How to Size a Genset for a Hospital ICU

Sizing a genset for a hospital ICU is not the same exercise as sizing one for an office floor, and treating it the same way is how facilities end up either underpowered during an outage or massively overspending on capacity they never use.

The standard formula is straightforward: total connected load in kW, divided by the power factor (typically 0.8 for a diesel genset), gives you the base kVA requirement. For an ICU, connected load includes ventilators, monitors, infusion pumps, HVAC for isolation rooms, lighting, and — critically — imaging equipment if it shares the same backup circuit.

A standard commercial calculation adds roughly 20-25% headroom above connected load to account for starting current on motors and future load growth. For a hospital critical-load circuit, that margin typically needs to go higher — 30-40% is a more realistic starting point — because:

• Simultaneous startup risk. Unlike an office where equipment switches on gradually through the day, a power-cut event can bring multiple high-draw devices (compressors, imaging equipment) online at once when the genset picks up load.
• Zero tolerance for brownout. A momentary voltage dip that an office barely notices can affect ventilator or monitoring equipment. Undersizing isn't just an inconvenience here.
• Regulatory and accreditation requirements. Many hospital accreditation standards specify minimum backup duration and load-bearing requirements independent of what a pure cost calculation would suggest.

A 40 kW connected ICU load, at a 0.8 power factor, gives a base requirement of 50 kVA. Apply a 40% critical-load margin instead of the standard 25%, and you land at roughly 70 kVA — a meaningfully different number than the 62.5 kVA a standard office calculation would suggest.

Sizing the genset correctly is only half the reliability question — the other half is what happens if that single genset itself fails to start, which every mechanical system can. Many hospitals, especially larger facilities with multiple critical circuits, plan around an "N+1" backup philosophy: enough generating capacity that even with one genset down for maintenance or failed on demand-start, the remaining capacity still covers the critical load. In practice this can mean two mid-sized sets configured to share critical load in parallel rather than one large set carrying everything alone, so a single point of failure doesn't take the whole backup system down with it. Whether N+1 makes sense for a given facility is a sizing and budget conversation with a consultant, not a default — but it's worth asking about explicitly rather than assuming a single genset, however well-sized, is automatically enough for a life-safety installation.

A correctly sized genset that runs out of diesel six hours into an extended outage hasn't actually solved the problem it was bought for. Runtime planning is a separate calculation from load sizing, and it matters more for a hospital than almost any other building type:

• Know your expected outage duration, not just your worst-case load. Grid reliability varies enormously by location — a facility in an area with frequent but short outages has different fuel-autonomy needs than one in a region prone to multi-day disruptions after storms or grid failures.
• Size the fuel tank around realistic runtime targets, typically expressed in hours of autonomy at full load, and confirm the dealer's runtime figures are based on your genset's actual fuel consumption curve, not a generic estimate.
• Plan the refuelling logistics, not just the tank capacity — who arranges emergency diesel delivery during an extended outage, and how quickly, is as much a part of the reliability plan as the tank size itself.

Our genset load calculator gives you a fast, equipment-level estimate, but for a hospital or any life-safety installation, that estimate should be the start of a conversation with a qualified electrical consultant and the genset dealer — not the final word. Get their sign-off on load calculations before you commit to a model.