Tower crane BESS sizing: how to calculate PCS power and battery capacity

Engineering guide · News blog

Tower crane BESS sizing starts with measured load data. Use it to calculate PCS power, usable battery capacity and recharge margin.

Tower crane BESS sizing starts with the site’s measured load profile, not the crane nameplate alone. A crane may use a moderate amount of energy over a shift, yet place a sharp demand on the supply for a few seconds when the hoist, trolley and slewing drives operate together.Size the source from average consumption and it may trip during a lift. Size a generator for the worst instant and it can spend most of the day lightly loaded. A hybrid battery energy storage system can cover the difference, provided its power rating, usable energy and controls are based on the site’s actual load profile.

The short answer

Start with a time-series load profile measured at the crane feeder or site point of connection. Find the highest coincident demand and subtract the power that the grid or generator can safely provide. The difference is the load the BESS power conversion system must carry. Check it in both kW and kVA, and match the event to the PCS overload duration.Next, calculate the accumulated energy deficit between recharge opportunities. That gives the minimum usable battery capacity. A full-shift simulation should then confirm state of charge, recharge time, voltage and frequency behavior, temperature derating, protection settings and any regenerative power from the crane drives. Power and energy must be checked separately. A national laboratory report on BESS sizing uses the same approach: the application defines both the power rating and the energy capacity. Oversizing either one means paying for capacity that the site rarely uses.For a broader comparison of applications and product ranges, see how to determine the right hybrid BESS size. This guide stays with the engineering calculation for tower cranes: measured load, PCS power, usable battery capacity and recharge margin.

Why crane nameplate power is not enough

A crane data sheet may show the rated power of the hoist motor. That number does not describe the complete load at the site supply. Trolley and slewing drives also draw power, and the same connection may serve site hoists, pumps, welders, lighting and welfare facilities. Some of these loads overlap. Others can be sequenced.Starting method changes the result. Caterpillar notes that an electric motor can draw two to eight times its normal running current while starting. That range is relevant to direct-on-line motors, but it should not be copied into a calculation for a modern VFD-controlled crane. A variable frequency drive controls acceleration and speed, so the supply sees a different demand profile. Engineers still need to check input current, ramp rate, harmonics and regenerative behavior.Cummins’ generator application guidance models running and starting requirements by load step. It notes that a VFD gives the designer more control over the rate of load application, but the drive can remain sensitive to voltage variation. Measured feeder data and the crane drive documentation are therefore more useful than a generic starting-current multiplier.

Step 1: build a usable load profile

Measure real power, apparent power, current, voltage, power factor and frequency. The sampling rate must be fast enough to capture the lifting events that the BESS will support. One-minute averages can hide a ten-second peak.Record several representative operating conditions:

• the heaviest planned lift;
• hoist, trolley and slewing operation at the same time;
• two or more cranes operating concurrently;
• other site equipment switching on during a lift;
• lowering or braking events that may return energy;
• shift start, breaks and low-load recharge periods.

When site data are not yet available, build the preliminary model from crane OEM current curves, VFD limits and an agreed operating sequence. Confirm those assumptions with temporary metering before selecting the final equipment.

Step 2: calculate the power deficit

For a grid-constrained site, the active-power deficit at each time step is:

P_bess(t) = max[P_load(t) - P_grid_limit, 0]

For a generator hybrid, replace the grid limit with the generator power assigned to the load by the control strategy. Do not assume this equals the generator nameplate rating. Site derating, the emissions configuration, acceptable load steps, battery charging demand and the chosen operating point all affect the available contribution.The highest value of P_bess(t) is the minimum active-power requirement before design reserve. Apparent power must be checked as well:

S = √(P² + Q²)

Here, P is active power in kW, Q is reactive power in kvar, and S is apparent power in kVA.For a balanced three-phase system, current and apparent power are related by:

S (kVA) = √3 × V × I ÷ 1000

In this equation, V is the line-to-line RMS voltage in volts and I is the line current in amperes.A PCS can reach its current or kVA limit while the measured kW still looks acceptable. Compare the load profile with the continuous rating and each short-duration overload rating, including the 10-second and 60-second limits where applicable.CLP Power’s construction-site BESS guideline uses the sum of coincident peak currents as an initial check. Final sizing still needs power factor, voltage dip, controls, protection and peak duration.

Step 3: calculate usable energy separately from nameplate capacity

Power is the maximum gap the BESS must fill. Energy is the accumulated gap over time.For each interval, calculate the energy discharged above the source limit:

E_deficit = Σ[max(P_load - P_source, 0) × Δt] ÷ η_discharge

Base the usable capacity on the largest cumulative energy deficit before the battery can recover. Then account for the permitted state-of-charge window, temperature, conversion losses, battery ageing and the project’s operating reserve.One peak is not enough to size the battery. Repeated crane cycles can lower state of charge across a shift even when each event lasts only seconds. Include charging during the valleys:

P_charge_available(t) = max[P_source_limit - P_load(t), 0]

If the battery receives less energy between lifts than it supplies during the peaks, its state of charge will continue to fall. The design must then provide more source power, more usable battery capacity, a different operating sequence or a dedicated charging window.

What the Malaysia project shows

At a Malaysian infrastructure site, three tower cranes operated behind a utility connection limited to 110 kW. Site demand reached 180 kW. During peak events, the Foxtheon P350 supplied between 40 kW and 70 kW.At the maximum recorded demand, the active-power calculation is direct:

180 kW site demand - 110 kW grid limit = 70 kW BESS contribution

The result is the maximum measured active-power deficit. It is not the complete system size. The design must still account for apparent power, peak duration, repeated cycles, charging time and operating reserve.Suppose that a 70 kW deficit accumulated for ten minutes before the next effective recharge period. The BESS would deliver:

70 kW × 10/60 h = 11.7 kWh

The 11.7 kWh figure applies only to this assumed interval. It is neither a product recommendation nor the project’s actual daily energy result. Capacity selection still requires a full-shift state-of-charge simulation.The deployed P350 has 376 kWh of battery capacity and worked as the energy buffer and site microgrid controller. The project reported that grid draw remained within the 110 kW limit while the three cranes operated. The 70 kW figure describes the largest reported power gap. Battery capacity and the control strategy determine whether the system can repeat that support through the working day.

Step 4: size the grid or generator contribution

The upstream source supplies the load below the control limit and recharges the energy discharged by the BESS.For a weak-grid project, check:

• the contractual or protective limit at the point of connection;
• the maximum charging current allowed without causing a trip;
• other site loads sharing the same connection;
• import and export rules;
• the recharge time available between crane cycles and shifts.

For a generator hybrid, check:

• continuous site demand plus the planned battery charging power;
• the largest remaining load step after the BESS responds;
• acceptable voltage and frequency dip;
• minimum and preferred loading stated by the generator manufacturer;
• ambient temperature and altitude derating;
• start, stop and minimum-run-time logic;
• fuel, noise and emissions constraints.

The article on reducing tower crane generator size by 50% compares three common hybrid configurations. Use the calculation method in this guide to test which configuration fits the measured site load.In a Belgium trenchless-construction project, a hybrid BESS and a 35 kVA Stage V generator supported a 75 kW soft-start crawler. This site result is not a general sizing ratio. It does show how storage can separate short equipment peaks from the generator’s steadier contribution.Load sequencing may reduce the source size. Cummins recommends allowing generator voltage and frequency to stabilize between controlled load steps. Construction sites also have unplanned overlap, so the model must test the intended sequence and credible worst-case combinations.

Step 5: check the electrical details that a kW calculation misses

A preliminary kW and kWh calculation cannot confirm safe operation. Before procurement, review these points with the crane OEM, BESS supplier and project electrical engineer: Check regenerative power separately. Some crane drives dissipate braking energy through resistors; others can return it to the AC bus. Where reverse power is possible, confirm that the PCS can absorb it at the expected state of charge and that the generator or grid interface supports the condition.

A practical tower crane BESS sizing workflow

For a preliminary design review:

1. Collect high-resolution load data at the intended connection point.
2. Define the source limit and the loads that may operate together.
3. Calculate the maximum kW, kVA and current deficit.
4. Match each peak to the PCS overload curve and response time.
5. Integrate the deficit over time to calculate usable energy.
6. Simulate charging and state of charge across the full shift.
7. Apply documented derating, ageing allowance and project reserve.
8. Check voltage, frequency, harmonics, fault current and protection.
9. Validate the model with commissioning tests under controlled loads.
10. Review operating data after deployment and tune the EMS limits.

The output is a power and energy envelope rather than a single kWh number. Match that envelope to a standard product or use it to define a project-specific configuration.

Frequently asked questions

Should tower crane BESS sizing start with kW or kWh?

Start with the coincident power deficit in kW and kVA. Confirm that the PCS can carry the peak for the required duration, then calculate the usable kWh needed between recharge opportunities. A large battery cannot compensate for an undersized PCS.

Does a VFD remove the starting surge?

A VFD controls motor acceleration and can reduce the abrupt demand of direct-on-line starting. A transient study is still required. The source model should use the crane drive’s input-current limits, ramp settings, power factor, harmonics and regenerative behavior.

Can a hybrid BESS allow a smaller generator?

Yes, when the BESS supplies short peaks and the generator covers steadier demand plus battery charging. The final generator size still depends on the measured load profile, acceptable load steps, charging strategy, site derating and the manufacturer’s operating limits.

How much load data is needed?

Capture enough representative shifts to include heavy lifts, concurrent crane operation and other site loads. Sample faster than the events under study. One-minute averages are not suitable when ten-second peaks affect the design.

Can the same method be used for construction hoists, pumps and crushers?

Yes. The power-deficit and energy-deficit calculations are the same, but the transient model must use each machine’s starting method, duty cycle, power factor and process constraints.

Plan the system around the load, not the label

Reliable sizing starts with measured demand. Crane nameplate power, generator rating and battery capacity each describe only part of the system. The final design must show that the PCS can carry the worst credible power event and that the battery can recover its state of charge during the shift.Foxtheon reviews load profiles, source limits and operating schedules for tower cranes and other high-surge construction equipment. Contact Foxtheon to discuss a preliminary configuration for a grid-constrained or off-grid project.