What size UPS do I need? Sizing capacity and runtime
My equipment adds up to 2000 W, so is a 2000 VA UPS enough? That calculation almost always ends in the wrong purchase. What is missing is not addition — it is three correction factors.
Step 1: count the real load, not the nameplate load
List the protected equipment one by one, with the power on its nameplate (in W or VA). Two rules:
- Count critical equipment only. Printers, water coolers and photocopiers should not be on a UPS — their heating elements draw a large amount of power in an instant.
- Convert VA to W. If a device is rated only in VA, multiply by the power factor (for computer-type loads, roughly 0.7–0.9) to get W.
Step 2: deal with start-up inrush
This is where sizing most often comes unstuck. The current drawn at the instant a motor, compressor, transformer or large-capacitance device starts can be 3–7 times its rated value.
There are three ways to handle it: size the UPS for the inrush; choose an online model with high overload capacity for those loads; or, for motors, move to a variable-speed drive or add a soft starter. If your list includes air conditioning, water pumps, pick-and-place machines or large photocopiers, this step cannot be skipped.
Step 3: leave headroom
Take the result of the first two steps and multiply by a headroom factor of 1.3–1.5, to cover:
- equipment added in future
- reduced output as the batteries age
- avoiding the overheating that comes from running at full load for long periods
Step 4: decide the runtime
Ask yourself one question: once the mains fails, what do I want to happen?
| Requirement | Suggested runtime |
|---|---|
| Just avoid the equipment restarting | 5–10 minutes |
| Complete a safe shutdown | 10–20 minutes |
| Hold on until a generator takes over | 15–30 minutes |
| Ride out a whole unplanned outage | 1 hour or more |
Runtime rises in proportion to battery count, and so does cost. Protecting only what genuinely needs it is often cheaper than buying a larger unit.
The three most common mistakes
- Adding up nameplate wattage and ignoring start-up inrush — the unit alarms on overload as soon as it is switched on.
- Treating VA as usable power — the unit you buy cannot carry the load you expected.
- Oversizing "for safety" — more money, more space, and a battery that sits in shallow discharge with poorer efficiency.
The most reliable approach is still a site survey. Send us your load list and a photo of the installation, and an engineer will size it against the real conditions and explain the basis for every number. You can browse the UPS range first, or contact us.