For a medium‑sized hospital, correctly sizing an uninterruptible power supply (UPS) is critical: undersizing risks overloads and shutdowns, while oversizing wastes capital and wastes energy. A good UPS strategy typically uses online double‑conversion UPS units backing up the most critical circuits (ICU, operating theatre, emergency rooms, IT, and lab equipment), sized with enough headroom and runtime until the generator or grid stabilizes.
Step 1: Identify critical loads
Before you can size UPS, you must know exactly what it will support. In a medium hospital, these usually include:
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ICU and operating‑theatre equipment (ventilators, monitors, operating‑room lights, anaesthesia machines).
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Emergency room and delivery‑room medical devices.
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Imaging and lab equipment (X‑ray, ultrasound, pathology analyzers) if these are considered life‑critical.
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IT infrastructure: servers, data‑centres, network switches, storage, and VoIP/PABX.
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Access‑control and nurse‑call systems on the same critical bus.
Make a list for each room, noting the nameplate power in watts or kVA, and separate these into “must‑stay‑on” and “can‑tolerate brief outage” zones. Work with clinical‑equipment vendors to capture real running and inrush currents, because devices like MRI or large imaging systems can draw much higher power at startup.
Step 2: Calculate total kVA/kW and add headroom
Once you have all critical‑load ratings, add them up in kVA or kW. It is common practice to:
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Convert all loads to a common unit (usually kVA).
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Multiply the total load by 1.2–1.3 to allow for spikes, future growth, and safe operation.
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Aim to keep the UPS operated around 70–80% of its rated capacity, not at 100%.
For example:
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Total critical load = 80 kW.
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With 20% headroom = 80 kW × 1.2 = 96 kW.
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A 100−120 kVA online UPS (or multiple smaller units in parallel) would be appropriate.
Medium hospitals often end up with UPS capacities in the 60–200 kVA range, depending on how many theatres, ICU beds, and IT systems are on the UPS bus.
Step 3: Decide runtime and battery sizing
For hospitals, UPS runtime is not about long‑term islanding but about bridging the gap until the generator stabilizes or the grid returns. Typical goals are:
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5–15 minutes for critical medical equipment if the generator is on ATS and starts quickly.
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15–30 minutes or more if the hospital relies on UPS for longer grid outages or wants to run key circuits off‑grid until the generator is dispatched.
Battery size is chosen based on:
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Total UPS‑supported load in kW.
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Desired runtime in minutes.
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Battery type (lead‑acid vs lithium‑ion) and manufacturer’s discharge curves.
Vendors and UPS manufacturers provide sizing tools or tables that map kVA × runtime to battery‑bank capacity (Ah), which you can use to select internal or external battery cabinets.
Step 4: Factor in redundancy and topology
Healthcare‑facility guidance strongly recommends redundancy in UPS systems. For a medium hospital, common configurations are:
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N+1 configuration: Two or more UPS modules where one can fail and the rest still carry the load.
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Dual‑path distribution: Critical devices fed from two separate UPS paths to avoid a single point of failure.
For example:
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If the calculated load is 100 kVA, you might install two 60 kVA UPS units in parallel (N+1) so the system can still run at 100% load if one unit is offline.
Parallel‑module setups also ease maintenance, since one unit can be taken offline while the others continue to protect patient‑care systems.
Step 5: Practical sizing tips for a medium hospital
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Use online double‑conversion UPS for all life‑support and IT‑critical loads; avoid cheaper line‑interactive units for surgical and ICU circuits.
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Group loads by zone (ICU, theatre, IT, lab) and size separate UPS units per zone where appropriate, instead of one monolithic UPS.
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Plan for future expansion: include extra breaker positions or a slightly larger UPS than today’s load, so you can add new equipment without re‑sizing.
At design stage, engage a specialist or UPS vendor to perform a detailed load‑survey, considering inrush currents, power‑factor, and local codes (such as NFPA or equivalent). When done correctly, a well‑sized UPS system ensures that power‑outage events in a medium hospital become a background issue rather than a crisis for both patients and staff.