A practical look at power architecture for low-load, long-runtime, and distributed security systems
Traditional AC UPS systems have earned their place in IT and network infrastructure, and they continue to perform well there. But security infrastructure is not IT infrastructure. Across airports, rail networks, correctional facilities, and large commercial sites, the combination of distributed system architectures, mixed AC and DC loads, extended runtime requirements, and non-conditioned installation environments creates a set of conditions that AC UPS was not designed to address well. This article examines where those limitations become consequential, and where a DC-centric Rectiverter architecture offers a more appropriate solution.
Not what you were looking for? If your requirement is DC power and battery backup for access control field devices (door controllers, card readers, and electric locks operating at 12Vdc or 24Vdc), that design problem is covered in detail in Designing Battery-Backed DC Power Systems for Electronic Security Applications. The two architectures serve different parts of a security system and are not interchangeable.
The Changing Shape of Security System Power Requirements
Modern security deployments are structurally different from the centralised, rack-based environments where AC UPS was developed. Across distributed infrastructure, power is consumed at dozens or hundreds of discrete points: field cabinets, edge processing nodes, access control panels, perimeter detection systems, each with its own load profile and often its own backup requirement.
Individual devices have become more efficient, but the overall system picture is more complex. The proliferation of analytics, edge processing, and high-density camera systems means that aggregate site loads are not necessarily lower than before, just distributed differently. The practical result is a mix of low-power field nodes, higher-density aggregation equipment, and a combination of AC and DC-powered systems, often spread across locations with very different environmental conditions.
At the same time, backup runtime expectations have grown. Requirements of 4-8 hours are increasingly common, and in some sectors (rail, utilities, corrections), longer runtimes are a contractual or standards-driven requirement rather than a preference. This combination of distributed loads, mixed power types, and long runtimes is where AC UPS architecture begins to show its limitations.
Where AC UPS Remains Appropriate
It is worth being precise about this. AC UPS systems remain well-suited to centralised loads that consume significant power and require short-to-medium runtime protection: servers and storage, control room equipment, and core network infrastructure including centralised PoE switching located in controlled comms room environments. In these applications, the UPS is protecting a defined, co-located load, the operating environment is usually controlled, and the architecture is a reasonable fit for the problem.
It is also worth noting that on sites where a Rectiverter handles AC and 48Vdc backup requirements, a separate DC-UPS may still be required for low-voltage access control loads (12Vdc or 24Vdc door controllers, electric locks, and similar field devices). These are complementary architectures serving different parts of the same site, not alternatives to each other.
The limitations of AC UPS emerge when the same approach is applied across the wider site, particularly to distributed field infrastructure, low-power loads with long runtime requirements, or equipment installed in uncontrolled environments. In those scenarios, the mismatch between the UPS architecture and the application requirements compounds across three related areas: system sizing, environmental performance, and operational management.
The W vs Wh Problem: Why AC UPS Sizing Gets Complicated
AC UPS systems are selected primarily on inverter rating (kVA capacity). This makes sense when the system is designed to support a clearly defined load. The problem arises when the runtime requirement, rather than the load itself, becomes the dominant design driver.
Consider a practical example: a site with a connected load of 200-300W that requires 6-8 hours of backup. A small UPS might technically handle the load, but its standard battery configuration will not come close to delivering that runtime. This forces one of two responses.
The first is to upsize the UPS platform. Rather than selecting a system matched to the actual 300W load, an engineer selects a 3kVA or 5kVA platform, not because the load demands it, but because a larger frame provides access to greater internal battery capacity and compatible external battery expansion modules. The power conversion infrastructure is now significantly oversized relative to the load, but that is the cost of accessing the energy storage capacity needed to meet the runtime requirement.
The second approach is to add external battery packs to a smaller UPS. This increases the available energy (Wh) without changing the inverter size, which seems more efficient until the charger becomes the limiting factor. As external packs are added, the fixed charge current becomes insufficient for the expanded battery bank: recharge times increase, batteries may not fully recover between events, and battery lifecycle is shortened by chronic under-charging.
Both paths lead to the same underlying problem: the system is no longer sized to the load. It is sized to the runtime requirement, and constrained by the architecture of the UPS platform rather than the actual application requirements. Power capacity (W) and energy storage (Wh) are structurally coupled in a way that makes it difficult to optimise either independently.
Environmental Reality and the Operational Burden That Follows
Security infrastructure is not deployed in data centres. Equipment is routinely installed in outdoor cabinets, unventilated plant rooms, riser cupboards, and confined spaces where ambient temperatures regularly reach 35-45°C and internal cabinet temperatures can exceed 50°C under load.
For VRLA batteries, the relationship between temperature and service life is well-established: every 10°C increase above the 25°C reference temperature approximately halves the expected battery life. A battery with a nominal 4-5 year design life, operating in a cabinet at 40°C, may deliver 2-3 years before degradation becomes significant.
The maintenance burden this creates is not trivial. Verifying battery condition in a VRLA system requires scheduled discharge testing on-site, at each location, by qualified personnel. Across distributed infrastructure, battery condition is validated periodically, not continuously, which means degradation can go undetected between maintenance cycles. In many cases, battery failures are only identified during a test or, worse, during an actual outage.
This is the operational burden of applying an architecture that was not designed for distributed, thermally stressed deployment. It is not a product quality issue. It is an architectural mismatch.
Rectiverter Architecture: A DC-Centric Approach
A Rectiverter system addresses the fundamental limitations of traditional AC UPS by restructuring the power architecture around a shared 48Vdc bus. Rather than building the system around an inverter and adapting battery infrastructure to meet runtime requirements, the DC bus becomes the central layer for both energy storage and power distribution, with AC and DC outputs treated as parallel consumers of the same energy reservoir. Powerbox Australia supplies Delta Eltek Rectiverter systems across Australia and New Zealand, and works directly with engineers, consultants, and integrators to specify and size these systems across security, rail, utilities, and industrial applications.
The power flow in a Rectiverter-based system works as follows: each Rectiverter module combines rectifier and inverter functions in a single unit, converting AC mains input to 48Vdc for the bus and supplying AC loads by inverting from that same bus; batteries are connected directly to the DC bus; and DC loads are supplied from the bus without additional conversion. This is a materially different topology from a traditional UPS, where all loads are effectively routed through the inverter, and batteries are connected to a high-voltage internal DC link that is not directly accessible to the rest of the system.
The practical consequence of this architecture is the separation of power capacity (W) from energy storage (Wh). Rectiverter modules are sized to match the actual load. Battery capacity is independently scaled to meet the runtime requirement. If the load is 300W but the runtime requirement is 8 hours, the system is engineered precisely to those parameters. Battery charging is handled by the Rectiverter modules themselves, which removes the fixed-charger bottleneck that constrains battery expansion in traditional UPS systems.
Transfer Time and the Elimination of the Static Switch
A point that is often overlooked in UPS architecture comparisons is transfer time: the brief interruption that occurs when a traditional offline or line-interactive UPS switches from mains to battery. Even at 10-20ms, this transition is long enough to cause microprocessor-controlled security devices to reset, fault, or generate nuisance alarms.
In a Rectiverter, each module draws AC input from the bus and supplies AC output from it simultaneously. The bus is maintained continuously by the Rectiverter modules from mains and by the battery. When mains fails, the bus voltage is sustained by the battery without interruption, and the AC output continues without any transition period. There is no transfer event for AC loads.
This also removes the need for a static bypass switch. In a modular Rectiverter with N+1 module redundancy, a single module failure simply shifts load to the remaining modules. The resilience is distributed across the parallel module topology rather than concentrated in a bypass switch that represents its own single point of failure.
Modular Construction and System Resilience
Rectiverter systems are built from independent plug-in Rectiverter modules, each combining rectifier and inverter functions, that operate in parallel on the DC bus. Each module can be added to increase capacity, removed for service, or replaced without interrupting the load. Systems are typically configured with N+1 redundancy: for a 2kW load, this might mean three 1kW modules, with the load supported by any two and the third providing redundant capacity.
Because DC loads are supplied directly from the bus without passing through the inverter, conversion losses are reduced, and heat generation is lower than in a system where all load is routed through a single conversion stage. In cabinet-based installations in high ambient temperature environments, this directly reduces thermal stress on batteries and improves long-term system stability.
System Monitoring: Delta Eltek Smartpack
The Delta Eltek Rectiverter platform uses the Smartpack Controller as its system management and monitoring interface. It provides a single point of visibility across Rectiverter modules, DC bus, and battery in real time. Smartpack enables direct integration with NOC and network management systems via SNMP v3, Modbus TCP, and HTTP/SSL, giving operators continuous visibility of system status without requiring a site visit.
For deployments spanning multiple sites, Delta Eltek's MSM (Multi-Site Monitoring) platform aggregates status and alarm data from multiple Rectiverter installations into a single monitoring portal. For security integrators managing power infrastructure across airport terminals, rail stations, or correctional facilities, this removes the site-by-site visibility problem that makes distributed infrastructure difficult to manage at scale.
Scalability: From Field Cabinet to Equipment Room
At the edge of the network, Rectiverter systems are available in compact subrack formats sized for integration within existing equipment cabinets. These are well-suited to field cabinets, trackside enclosures, and distributed security nodes where space is limited. The modular, hot-swappable construction and N+1 capability apply at this scale as much as at larger installations.
For higher-capacity applications, the same architecture scales into purpose-built cabinetised systems integrating Rectiverter modules, DC and AC distribution, battery interconnection, and monitoring interfaces. Whether a system is installed in a roadside cabinet or an equipment room, the DC bus architecture, the modular power conversion approach, and the monitoring interface are the same. That reduces engineering complexity at the design stage and simplifies maintenance across the lifecycle of a large, distributed deployment.
It is worth noting that Rectiverter platforms are not limited to the load ranges typical of security infrastructure. The same modular architecture scales to hundreds of kilowatts and is deployed across telecommunications, utilities, and large industrial applications. The 0.5-5kW framing in this article reflects the sweet spot for distributed security deployments, not a ceiling on what the platform can support.
Battery Strategy: VRLA vs Lithium
The advantages of a Rectiverter architecture are significantly enhanced when paired with lithium battery systems rather than VRLA. The preferred battery partner for Rectiverter deployments at Powerbox Pacific is Polarium, a Swedish manufacturer whose 48Vdc lithium modules are specifically designed for critical infrastructure standby applications. The shift to a 48Vdc bus architecture makes this pairing technically straightforward: Polarium modules connect as parallel units on the DC bus rather than as series strings, and the operational benefits over the system lifecycle are substantial.
| Parameter | VRLA | Polarium Lithium |
| Design life | 3-5 years | Up to 20 years |
| Warranty | 1-2 years | Up to 7 years |
| Usable capacity | ~50–70% | ~90–100% |
| Temperature tolerance | Poor | Strong |
| Maintenance | Regular testing required | Minimal |
| Monitoring | Limited | Integrated via BMS (SoC, SoH, Temperature, Voltage, Current) |
| Replacement cycles (10–15 yr horizon) | 3-5 | 1 |
Chemistry Selection: LFP or NMC
Polarium offers both Lithium Iron Phosphate (LFP) and Nickel Manganese Cobalt (NMC) cell chemistries across their SLB48 module range. NMC modules are available from 50Ah through to 250Ah, making them particularly well-suited to space-constrained security cabinet installations where energy density is the binding constraint. For distributed security infrastructure in New Zealand conditions, LFP is generally the appropriate choice where enclosure space permits, offering superior thermal stability and longer cycle life that directly addresses the high-temperature cabinet environments discussed in this article.
Parallel Architecture vs VRLA Series Strings
The VRLA battery architecture in a conventional UPS system relies on high-voltage series strings. A failure or significant degradation in a single cell or unit within that string affects the performance of the entire string. Polarium modules connected in parallel on the 48Vdc bus do not share this failure mode. Individual modules can be assessed, isolated, or replaced without affecting the rest of the battery bank. A single module failure reduces available capacity proportionally, rather than taking down the entire bank.
Safety Architecture
Polarium’s design is built around five independent layers of protection: cell-level qualification and chemistry selection, module-level mechanical isolation and independent fusing, a redundant BMS with watchdog function, integrated Current Limiting Devices (CLD™) that operate independently of external site controllers, and system-level compliance with IEC 62619. The independence of the protection layers means electrical protection operates regardless of whether the site controller or rectifier system is functioning correctly.
From Periodic Testing to Real-Time Visibility
VRLA battery condition is inferred from periodic discharge testing, a manual, scheduled activity that provides a point-in-time assessment of battery health. Polarium’s BMS monitors state-of-charge (SoC) and state-of-health (SoH) continuously at both module and cell level, with redundant monitoring channels verifying all key parameters in real time. Performance data can be reported directly to a NOC or building management system, and historical data supports predictive maintenance without requiring a site visit.
Over a 10-15 year system lifecycle, VRLA batteries in a typical security infrastructure deployment will require 3-5 replacement cycles. A Polarium system installed at the same time may require one. The capital cost of lithium is higher at the point of installation, but that differential is typically recovered within the first or second VRLA replacement cycle, before accounting for the ongoing labour cost of battery testing and maintenance coordination across distributed locations.
When Does a Rectiverter-Based Approach Make Sense?
The case for a Rectiverter-based architecture is strongest where the combination of low load, long runtime, challenging environment, and distributed deployment puts traditional AC UPS under the greatest stress. Specifically:
• System loads in the 0.5-5kW range where extended runtime is the primary design driver
• Backup runtime requirements of 2-3 hours or more
• Installation in uncontrolled environments with high ambient temperatures
• Distributed deployments across multiple locations where maintenance access is limited or costly
• Applications where mixed AC and DC load support is required from a single power platform
Where the load is substantial, the environment is controlled, and the runtime requirement is moderate, a conventional AC UPS remains a practical solution. The selection decision is not ideological; it is application-driven.
Frequently Asked Questions
What is an Delta Eltek Rectiverter and how does it work?
The Delta Eltek Rectiverter is a modular power system from Delta Electronics built around a shared 48Vdc bus. Each Rectiverter module combines rectifier and inverter functions in a single unit: AC mains input is converted to 48Vdc for the bus, and AC loads are supplied by inverting from that same bus. Batteries connect directly to the bus, and DC loads are supplied from the bus without further conversion. Power capacity (W) and energy storage (Wh) can be sized independently, which is the fundamental advantage over traditional AC UPS architecture.
What is the difference between a Rectiverter and a traditional AC UPS?
A traditional AC UPS routes all load through a central inverter and stores energy in a high-voltage battery string tied to that inverter. A Rectiverter uses a 48Vdc bus as the central energy layer, with Rectiverter modules operating in parallel, each combining rectifier and inverter functions in a single unit. Power conversion capacity and battery capacity are independently scalable, there is no transfer time on mains failure, and N+1 module redundancy eliminates the single-inverter failure point that characterises most standalone UPS systems.
What system load range is the Delta Eltek Rectiverter suited to for security and critical infrastructure applications in New Zealand?
For distributed security infrastructure, the Delta Eltek Rectiverter is most commonly applied to system loads in the 0.5-5kW range where extended runtime (typically 2 hours or more) is the primary design driver. At these load levels, traditional AC UPS systems tend to be significantly oversized to access the battery capacity needed for long runtimes. The Rectiverter platform itself scales to hundreds of kilowatts, but for security infrastructure the 0.5-5kW range represents the point where the architecture economics align most clearly.
Can Polarium lithium batteries be used with the Delta Eltek Rectiverter?
Yes. Polarium's SLB48 series lithium modules are designed for 48Vdc bus integration and connect as parallel units on the Rectiverter DC bus. Available in LFP and NMC chemistries, with NMC modules covering 50Ah to 250Ah for space-constrained security installations, they are sized and paralleled to meet specific runtime requirements independently of the Rectiverter module capacity. Powerbox supplies integrated Rectiverter and Polarium systems and handles system design, battery sizing, and configuration for both subrack and cabinetised deployments.
What is Delta Eltek Smartpack and how does it support remote monitoring of power systems?
Smartpack is the system controller for the Delta Eltek Rectiverter platform. It provides real-time visibility of Rectiverter module status, DC bus voltage, battery state-of-charge, state-of-health, and system alarms from a single interface. Smartpack supports integration with NOC and network management systems via SNMP v3, Modbus TCP, and HTTP/SSL. For multi-site deployments, Delta Eltek's MSM platform aggregates Smartpack data from multiple installations into a single portal, allowing fleet-wide visibility across distributed infrastructure.
Does a Rectiverter system require a static bypass switch?
No. In a traditional online UPS, a static bypass switch provides a hardwired path to maintain supply if the inverter fails. In a modular Rectiverter with N+1 inverter redundancy, a single inverter module failure shifts load to the remaining parallel modules without interrupting output. The resilience is distributed across the module topology rather than dependent on a bypass switch, which itself represents a potential single point of failure in a conventional UPS design. This also means there is no transfer time on mains failure: the inverter modules draw from the DC bus continuously, and the bus is sustained by the battery when mains is lost.
How Powerbox Approaches Power System Design
Powerbox is a specialist distributor and manufacturer of power electronics, focused on critical infrastructure sectors including security, rail, utilities, defence, mining, and telecommunications. Our role is to work with engineers, consultants, and integrators at the design stage to ensure the power architecture is correct before product selection begins.
For Rectiverter-based security systems, this means starting with the full picture: connected load, runtime requirement, installation environment, maintenance access constraints, and monitoring integration requirements. From those parameters, we specify the Rectiverter module capacity, battery configuration, and deployment format: subrack for field cabinet integration or cabinetised for equipment room and shelter installations.
If you are specifying power infrastructure for a security or critical infrastructure application, contact Powerbox Pacific to discuss your project.
About the Author
James Rutty, Director, Powerbox Australia
James Rutty is a Director at Powerbox Australia, with over 15 years of experience in power electronics for critical infrastructure across Australia and New Zealand. He works with engineers, consultants, and integrators at the architecture level, from initial load assessment and system design through to product specification, commissioning support, and lifecycle management.
