Selecting and sizing a hydraulic power unit (HPU) correctly is one of the most critical decisions in any hydraulic system design. An undersized unit leads to overheating, pressure drops, and premature failures. An oversized one wastes energy, space, and budget. Whether you’re designing a compact machine for mobile use or integrating a power pack into an industrial automation line, this guide walks you through the key parameters every engineer needs to evaluate before.

1. Define the load requirements for size a hydraulic power unit: start with the actuator

The sizing process of a hydraulic power unit always begins at the actuator , the cylinder, hydraulic motor, or rotary actuator the HPU must drive and works backwards toward the power source.

For a linear cylinder, you need:

• Required force (F) in kN or tonnes
• Cylinder bore diameter (D) and rod diameter
• Stroke length (L)
• Cycle time (t) — how fast the stroke must complete

From these, you can calculate the required flow rate (Q):

Q (L/min) = (π/4 × D² × stroke) / cycle time

And the required working pressure (P):

P (bar) = F / effective piston area

Always add a 10–20% safety margin to both pressure and flow to account for line losses, viscosity changes, and future system adjustments.

For a hydraulic motor, define the required torque and rotational speed. The flow rate is determined by the motor’s specific displacement (cc/rev) multiplied by the desired RPM.

2. Establish the operating pressure range in order to size a hydraulic power unit

Hydraulic power units are typically rated across a broad pressure range from as low as 50 bar for light-duty applications up to 370 bar for high-force compact systems. Choosing the correct pressure class is not just a safety issue: it directly impacts pump size, motor power, component selection, and the physical envelope of the entire system.

Higher pressure = smaller actuators for the same force. This is one of the core reasons why modern OEM machine builders choose high-pressure compact HPUs: a 370 bar system can deliver the same force as a low-pressure system with a cylinder half the size. For space-constrained applications — think mobile cranes, vehicle-integrated equipment, aerial work platforms, or marine deck machinery — this is a decisive advantage.

Key rule: Never size your hydraulic power unit at its absolute maximum pressure rating. Operating continuously near the upper limit shortens pump and seal life. A well-sized system typically operates at 70–85% of the rated maximum pressure.

3. Calculate the required hydraulic power and motor size

Once you have flow rate (Q in L/min) and pressure (P in bar), calculating the required hydraulic power is straightforward:

Power (kW) = (P × Q) / 600

This formula gives you the theoretical hydraulic power. To find the actual electric motor power, you must account for pump efficiency (typically 85–92% for gear pumps, 90–95% for piston pumps) and motor efficiency:

Motor Power (kW) = Hydraulic Power / (η_pump × η_motor)

For example: a system requiring 160 bar at 8 L/min needs approximately 2.13 kW of hydraulic power. With a combined efficiency of 85%, you would specify at minimum a 2.5 kW motor.

4. Choose the right pump type for your hydraulic power unit

The pump is the heart of the HPU. The main types to consider:

• Gear pumps : robust, cost-effective, suitable for continuous duty at moderate pressures (up to ~250 bar). Ideal for industrial and general-purpose applications.
• Piston pumps : high efficiency, suitable for high-pressure applications (up to 370 bar and beyond), best for demanding or precision systems.
• Vane pumps : low noise, good for servo applications, medium pressure range.

For compact hydraulic power unit design — especially units destined for integration inside machines or mobile equipment — gear pumps and piston pumps are the dominant choice, paired with carefully engineered manifold blocks to minimize external piping and potential leak points.

5. Size the reservoir: thermal capacity and residence time

The hydraulic reservoir serves two functions: fluid storage and heat dissipation. A common rule of thumb is to size the tank at 3–5× the pump flow rate per minute (e.g., a 10 L/min pump → 30–50 L tank). However, for compact HPUs operating in thermally demanding environments, this rule must be balanced against physical space constraints.

In such cases, external heat exchangers (oil-to-air or oil-to-water coolers) become necessary. Thermal analysis should always verify that the fluid temperature stays below 60–70°C under worst-case continuous operation.

For compact power packs — where tank volume is intentionally minimized — selecting a high-quality hydraulic fluid with good thermal stability is equally important. Biodegradable fluids such as synthetic esters can offer performance advantages in environmentally sensitive applications.

6. Select the power supply: AC vs. DC Drive

For industrial stationary applications, three-phase AC motors (230/400V) are standard. However, the rapid growth of mobile machinery, autonomous vehicles, and off-grid equipment has made DC-powered HPUs increasingly important.

DC hydraulic power units operating at 12V, 24V, or 48V are now used across a wide range of sectors: emergency vehicles, rail maintenance equipment, maritime applications, agricultural machinery, and battery-electric construction machines. When sizing a DC HPU, pay careful attention to:

• Inrush current at startup (can be 5–8× rated current)
• Cable cross-section and voltage drop over long cable runs
• Duty cycle — many DC motors are rated for intermittent operation

7. Consider safety, certifications, and control architecture when you size your hydraulic power unit

Modern HPU sizing is not just a hydraulic and mechanical exercise — it increasingly involves functional safety and control system integration.

For applications in potentially explosive environments (ATEX zones), HPUs must be specified with ATEX-rated components from the outset. For safety-critical applications — cranes, elevators, rescue equipment, subsea machinery — SIL (Safety Integrity Level) ratings per IEC 61508 may be a contractual or regulatory requirement.

Advanced compact HPUs can integrate a built-in electronic controller that manages pressure, flow, temperature monitoring, fault detection, and communication with the machine’s PLC or BMS. This “smart” architecture reduces external wiring, simplifies commissioning, and enables remote diagnostics — a key differentiator for OEM integrators working on connected machines.

Summary: Hydraulic power unit sizing checklist

Working with a compact hydraulic power unit specialist

For OEM engineers designing space-constrained or safety-critical machines, working with a specialist manufacturer from the early sizing stage — rather than adapting a standard unit at the end — can significantly reduce both development time and total system cost.

At Hydronit, we support engineers through the full HPU selection and customization process: from hydraulic calculations to control architecture and certification. Our product range covers operating pressures up to 370 bar, with configurations for AC, DC, and mixed drive systems, distributed across more than 70 countries worldwide.

→ Contact our technical team to discuss your HPU sizing requirements.