An AC load bank applies load to alternating-current sources such as generators, alternators and the inverter output of a UPS, while a DC load bank applies load to direct-current sources such as battery strings, telecom power plant and battery energy storage systems (BESS). The core difference is the type of power being drawn: AC load banks verify that a machine can produce clean, rated AC power; DC load banks verify that a store of DC energy can deliver its rated capacity over time.
Key takeaways
- AC load banks test AC sources (generators, alternators, turbines, UPS output); DC load banks test DC sources (battery strings, telecom -48 V plant, UPS DC bus, BESS, chargers, fuel cells).
- AC load banks come in resistive (unity pf), resistive-reactive (typically 0.8 pf) and reactive types; a full generator proof uses 0.8 pf, needing about 75 kVAR per 100 kW.
- DC load banks measure true battery capacity by controlled discharge to an end-of-discharge voltage, the reference method in IEEE 450/1188/1106 for stationary batteries.
- Control modes decide what stays constant: constant current (CC) for battery capacity, constant power (CP) for inverter-like loads, constant resistance (CR) for simple resistive steps.
- AC ratings follow standard system voltages (208-600 V and up, 50/60 Hz); DC ratings span roughly 12 V to over 1000 V and must be specified by both voltage and current.
- A full standby chain (generator, UPS, battery) usually needs both AC and DC load banks, tested in sequence; equipment should carry CE, UL and IEC approvals.
What is the difference between an AC and a DC load bank?
An AC load bank draws power from an alternating-current source and dissipates it, usually as heat, to prove that the source can supply its rated output under real load. It is the standard tool for testing diesel and gas generators, gensets in parallel, alternators, turbines, switchgear and the AC output of an uninterruptible power supply (UPS). Because generators are rated in kW and kVA at a given power factor, AC load banks are built to reproduce that load accurately.
A DC load bank draws power from a direct-current source instead. Its job is not to prove a machine can generate power, but to prove that a store or supply of DC energy can deliver its rated current and capacity. Typical subjects are battery banks, the DC bus and batteries inside a UPS, -48 V telecom power plant, DC chargers and rectifiers, fuel cells, and the cells or racks of a battery energy storage system.
In short: use an AC load bank when the source produces AC (generation and UPS output); use a DC load bank when the source stores or supplies DC (batteries, telecom plant, BESS). Many facilities need both, because a full standby power chain contains an AC generator, a UPS with an AC output, and a DC battery string behind that UPS.
What does an AC load bank test?
An AC load bank tests the ability of an AC source to accept and sustain load at its rated voltage, frequency and power factor. The most common applications are commissioning and periodic testing of standby generators, exercising gensets to prevent wet-stacking, proving UPS output capacity, and verifying automatic transfer switches, switchgear and paralleling controls under realistic conditions.
There are three broad families of AC load bank, defined by the kind of load they present. A resistive load bank presents a unity power factor (1.0 pf) load rated in kW; it loads the engine and alternator and is the workhorse for basic generator testing. A resistive-reactive load bank adds inductive (and sometimes capacitive) elements so the set can be tested at a lagging power factor, typically 0.8 pf, which is the rating point of most generators. A reactive load bank supplies the kVAR component on its own, usually alongside a separate resistive unit.
The power-factor relationship is fixed by geometry: to reach a 0.8 lagging power factor, a load bank supplies roughly 75 kVAR of inductive load for every 100 kW of resistive load. Testing only with a resistive load loads the engine but not the alternator, voltage regulator or excitation system to their full design duty, which is why 0.8 pf resistive-reactive testing is preferred for a complete generator proof.
What does a DC load bank test?
A DC load bank tests whether a DC source can deliver its rated current and, crucially, its rated capacity in amp-hours over a defined discharge period. The classic application is a battery capacity or discharge test: the string is discharged at a controlled current representing its design duty cycle, and the time taken to reach the end-of-discharge voltage reveals the true remaining capacity as a percentage of nameplate.
Because a battery ages invisibly, a controlled discharge into a DC load bank is the most reliable way to know real capacity. Constant-current discharge is the reference method used by the stationary-battery standards (for example IEEE 450 for vented lead-acid, IEEE 1188 for VRLA and IEEE 1106 for nickel-cadmium), because a steady current cleanly reproduces the load the battery must actually support in an outage. A good DC load bank stops automatically when the string reaches its low-voltage limit, a set time, or a target amp-hour figure, and logs cell or block voltages throughout.
Beyond batteries, DC load banks test telecom -48 V power plant and rectifiers, UPS DC buses, DC generators, chargers, fuel cells and fuel-cell stacks, and increasingly the modules, racks and containers of battery energy storage systems, where discharge testing verifies both usable capacity and the behaviour of the battery management system under load.
What voltage and power ranges do AC and DC load banks cover?
AC load banks are built around the standard low-voltage system voltages and both mains frequencies. Common ratings include 208, 230, 240, 400, 415, 480 and 600 V AC, three-phase (single-phase versions exist for smaller units), at 50 Hz or 60 Hz. Medium-voltage load banks extend testing to distribution voltages such as 3.3, 6.6 and 11 kV for large plant and utility work. Capacity ranges from a few kW for portable units to many megawatts for data-centre and utility testing, and units are designed to switch load in defined steps for controllability.
DC load banks span a much wider spread of voltages because DC sources vary so widely. Typical points include 12, 24 and 48 V for automotive, telecom and small UPS batteries; 110/125 V and 220/250 V for switchgear tripping batteries and larger UPS strings; and higher voltages up to several hundred or over a thousand volts DC for data-centre battery systems and BESS. Current ratings matter as much as voltage on the DC side, since capacity testing is driven by discharge current, so DC units are specified by both maximum voltage and maximum current.
When choosing a unit, match not only the nominal voltage but the full operating window. A battery's terminal voltage falls as it discharges, so a DC load bank must keep controlling load down to the end-of-discharge voltage; an AC load bank must hold its rating across the voltage and frequency tolerance band of the source it is testing.
What are load bank control modes (CC, CP, CR)?
Control mode describes what quantity the load bank holds constant as conditions change. The main modes are constant current (CC), constant power (CP), constant resistance (CR) and, on some electronic loads, constant voltage (CV). Choosing the right mode is what makes a test meaningful, because a battery's voltage sags during discharge and the mode determines how the drawn load responds.
In constant current (CC) mode the load draws a fixed current regardless of input voltage. This is the standard mode for battery capacity and discharge testing, because a steady current cleanly reproduces the battery's duty cycle and lets you time the discharge to the end-voltage. In constant power (CP) mode the load holds the drawn power (watts) steady, so as the battery voltage falls the current is automatically increased to keep power constant; this best mimics loads such as inverters and DC-DC converters that demand fixed power. In constant resistance (CR) mode the load behaves as a fixed resistor, so current rises and falls in proportion to voltage (I = V/R).
Traditional AC and resistive DC load banks are inherently constant-resistance devices: they are banks of resistor elements switched in kW steps, so the load naturally follows voltage. Programmable electronic DC loads add true CC, CP and CV control with fine resolution and data logging. Match the mode to the question you are asking: CC for how much capacity a battery really has, CP for how a source behaves under a fixed-power demand, and CR for a simple, robust resistive load step.
Which load bank do you need, and can one unit do both?
Start from the source, not the load bank. If you are proving a generator, genset paralleling scheme, turbine or the AC output of a UPS, you need an AC load bank, and for a complete generator proof you generally want resistive-reactive capability so you can test at 0.8 power factor rather than resistive-only. If you are proving a battery string, telecom plant, DC UPS bus or a BESS, you need a DC load bank sized in both volts and amps for your string, run in constant-current mode for capacity work.
A full standby power chain often needs both, tested in sequence: an AC load bank on the generator and on the UPS output, and a DC load bank on the battery behind the UPS. Some combined and modular systems can address both AC and DC testing, and manufacturers offer resistive, resistive-reactive, DC, liquid-cooled, data-centre and server-emulator families so the load bank can match the exact source, environment and footprint. Whatever the type, the equipment itself should carry the relevant approvals (CE, UL and IEC compliance) for the market it is used in.
Ashford Energy designs and supplies the full range of load banks, from AC resistive and resistive-reactive units to DC, liquid-cooled, data-centre and server-emulator models, so the test matches the source rather than forcing the source to fit the test.