A Mini UPS (Uninterruptible Power Supply) is a compact backup power device used to keep low-power equipment — routers, IP cameras, IoT controllers, industrial sensors — alive during grid outages. Sizing a UPS incorrectly is one of the most common mistakes in product and system design: an undersized unit fails to deliver the required runtime, while an oversized unit adds unnecessary cost, weight, and thermal burden.
This guide walks through the complete, engineering-accurate process for sizing a Mini UPS — from load analysis to battery selection to efficiency-adjusted runtime calculation.
Step-by-Step Sizing Process
Calculate Total Load Power (W)
List every device connected to the UPS and note its power consumption in Watts. Sum all devices for total load power.
Define Required Backup Runtime (hours)
Determine how long the UPS must sustain the load — e.g. 2 hours, 4 hours, or until generator start (typically 10 to 15 minutes).
Account for System Efficiency
The UPS inverter and battery discharge are not 100% efficient. Apply an efficiency factor — typically 0.80 to 0.90 for mini UPS systems.
Calculate Required Battery Capacity (Ah)
Apply the core sizing formula adjusted for battery voltage, runtime, and system efficiency.
Apply Depth of Discharge (DoD) Derating
Never discharge a battery to 0%. Apply DoD limit — 80% for Li-ion/LFP, 50% for VRLA/lead-acid — to protect cycle life.
The Core Sizing Formula
The fundamental equation for UPS battery sizing is:
Capacity (Ah) = (Load Power (W) x Runtime (h)) / (Battery Voltage (V) x Efficiency x DoD)
// Example: 20W load, 4h runtime, 12V battery, 85% efficiency, 80% DoD
Capacity = (20 x 4) / (12 x 0.85 x 0.80)
Capacity = 80 / 8.16
Capacity = 9.8 Ah → Select a 10 Ah battery
Worked Example: IoT Gateway UPS
Let us size a mini UPS for a typical IoT gateway installation with the following connected loads:
| Device | Power (W) | Qty | Total (W) |
|---|---|---|---|
| 4G / LTE Router | 12 W | 1 | 12 W |
| IoT Gateway Controller | 5 W | 1 | 5 W |
| IP Camera (PoE) | 8 W | 1 | 8 W |
| 5-Port Network Switch | 4 W | 1 | 4 W |
| Total Load | 29 W | ||
Requirements: 4-hour backup runtime, 12V LFP battery, system efficiency = 85%, DoD = 80%.
Capacity = (29W x 4h) / (12V x 0.85 x 0.80)
Capacity = 116 / 8.16
Capacity = 14.2 Ah → Select a 15 Ah, 12V LFP battery pack
Battery Chemistry Comparison
| Chemistry | Nominal Voltage | Max DoD | Cycle Life | Best For |
|---|---|---|---|---|
| VRLA / Lead-Acid | 12V | 50% | 200 – 500 | Low cost, stationary backup |
| LFP (Lithium Iron Phosphate) | 12.8V | 80% | 2000 – 4000 | Long life, safe, industrial |
| NMC Lithium-ion | 11.1V / 14.8V | 80% | 500 – 1000 | Compact, high energy density |
| NiMH | 12V | 70% | 500 – 1000 | Moderate cost and life |
For most modern Mini UPS designs, LFP (Lithium Iron Phosphate) is the recommended chemistry — offering the best balance of cycle life, thermal safety, and usable capacity at 80% DoD, ideal for always-on industrial and IoT installations.
Key Design Considerations
1. Charging Current and BMS
The battery charger must supply sufficient current to recharge the battery within an acceptable window — typically 8 to 12 hours for a mini UPS. A Battery Management System (BMS) is mandatory for all lithium chemistries, protecting against overcharge, over-discharge, over-temperature, and short circuit conditions to ensure both safety and long battery life.
2. Output Voltage Regulation
Mini UPS systems typically output 12V DC (passthrough) or selectable 5V / 9V / 12V via USB-PD or barrel connectors. The inverter or DC-DC converter must maintain tight output voltage regulation — within plus or minus 5% — across the full battery discharge curve from 100% down to the DoD cutoff point.
3. Transfer Time
For sensitive equipment, the switchover time from mains power to battery must be below 20ms — the maximum tolerable for most routers, controllers, and PoE cameras. A MOSFET-based or ideal diode switching topology is strongly preferred over relay-based switching for faster, cleaner transitions with no contact bounce.
4. Temperature Derating
Battery capacity decreases significantly at low temperatures. LFP cells lose approximately 15 to 20 percent of rated capacity at 0 degrees Celsius. For outdoor installations or uncontrolled environments, always apply a temperature derating factor and upsize the battery accordingly.
Quick Reference: Sizing Table
A starting-point reference based on 12V LFP battery, 80% DoD, and 85% system efficiency:
| Load Power (W) | Required Runtime | Recommended Battery |
|---|---|---|
| 10 W | 2 hours | 3 Ah, 12V LFP |
| 20 W | 2 hours | 6 Ah, 12V LFP |
| 30 W | 4 hours | 18 Ah, 12V LFP |
| 50 W | 4 hours | 29 Ah, 12V LFP |
| 100 W | 2 hours | 29 Ah, 12V LFP |
Conclusion
Properly sizing a Mini UPS requires understanding your load, runtime requirements, battery chemistry, and system efficiency. The formulas are straightforward — the discipline is in accurately measuring real loads and applying the right derating factors for DoD, temperature, and ageing. Always add a 20 to 25 percent capacity buffer above your calculated requirement to account for battery ageing over time and unexpected load spikes.
At Infigrace Technologies, we design custom Mini UPS systems for industrial IoT, EV charging infrastructure, and embedded applications — using LFP packs with integrated BMS, fast-transfer MOSFET switching, and multi-output USB-PD support.