Why We Built India’s First Free ERD Battery Sizing Calculator — Zero-Switch Lift Inverter Technology Explained
Why We Built India's First Free
ERD Battery Sizing Calculator
Zero-switch Emergency Rescue Device technology explained — and a free open tool for lift engineers, architects and building managers across India.
In This Article
- Why we built this calculator
- What is an ERD? How is it different from ARD?
- Zero-switch technology — why 3 milliseconds matters
- SMF vs Tubular vs Lithium — which battery for your lift?
- How to size an ERD — the 3-phase calculation explained
- The free calculator — who it's built for
- Haryana mandate and what's coming across India
- Real installations — data from the field
Why We Built This — The Problem Nobody Was Solving
I have been building inverter and power backup technology for over 25 years. When we developed the Emergency Rescue Device at Su-vastika, we solved the core engineering problem: a lift motor should never feel a grid failure. But we discovered a second problem that nobody was addressing — nobody could easily calculate what battery they needed.
Every time a housing society or architect asked "how many batteries do we need for 30 minutes backup?" — they had to call a salesperson, wait for a quote, and hope the numbers were correct. The data existed in our internal documents and Su-Kam training materials. It was never public.
So we built India's first open ERD sizing calculator on LiftInverter.com — completely free, based on field data from 200+ real installations across India.
This calculator is independent and neutral
LiftInverter.com is an independent editorial resource. The calculator works for any 3-phase ERD — not just Su-vastika. We believe the industry grows when engineers have the right tools.
What is an ERD? How It Replaced ARD Technology
An Emergency Rescue Device (ERD) is a specialised 3-phase UPS built exclusively for elevator motors. When mains power fails, the ERD instantly delivers clean 3-phase power to the lift motor, allowing it to travel to the nearest floor safely, open its doors, and release passengers.
Before ERD, lifts used ARD (Automatic Rescue Device) — a basic DC battery system that reacted to power failure. The old ARD technology had serious limitations that drove lift manufacturers and building owners mad:
| Feature | Old ARD Technology | Modern ERD (3-phase) |
|---|---|---|
| Switchover time | ~30 seconds | 3 milliseconds |
| Cabin jerk on switchover | Yes — passengers feel it | Zero — invisible transition |
| Light continuity | Lights cut out | 100% continuous |
| Motor wear & tear | High — voltage spikes | Minimal — clean sine wave |
| Can replace generator | No | Yes |
| Solar compatible | No | Yes |
| Centralised (10–20 lifts) | No | Yes |
| IoT / Remote monitoring | No | Yes — GSM / WiFi |
| Battery type support | Lead acid only | SMF / Tubular / Lithium |
Zero-Switch Technology — Why 3 Milliseconds Changes Everything
The most common question we get from engineers is: "Why does the switchover time matter? The lift will stop at the next floor anyway."
This misses the fundamental point. It's not about rescue time — it's about what happens to the motor, the passengers, and the lift mechanics in that gap.
The Physics Behind Zero-Switch
Su-vastika's ERD uses IGBT (Insulated Gate Bipolar Transistor) switching technology combined with a bi-directional power architecture. The system continuously monitors grid voltage and pre-positions its output. When the grid drops, the IGBT switches in under one AC cycle — approximately 3 milliseconds at 50Hz.
The lift motor's flywheel inertia takes several hundred milliseconds to decay. At 3ms switchover, the motor never sees a power interruption — it continues rotating as if nothing happened.
Technical Note for Lift Engineers
The 3ms switchover is sub-cycle (one 50Hz AC cycle = 20ms). The motor's back-EMF at 3ms is still within the inverter's input range. This is why there is no torque discontinuity — the motor shaft never decelerates. Older ARD systems operated on DC relay switching which introduced 20–30 second delays.
SMF vs Tubular vs Lithium — Which Battery for Your Lift?
This is the most practical question lift engineers ask us. The ERD itself is standard — the battery choice determines your backup duration, space requirement, and total cost of ownership over 10 years.
Battery Backup Comparison — 20 KVA ERD / 11.5 KW Lift / 30 Minutes Backup
Batteries Required for 30 Min Backup (100 Ah blocks, 360V bank)
Source: Su-vastika battery sizing methodology · Field data 2017–2024 · Calculate your own at LiftInverter.com
| Battery | Typical Cost | Cycle Life | Backup per Ah | Space | Best For |
|---|---|---|---|---|---|
| SMF Lead Acid | ₹3,000–₹6,000/unit | 200–300 cycles | ~50% DoD usable | Large | Budget installations, well-ventilated rooms |
| Tubular Lead Acid | ₹5,000–₹9,000/unit | 500–800 cycles | ~60% DoD, better curve | Large + ventilation | Longer backup on budget |
| Lithium LiFePO4 | ₹18,000–₹30,000/unit | 2,000–3,000 cycles | 80–90% DoD usable | 60% smaller | Premium builds, limited space, lowest TCO |
Important: De-rating Factor for Lead Acid at Short Backup Times
SMF batteries have a de-rating factor of 0.45 at 15 minutes and 0.52 at 30 minutes. This means a nominally 100Ah battery only delivers ~45Ah in 15 minutes. Lithium batteries have a flat discharge curve — their full capacity is available at any duration. This is why you need fewer Lithium batteries for the same backup time.
How to Size an ERD — The 3-Phase Calculation Explained
Most lift engineers and architects don't know how to calculate ERD KVA. Here's the complete methodology — the same formula our calculator uses.
Step 1: Calculate Lift Motor KW
Motor KW Formula
Motor KW = (Load kg × Speed m/s × 9.81) ÷ (1000 × Efficiency)
Where: Efficiency = 0.85 for gearless (MRL) lifts, 0.70 for geared lifts
Example: 630 kg lift at 1.0 m/s, gearless → (630 × 1.0 × 9.81) ÷ (1000 × 0.85) = 7.27 KW
Step 2: Calculate Required ERD KVA
ERD KVA Formula
ERD KVA = (Total Motor KW ÷ 0.8 PF) × 1.25 Safety Factor
Example: 7.27 KW motor → (7.27 ÷ 0.8) × 1.25 = 11.36 KVA → Select 15 KVA ERD
Step 3: Calculate Battery Ah for Backup Duration
Use the de-rated discharge current method. The battery Ah required depends on the ERD's DC draw current and the desired backup duration, corrected by the de-rating factor for your backup time.
| Backup Time | SMF De-rate Factor | 20 KVA ERD / 50A DC Draw | Batteries (100Ah, 360V bank = 30 in series) |
|---|---|---|---|
| 15 minutes | 0.45 | 32 Ah required | 30 × 100Ah (1 string) |
| 30 minutes | 0.52 | 45 Ah required | 30 × 100Ah (1 string) |
| 1 hour | 0.62 | 88 Ah required | 60 × 100Ah (2 strings) |
| 2 hours | 0.74 | 125 Ah required | 60 × 150Ah (2 strings) |
| 3 hours | 0.80 | 180 Ah required | 90 × 200Ah (3 strings) |
Don't want to do this manually? Use our free calculator → Select your lift brand, capacity, speed, and backup duration — it handles all the math instantly.
The Free Calculator — Built for These Professionals
The ERD Sizing Calculator at LiftInverter.com was built specifically for three groups of professionals who struggle with ERD specification today:
🏗️ Lift Engineers & Technicians
Specify ERD KVA and battery count per lift in 30 seconds. Supports all major Indian brands: Johnson, Omega, Escon, Otis, Schindler, KONE, Mitsubishi, ThyssenKrupp, Fujitec.
🏢 Architects & PMC Teams
Size ERD room space requirement, compare SMF vs Lithium footprint. Generate data for your BOQ and compliance documents (Haryana, RERA, BIS).
🏘️ Housing Society Managers
Understand what you're being quoted. Input your lift's specs and verify if the battery bank your contractor proposed actually meets the 15-minute Haryana mandate.
⚡ Electrical Consultants
Full comparison output: SMF vs Tubular vs Lithium — batteries, strings, configuration, space estimate, Haryana compliance status. One click.
Try the Calculator Free — No Login Required
Select your lift brand · Enter capacity and speed · Choose backup duration · Get exact battery specification instantly
⚡ Open ERD Calculator 📚 LiftInverter.com Knowledge HubHaryana Mandate — And What's Coming Across India
In 2020, Haryana became the first Indian state to legally mandate ERD systems for all housing and commercial projects. The Haryana Renewable Energy Development Authority (HAREDA) went further, specifying:
Haryana HAREDA Mandate
Solar-powered Emergency Rescue Device with minimum 15 minutes emergency backup — mandatory for all lift and escalator installations in housing and commercial projects in Haryana.
This mandate was driven by daily entrapment incidents reported across Haryana's rapidly urbanising districts — Gurugram, Faridabad, Panchkula. Government installations were some of our earliest projects: DC Office Gurugram, Haryana Police Housing in both Gurugram and Manesar, and the HAREDA department office in Rewari itself.
Maharashtra, Karnataka, Delhi NCR, Tamil Nadu, Gujarat and Rajasthan are all expected to follow. Builders in these states who specify ERD now are ahead of the mandate — not behind it.
Read more: BIS Certificate for Lift Inverter/ERD · State-by-state policy tracker at LiftInverter.com
Real Installations — Data from the Field
The calculator is not theoretical. Every Ah figure in it is validated against real installations. Here are selected projects that shaped the data:
Chintel Pardiso, Gurugram
2 centralised ERDs (100 KVA + 200 KVA) running 18 lifts of 11.5 KW each. Oct 2021.
M3M Latitude, Gurugram Sec 65
6 × 50 KVA ERDs for 32.3 KW high-speed lifts. Jun 2021.
BPTP Terra, Gurugram
9 × 20 KVA ERDs, 9 lifts of 11.4 KW. Feb 2021.
Runwal Seagull, Pune
9 × 30 KVA ERDs with Lithium batteries, 2 lifts of 7.2 KW each. Mar 2024.
Spaze Tristaar Mall, Gurugram
7 × 20 KVA ERDs for 7 lifts of 11.4 KW. Jan 2021.
TATA ITPG, Sector 59, Gurugram
ERD installation for corporate campus. Mar 2022.
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Part of the Series
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Su-vastika Lithium Batteries
25+ patents · Made in India · 10,000+ installations