UPS and Battery Sizing for Control Systems
List the loads on a control system UPS. The sheet totals the load, sizes the UPS kVA with margin and calculates the battery capacity for the required backup time.
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- Calculator sheet with a worked example
- How to Use sheet with steps
- 19 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
UPS loads
| No | Load | Qty | Power each (W) | Total (W) | Critical? | Remarks |
|---|---|---|---|---|---|---|
| 1 | DCS controller cabinets | 2 | 600 | 1,200 | Yes | |
| 2 | SIS cabinet | 1 | 500 | 500 | Yes | |
| 3 | Operator stations | 3 | 350 | 1,050 | Yes | |
| 4 | Network switches | 4 | 60 | 240 | Yes | |
| 5 | Field 24 V DC supplies | 2 | 480 | 960 | Yes |
About this Calculator
Control systems must ride through power failures long enough to shut down safely or switch to backup power. This sheet gives a documented first estimate of UPS and battery size.
Basis: UPS kVA = Σ W ÷ PF × (1 + margin). Battery Ah = P × t ÷ (Vdc × η) × ageing factor ÷ depth of discharge. Simplified constant power method; final sizing per battery maker discharge tables (for example IEEE 485 approach).
How to use
- List all loads fed by the UPS.
- Set power factor, margin and backup time.
- Enter battery voltage, inverter efficiency, ageing and depth of discharge.
- Read UPS kVA and battery Ah.
- Confirm with vendor discharge data.
Formulas used
- kVA = ΣW ÷ PF × (1 + margin) ÷ 1000.
- Ah = ΣW × t ÷ (Vdc × η) × ageing ÷ DoD.
Related tools
Frequently Asked Questions
What ageing factor is usual?
1.25 is common, so a battery still meets the load at 80 % capacity at end of life.
Why depth of discharge?
Deep discharge shortens battery life; limiting it keeps the battery healthy.
Does temperature matter?
Yes, cold reduces capacity; apply a temperature factor from the maker.
Learn more on the blog
Redundant power supply for PLC
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
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