Scope and method of UPS and battery bank sizing
Based on the load list and required backup time, it estimates the UPS rating (kVA and kW), battery voltage and configuration, required amp-hours, charger current, input breaker, room heat load and minimum generator size. Calculations run in your browser. Using the calculation and report service requires signing in and confirming your email. When a report is saved, the input data and results are sent to the site server and stored in your account; a copy of the report is also sent to your confirmed email. See the privacy policy for details.
- Watts and volt-amperes: Apparent power of each load = watts ÷ that load's power factor. The UPS must satisfy both the kVA and the kW constraints, and motor starting loads are compared against the unit's overload capability.
- Margin and altitude: The default growth margin is 20 percent; at high altitude the unit's allowable power is derated, and the calculation applies that.
- Battery amp-hours: Battery power = load power ÷ inverter efficiency. In a short discharge (such as 15 minutes) a lead-acid battery gives back only part of its rated amp-hours; this behavior is included in the calculation, after which the temperature factor, aging (battery replacement at 80 percent capacity) and a design margin of 1.10 are applied.
- Input and generator: The input breaker is sized for the unit's own rated current at low grid voltage (rated load + full charging), and the minimum generator size is based on the unit's input power, not its output rating.
- Room heat: Whatever input power does not reach the output is heat; it is reported in watts, BTU per hour and tons of refrigeration.
Sample calculation results for common loads
| Load | Load power (W) | Requested time | UPS | Battery | Design backup time | Input breaker | Minimum generator size |
|---|---|---|---|---|---|---|---|
| Office: 8 computers and 8 monitors, modem and switch | 1,900 | 15 minutes | 3 kVA | 6 blocks 42Ah · 72V | 18 minutes | 20 A | 4.9 kVA |
| Small server room: 4 1U servers, 2 switches and a firewall | 1,940 | 30 minutes | 3 kVA | 6 blocks 65Ah · 72V | 38 minutes | 25 A | 5.2 kVA |
| CCTV: 64 cameras, 4 NVRs and 4 access controllers | 912 | 60 minutes | 1.5 kVA | 4 blocks 65Ah · 48V | 70 minutes | 16 A | 2.8 kVA |
Estimate using general market values, a lead-acid battery at 25 °C and a 20 percent growth margin; these results are not a final design or a performance commitment. For a project-specific estimate, enter your load data in the calculator.
Frequently asked questions
What is the difference between watts and volt-amperes when choosing a UPS?
Watts are real power and volt-amperes are apparent power. The load must fall within both the real-power and apparent-power limits of the UPS at the same time; for today's IT equipment with a high power factor, the kW ceiling is usually the decisive one.
What UPS do I need for CCTV cameras?
Add up the power of the cameras, the recorder (NVR/DVR) and network equipment, and define how long recording must continue during an outage. For small systems, a 1 kVA UPS with one battery is considered based on load and backup time; the calculator gives an estimate matched to your inputs.
How many batteries and how many amp-hours are needed?
The battery bus voltage depends on the UPS rating and determines the number of blocks; the amp-hours come from the load power, backup time and the temperature and aging factors. A sample calculation is given in the table.
How long does recharging take after an outage?
It depends on the charger current and how much was discharged in that outage. The calculator estimates the time for the main charging stage; reaching full charge takes a few more hours.
Why do some results show a warning?
If the inrush current exceeds the unit's tolerance, the battery discharge rate goes beyond the allowable limit, or the load exceeds the calculator's largest rating, the calculator reports that the design needs technical review along with the relevant warning.
How much do the UPS and batteries cost?
Prices are not published on this page. Send the result via the WhatsApp option to receive a technical proposal based on the recorded results.
Related pages
Industrial UPS·Step-by-step UPS sizing guide·Types of UPS·Online UPS·Lithium UPS·Contact us·Privacy Policy