UPS Sizing Calculator
Enter the load in watts, the backup runtime you need, the battery bank voltage and the chemistry — the calculator returns the required UPS capacity in VA, the recommended rating at 80% loading, and the battery bank size in ampere-hours with a 25% headroom already applied. No sign-up, instant results.
Typical loads in a control cabinet (nameplate ranges)
| Equipment | Typical continuous load |
|---|---|
| Unmanaged network switch (8–24 port) | 10–30 W |
| PoE switch (24-port, depends on powered devices) | 100–600 W |
| NVR with drives (16-channel) | 40–80 W |
| PLC power supply (mid-size rack) | 30–100 W |
| HMI panel (7–15 in) | 15–50 W |
| Industrial PC / engineering workstation | 100–300 W |
| Whole small control cabinet (PLC + HMI + switch + relays) | 100–300 W |
Typical nameplate ranges for planning only — always confirm against the datasheets of the exact equipment you will power. The calculation is a first-order estimate: discharge-rate, temperature and aging corrections belong to the battery manufacturer's curves.
How the Calculator Works
UPS sizing starts with two separate numbers: how big the UPS must be, and how much battery it must carry.
UPS capacity (VA). Equipment nameplates state watts (real power) while UPS ratings are stated in VA (apparent power). The bridge is the load power factor — 0.6–0.8 for older equipment, 0.95+ for modern supplies with power factor correction:
Required VA = load watts ÷ power factor
Then apply a loading target of no more than about 80% of the UPS rating — headroom for inrush, future additions, and battery-charging behavior right after an outage:
Recommended rating = required VA ÷ 0.8
Battery bank (Ah) — the battery sizing half. The energy the load draws over the backup window, divided by what the bank can actually deliver:
Ah = load W × backup hours ÷ (bank voltage × overall efficiency)
Overall efficiency folds inverter conversion losses and battery discharge losses together — 0.85 is the conservative figure for VRLA (sealed lead-acid) strings, 0.95 for lithium (LiFePO4) banks with their integrated battery management. The result is then grown by 25% headroom before it is shown, because batteries age, run colder than rated, and deliver less than nameplate at high discharge rates. A 24 V bank is two 12 V blocks in series, a 48 V bank is four — series blocks must all have the same Ah rating.
When Engineers Use This Calculator
Sizing control-power backup for a PLC cabinet. The most common job: a control cabinet with a PLC supply, an HMI and a network switch totals 100–300 W, and fifteen minutes of control power is typically enough for a clean, sequenced shutdown. Enter those two numbers and the calculator does the rest.
Riding through a transfer to generator. Where a standby generator exists, the UPS only has to bridge the start window — often five to ten minutes. Sizing the bank for that window instead of a full outage saves a significant amount of battery.
Securing remote and edge installations. NVRs, PoE switches and RTUs in water, solar and telecom enclosures ride through dips and short outages on a small UPS; the load table above gives starting figures when nameplates are not yet at hand.
Checking an existing bank. If a site survey shows a UPS already installed, entering its measured load and the original design runtime shows whether the battery bank is still adequately sized — or has aged past its service life (roughly 3–5 years for VRLA, 10+ for lithium).
Frequently Asked Questions
How do I calculate what size UPS I need?
Add up the watts of everything the UPS will carry, divide by the power factor to get VA, then divide by 0.8 to apply the 80% loading target, and pick the next standard rating above that. Example: 500 W at 0.9 PF → 556 VA → 695 VA recommended → a 700–750 VA single-phase unit. The uninterrupted power supply calculator above does all three steps in one pass.
How do you calculate battery Ah for a UPS?
Battery ampere-hours = load watts × backup hours ÷ (battery bank voltage × overall efficiency), then add headroom for aging. Example: 300 W for 15 minutes on a 24 V VRLA bank → 300 × 0.25 ÷ (24 × 0.85) ≈ 3.7 Ah minimum, 4.6 Ah with 25% headroom.
Why add 25% battery headroom?
Three effects eat into nameplate capacity: batteries deliver less at high discharge rates (the Peukert effect, strongest for lead-acid), capacity drops in cold environments, and every year of service takes a little more. The 25% margin covers the combination at the design stage; the manufacturer’s discharge curves remain the final word.
Lead-acid or lithium batteries for the UPS?
VRLA (sealed lead-acid) banks are inexpensive and well understood, with a service life of roughly 3–5 years in standby duty and overall efficiency around 0.85. LiFePO4 lithium banks cost more up front but run near 0.95 efficiency, last ten years or more in favorable conditions, and need less enclosure volume — which matters when the batteries live inside a control cabinet with thermal limits.
How many 12 V batteries make the bank?
Divide the bank voltage by twelve: a 24 V bank is 2 × 12 V blocks in series, 48 V is 4 blocks. Blocks in series must share the same Ah rating and age; the bank’s capacity in Ah equals the per-block rating, while the energy stored multiplies with voltage. For longer runtimes at low voltage, adding parallel strings (each with its own fusing) is the usual practice.
Related Guides
- Uninterruptible Power Supply (UPS) Guide — topologies, kVA sizing theory and battery backup-time formulas.
- UPS vs Surge Protector — which device solves which power problem, and how they coordinate in a cabinet.
- Transformer kVA Calculator — sizing the source upstream of the UPS.
- Voltage Drop Calculator — checking the branch circuit that feeds the cabinet.
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