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ISO 25745-2:2015 — Energy Performance of Lifts — Part 2: Energy Calculation and Classification
Advanced methodology for calculating lift energy consumption based on traffic analysis and design parameters
Overview of ISO 25745-2
ISO 25745-2:2015 extends the ISO 25745 framework by providing a calculation-based methodology for determining lift energy performance. While Part 1 relies on physical measurement of installed lifts, Part 2 enables architects, consultants, and lift manufacturers to predict energy classification at the design stage — before a lift is manufactured or installed. This predictive capability is invaluable for new building design, lift modernization planning, and comparative technology assessment.
The calculation methodology incorporates traffic analysis parameters (building type, population, traffic patterns), lift design specifications (rated load, speed, drive type, machine technology), and building characteristics (travel height, number of landings, machine room conditions) to produce an estimated annual energy consumption and corresponding energy classification from A to G.
ISO 25745-2 is the preferred tool for building energy modelers incorporating vertical transportation into whole-building energy simulations. The calculation methodology aligns with the inputs required by energy simulation software such as EnergyPlus, IES VE, and DesignBuilder.
Calculation Methodology
The standard defines a multi-step calculation procedure that builds from basic lift parameters to annual energy estimation:
| Step | Parameter Calculated | Inputs Required | Formula Reference |
|---|---|---|---|
| 1 | Reference running energy (E_r_ref) | Rated load, rated speed, travel height, drive type | Section 5.2, Equation (1) |
| 2 | Standby power components | Controller, door operator, lighting, ventilation, display power | Section 5.3, Table 2 |
| 3 | Daily trips estimation | Building type, population served, traffic profile | Annex A, Table A.1 |
| 4 | Annual running energy | E_r_ref × trips/day × operating days × load factor | Section 5.4, Equation (4) |
| 5 | Annual standby energy | Standby power × operating hours × standby factor | Section 5.5, Equation (5) |
| 6 | Regenerative energy credit | Regen efficiency, counterweight ratio, traffic distribution | Section 5.6, Annex B |
| 7 | Total annual energy (E_a) | Running + standby − recovered energy | Section 5.7, Equation (7) |
| 8 | Energy classification index | E_a normalized to reference installation | Section 7, Annex C |
Reference running energy. The calculation begins with the reference running energy — the energy consumed by the lift’s drive system during one complete reference round trip at rated load. This is calculated from the potential energy change of the car and counterweight system (ΔE = m × g × h), divided by the drive system efficiency. For example, a 1600 kg lift serving 10 floors (30 m travel) at 2.5 m/s has a potential energy component of approximately 157 kJ per round trip, but actual electrical consumption depends on the drive efficiency: 0.75 for geared AC drives, 0.85 for VVVF geared, 0.88 for VVVF gearless PM, and 0.65 for hydraulic systems.
Traffic analysis for trip estimation. Annex A provides traffic profiles for different building types: offices (peak at 8:00–9:30 and 17:00–18:00), hotels (distributed throughout the day with peaks at check-in/check-out), residential (morning and evening peaks), and hospitals (relatively constant throughout 24 hours). The total daily trips are calculated using the building population, the up-peak handling capacity of the lift group, and the traffic distribution profile. A standard office building typically generates 80–120 trips per person per year in the lift system.
A common error in lift energy calculation is using average daily traffic without accounting for seasonal variation. Office buildings in temperate climates show 30-50% higher lift traffic in winter months (when occupants avoid stairs) compared to summer. ISO 25745-2 provides monthly adjustment factors in Annex A to account for this effect.
Engineering Design Insights
ISO 25745-2 reveals several important engineering relationships between design choices and energy performance:
Counterweight ratio optimization. The standard’s calculation methodology clearly demonstrates the impact of counterweight ratio on energy consumption. The theoretical optimum is 50% (counterweight mass = car mass + 50% rated load), which minimizes the net unbalanced mass across all load conditions. Practical installations typically use 45–50%. Moving from 40% to 50% counterweight ratio reduces annual energy consumption by 8–12% in a typical office installation. For regenerative drive systems, the effect is partially offset by reduced energy recovery potential, making lower counterweight ratios (40%) more efficient in some high-traffic regenerative installations.
Door system energy. Door operator energy consumption is a significant and often overlooked component. The standard specifies typical power values: 150–250 W for AC door operators, 80–150 W for DC/VVVF door operators, and 40–80 W for servo-controlled door operators with standby management. Door energy contributes proportionally to traffic volume — a busy 20-floor office building with 12-person lifts may see doors accounting for 15–25% of total running energy. Specifying servo-controlled door operators with standby shutoff can reduce door-related energy consumption by 50-60%.
Accuracy of calculation vs. measurement. The standard acknowledges that calculation-based results may differ from field measurements by ±15% due to installation-specific factors (guide rail friction, rope bending stiffness, sheave bearing condition). However, the classification bands are sufficiently wide (±20% per class boundary) that the calculated classification is reliable for design decisions. When a calculated classification falls within 5% of a class boundary, the standard recommends confirming through physical measurement per ISO 25745-1 before making binding energy performance commitments.
For lift modernization projects, ISO 25745-2 provides a powerful business case tool. By calculating the energy performance of the existing installation and comparing it with the predicted performance of proposed modernization options (e.g., replacing the drive system, adding regenerative capability, implementing standby management), building owners can generate accurate ROI projections. A typical modernization from Class E to Class B in a 15-floor office building shows a payback period of 3-5 years based on energy savings alone, excluding maintenance and reliability benefits.
FAQs
Q: Can ISO 25745-2 be used for lifts with destination dispatch control?
A: Yes. The standard includes provisions for advanced dispatch systems, which typically reduce the number of trips required to serve a given traffic demand by 15-30% through improved car grouping and passenger batching, thereby reducing total running energy proportionally.
A: Yes. The standard includes provisions for advanced dispatch systems, which typically reduce the number of trips required to serve a given traffic demand by 15-30% through improved car grouping and passenger batching, thereby reducing total running energy proportionally.
Q: How does the calculation account for differences in machine room temperature?
A: The standard provides temperature correction factors in Annex D. Energy consumption of drive equipment increases by approximately 1.5% per °C above 25°C ambient. Machine rooms in hot climates or with inadequate ventilation can significantly degrade drive efficiency and increase controller standby consumption.
A: The standard provides temperature correction factors in Annex D. Energy consumption of drive equipment increases by approximately 1.5% per °C above 25°C ambient. Machine rooms in hot climates or with inadequate ventilation can significantly degrade drive efficiency and increase controller standby consumption.
Q: Is the calculation method valid for high-speed lifts (≥4 m/s)?
A: Yes, but additional factors must be considered: air resistance in the hoistway, which becomes significant above 4 m/s; rope compensation effects; and the increased standby consumption of high-performance controllers. The standard provides supplementary calculation guidance for high-speed installations in Annex E.
A: Yes, but additional factors must be considered: air resistance in the hoistway, which becomes significant above 4 m/s; rope compensation effects; and the increased standby consumption of high-performance controllers. The standard provides supplementary calculation guidance for high-speed installations in Annex E.
Q: What is the minimum set of input data required for classification?
A: The minimum data set includes: rated load, rated speed, travel height, number of landings, drive type (AC VVVF, DC, hydraulic, etc.), machine type (geared, gearless, PM), counterweight ratio, car mass, and building type. Without these parameters, calculation uncertainty exceeds acceptable limits.
A: The minimum data set includes: rated load, rated speed, travel height, number of landings, drive type (AC VVVF, DC, hydraulic, etc.), machine type (geared, gearless, PM), counterweight ratio, car mass, and building type. Without these parameters, calculation uncertainty exceeds acceptable limits.
