Last updated: September 2026 | SmartPower Electronics — manufacturer of load banks & power testing equipment since 2010

Quick answer: what size load bank do you need?

Size your load bank at 125% of the rated power of the device under test. For a 400 kW diesel generator, choose a 500 kW load bank; for a 100 kW generator, a 125–150 kW unit is the practical sweet spot. The 25% margin lets you reach true 100% load without running the equipment at its absolute limit, and leaves headroom for voltage/frequency tolerance and future expansion. For UPS and data-center systems, size on real power (kW = kVA × power factor, typically 0.8) and confirm the load type (resistive vs reactive vs electronic) matches what you are testing.

Read on for the step-by-step method, worked examples, and the mistakes that cost buyers the most.

What is a load bank and when do you need one?

A load bank is a device that consumes electrical power and converts it to heat, creating a controlled, adjustable load for testing a power source — a generator, UPS, inverter, or battery system. It answers one question with proof: "Can this power system actually deliver what its nameplate promises, under real load?"

You need one when you must:

• Acceptance-test a new generator, UPS, or data-center PDU at commissioning
• Run routine loaded exercises required by codes and standards (e.g., NFPA 110 for emergency power systems)
• Verify repairs after alternator, AVR, or cooling-system work
• De-rate or validate equipment in hot or high-altitude sites
• Aging-test products on a production line (EV chargers, inverters, power supplies)

Rule of thumb used by field engineers: load bank capacity = rated power of the device under test × 1.25. Example: for a 400 kW diesel generator → 500 kW load bank.

Step-by-step: how to size a load bank for a generator

Step 1 — Identify the generator's rated power and your test objective

Find the continuous prime/standby rating on the genset nameplate (kW or kVA). Then define the test:

Step 2 — Apply the 1.25 sizing margin

Why not exactly 100%? Three reasons:

1. Alternator voltage drops under full load — a unit sized at exactly 100% may not let you reach rated kW.
2. Ambient conditions (heat, altitude) derate both the genset and the load bank's cooling.
3. Headroom lets you test future upsized gensets instead of buying a second unit.

Step 3 — Match load type to the device

See the comparison table below. A pure generator test usually needs resistive elements; UPS and rotary systems need reactive (RLC) capability to simulate motors and lagging power factor.

Step 4 — Match voltage, phase, frequency and connector

Confirm the load bank's input matches your site: AC 230/400 V, single/three-phase, 50/60 Hz (dual-frequency units exist), plus the correct plug/socket standard. For permanent data-center installations, choose terminal-box or busbar connection; for field work, a portable unit with industrial sockets.

Step 5 — Choose control mode and step resolution

Modern load banks offer:

• Local manual / touch screen — for on-site commissioning
• Remote PC / host software — for automated load profiles, ramp tests, and report generation
• Step resolution matters: 5 kW or smaller steps on a 100 kW unit let you test precisely at 30%, 75%, 100%

Step 6 — Plan the physical installation

• Clearance: at least 2 m at the cold-air intake and 5 m at the hot-air exhaust from walls or obstacles
• Indoor vs outdoor: containerized units typically meet IP54–IP55 (dust-protected, water-jet resistant); indoor units are usually IP20
• Noise: resistive units commonly run 65–85 dB depending on power — check site limits
• Cooling: never block the outlet — hot-air recirculation causes derating and element damage

Resistive vs. reactive vs. electronic: which load bank do you need?

How to size a load bank for UPS and data-center systems

UPS output testing is about real power, not apparent power:

• kW = kVA × power factor. A 250 kVA UPS at PF 0.8 needs a 200 kW load bank to reach full output.
• For inverter/UPS output testing, resistive load verifies steady-state; add reactive capability to test under realistic lagging power factor.
• For data-center generator plants, size per Tier requirements and test resistive + reactive to mirror mixed IT load — don't rely on resistive-only unless the site is purely resistive.

Data-center sizing shortcut: total IT load × 1.25, then round up to the next standard unit size, and plan for parallel connection if you may exceed 1,000 kW later.

How often should you test? (and which standards apply)

• Emergency power (EPSS): NFPA 110 outlines loaded exercise schedules — commonly 30% load for 30 minutes monthly, with full-load testing at commissioning and annually in many jurisdictions. Always verify the edition applicable to your site.
• Genset performance: ISO 8528 series governs rating and performance definitions for generating sets.
• UPS/battery: industry practice recommends full-load testing at commissioning and quarterly loaded runs to expose failing battery strings before they fail under real outage.
• Calibration: load-bank sensors and controllers should be calibrated at an accredited lab annually (or per manufacturer guidance).

FAQ

What size load bank do I need for a 400 kW generator?

A 500 kW load bank (400 kW × 1.25). This reaches 100% load comfortably and leaves margin for voltage drop and ambient derating.

Why can't I just size the load bank at exactly the generator's rating?

At exactly 100%, alternator voltage drop and ambient conditions can prevent you from ever reaching full rated kW — the test fails for the wrong reason. The 25% margin is standard field practice.

Can one load bank test both a generator and a UPS?

Often yes, if voltage/frequency match. Use a resistive unit for steady-state generator and UPS inverter tests; add reactive (RLC) capability if you need to simulate lagging power factor for rotary UPS or data-center plants.

Resistive, reactive, or electronic — which one do I actually need?

Routine generator/UPS maintenance → resistive (best value). Data center and marine backup → resistive + reactive. R&D and production aging of batteries/chargers/inverters → electronic.

How do I size for a UPS instead of a generator?

Convert to real power first: kW = kVA × PF (typically 0.8). A 250 kVA UPS needs a 200 kW load bank to be fully exercised.

What IP rating do I need for outdoor use?

At least IP54–IP55 for containerized outdoor units. For permanent outdoor installations in humid climates, choose a model with built-in dehumidification/heating, and perform an insulation-resistance test before first use.

Should I buy or rent a load bank?

Rent if you test once or twice a year and transport is easy. Buy if you have recurring code-required testing, a production line, or multiple sites — a 500 kW unit typically pays for itself in under two years of rental-equivalent usage.

How do I size for future growth?

Size for your next genset or UPS, not the current one — one load bank can serve a fleet of smaller units, so oversizing once (e.g., 1000 kW instead of 800 kW) avoids a second purchase later. Ensure the unit supports parallel connection for further expansion.

Summary

Sizing a load bank is a 6-step process: know the rated power → apply the 1.25 margin → pick the load type → match voltage/phase/frequency → choose control & step resolution → plan cooling and clearances. For generator testing, start at 125% of rated power with a resistive unit; for UPS and data centers, convert kVA to kW at PF 0.8 and add reactive capability when the load profile demands it.

Since 2010, SmartPower Electronics has manufactured load banks from 3 kW to 2,000 kW (AC/DC, resistive, RLC and electronic types) with CE, CCC and UL certification, trusted in 80+ countries for generator, UPS, data-center, solar, and EV-charger testing. Our engineering team can review your sizing sheet and recommend a standard or custom configuration — including touch-screen control, remote PC software, and IP54 outdoor containerized builds.

Need a second opinion on your sizing calculation? Contact our engineers or browse the full load bank range. For day-to-day questions, the load bank FAQ covers capacity, IP ratings, maintenance and calibration.