rated power factor: use a resistive load bank rated to the genset's kW (which loads it to 100% of kW but only about 80% of kVA), or a resistive-plus-reactive load bank rated to the full kVA at 0.8 power factor to test the alternator fully. Then derate for altitude and ambient temperature at the test site.

How do you size a load bank for a generator?

Sizing a load bank means choosing a unit whose capacity and type let you load the generator to the level you need to test - typically between 80% and 100% of its rated output. The starting point is always the generator nameplate, which lists a kVA rating, a kW rating and a rated power factor (almost always 0.8 lagging for a genset).

The single most important rule is to size against the correct quantity. A resistive-only load bank operates at unity power factor (1.0), so every kilovolt-amp it draws equals one kilowatt. That means a resistive load bank rated to the generator's nameplate kW will fully load the engine (100% kW) but only load the alternator to about 80% of its kVA. To test the alternator and voltage regulator to full nameplate kVA, you need a combined resistive-plus-reactive load bank that reproduces the 0.8 power factor.

In practice, sizing comes down to four decisions: (1) what percentage of rated output you must reach; (2) whether a resistive load bank is sufficient or a reactive component is required; (3) the correct kW and kVAR figures once power factor is applied; and (4) how much extra headroom to add for altitude and temperature derating at the test location.

kW vs kVA vs kVAR: which rating do you size against?

Getting the units right is where most sizing errors happen. A generator's real power is measured in kilowatts (kW) - this is what the engine produces and what does useful work. Apparent power is measured in kilovolt-amps (kVA) - this is what the alternator must supply. Reactive power is measured in kilovolt-amps reactive (kVAR) - the magnetising component drawn by motors, transformers and similar inductive loads. Power factor is the ratio of real to apparent power: kW divided by kVA.

Because gensets are rated at 0.8 power factor, kW = kVA x 0.8. A 500 kVA generator is therefore a 400 kW generator. A resistive load bank rated at 400 kW will load that machine to 100% of its kW (fully exercising the engine) but only to 80% of its 500 kVA (leaving 20% of the alternator's capability untested).

To load the alternator to a genuine 100% of nameplate kVA at 0.8 power factor, add inductive (reactive) load. The standard relationship is 75 kVAR of inductive load for every 100 kW of resistive load, which resolves to a 0.8 power factor. So a full-kVA test of that 500 kVA / 400 kW set needs roughly 400 kW resistive plus 300 kVAR inductive, combining to 500 kVA at 0.8 pf.

Should you test at 80% or 100% of rated kW?

For a routine standby generator, the widely accepted minimum is a resistive load bank rated to at least 80% of the generator's nameplate kW. Running a diesel engine at low load for extended periods causes wet stacking - unburned fuel and carbon deposits in the exhaust and turbocharger - and a load bank test at 80% or above burns this off and confirms the set can carry real load.

For commissioning, acceptance testing and full performance verification, size for 100%. A resistive load bank at 100% of nameplate kW proves the engine, cooling system, fuel delivery and exhaust at full thermal load. A combined resistive-plus-reactive load bank at 100% of nameplate kVA (0.8 pf) additionally proves the alternator, automatic voltage regulator and how the machine holds voltage and frequency under a realistic lagging load.

Compliance testing is often stepped rather than a single value. Under NFPA 110, the annual load-bank exercise has historically used a three-step protocol (30% or 25%, then 50%, then 75%); the 2025 edition streamlines this to 50% of nameplate for 30 minutes followed by 75% for one hour. Because editions and local authorities having jurisdiction differ, always size the load bank to reach 100% and step down, and confirm the exact protocol your inspector enforces. Sizing for the maximum you might need is safer than discovering mid-test that the unit cannot reach the required step.

How do altitude and temperature derating affect load bank sizing?

Both generators and load banks lose capacity in thin, hot air, and both must be accounted for so the load bank can still reach the target percentage at the actual site conditions. Reduced air density at altitude and high ambient temperature cut the generator's output and, for forced-air-cooled load banks, can reduce the load the bank can safely dissipate.

A common engineering rule of thumb for generator derating is roughly 3.5% loss of power for every 1,000 feet (about 300 m) above sea level, and approximately 1% for every 5 to 10 degrees C of ambient temperature above the rated condition (typically 40 degrees C for gensets, or 25 degrees C for some ratings). These are guidelines only - the manufacturer's derating curves for the specific engine, alternator and load bank always take precedence.

The practical consequence for sizing is twofold. First, the generator's real available output at a high, hot site may be well below its sea-level nameplate, so 100% of the derated output is a smaller number - do not oversize the load bank to the nameplate if you only intend to reach derated full load. Second, a fan-cooled load bank at altitude has less cooling air, so its own capacity may need derating; choose a bank with headroom or one specified for the site conditions. When conditions are severe, natural- or forced-convection ratings and liquid-cooled designs behave differently, so check the load bank datasheet against the site elevation and maximum ambient (many resistive elements are rated to around 50 degrees C ambient).

Worked examples: sizing a load bank step by step

Example 1 - Resistive-only compliance test of a 500 kVA standby set. The nameplate reads 500 kVA, 400 kW, 0.8 pf. For an 80% minimum exercise you need at least 320 kW of resistive load (0.8 x 400 kW). To retain the option of a full 100% kW test and stepped loading, specify a 400 kW resistive load bank with adjustable steps. This loads the engine to 100% of kW but only to 80% of the 500 kVA, which is acceptable for routine anti-wet-stacking and NFPA-style exercises.

Example 2 - Full-kVA acceptance test of the same 500 kVA set. To test the alternator and AVR to full nameplate at 0.8 pf, combine resistive and reactive load: about 400 kW resistive plus about 300 kVAR inductive, which sums to 500 kVA at 0.8 power factor. Specify a resistive/reactive load bank rated 400 kW / 300 kVAR (or the nearest standard size above), giving a genuine 100% kVA test that a resistive-only bank cannot deliver.

Example 3 - Derating at a high-altitude, hot site. Take a 1,000 kW (1,250 kVA) generator installed at 5,000 ft and 45 degrees C. Applying roughly 3.5% per 1,000 ft gives about 17.5% altitude loss; a further allowance of a few percent for the elevated ambient brings the derating to roughly 20-25%. The set may therefore deliver only around 750-800 kW at site. Size the resistive load bank to reach that derated full load (around 800 kW) rather than the 1,000 kW nameplate, and confirm the load bank's own cooling is rated for 5,000 ft and 45 degrees C so it can actually dissipate that load. If in doubt, add 10-15% capacity headroom.

What load bank type should you choose for the test?

The load bank type follows from what you are trying to prove. A resistive AC load bank (unity power factor) is the standard choice for engine exercising, anti-wet-stacking runs and kW verification. It is the simplest and most economical option and is sufficient for the great majority of routine standby generator tests.

A resistive/reactive (inductive) load bank is required when you must test the alternator, voltage regulation and full kVA at the rated 0.8 power factor - typically at commissioning, factory acceptance and for critical installations. Reactive load banks reproduce the lagging load that motors and transformers impose in service, which a resistive bank cannot. For sites with significant capacitive load, such as long cable runs or lightly loaded UPS systems, a capacitive (leading) capability may also be relevant.

Other specialised types map to specific applications: DC load banks for batteries and DC plant, liquid-cooled load banks where air cooling is impractical or noise and footprint matter, and data-centre or server-emulator load banks that mimic IT load profiles. Whichever type you choose, size it against the correct quantity (kW for resistive, kVA and kVAR for reactive), apply site derating, and verify it meets the relevant standards - IEC/UL/CE for the equipment and ISO 8528-6 (BS 7698-6) for genset acceptance testing. As a load bank manufacturer, Ashford Energy can advise on the right rating and configuration for a specific generator and site.

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