🖥️How much cooling does this server room need?

Take the rack’s actual draw in watts and multiply by 3.412. A rack pulling 3 kW needs 10,240 BTU/h of cooling for the equipment alone, plus the envelope and anyone standing in there. Use measured draw from the PDU if you have it — nameplate ratings are usually double what the hardware really pulls.

For a small closet, a mini-split can work, and many small offices run exactly that way. Two things differ from a home install: it has to run all year, including when it is −10 °C outside, so it needs a low-ambient kit; and it should not be the only one. A portable unit with a hose is the worst option — it dumps its condenser heat through a leaky duct and pulls make-up air from the room it is meant to cool.

ASHRAE’s recommended envelope for data equipment is 18 to 27 °C at the equipment inlet, which is warmer than most people run. Chasing 18 °C costs real money and buys nothing; 22 to 24 °C at the inlet is comfortable for the hardware and much cheaper. Measure at the front of the rack, not on the wall.

If the room matters, yes. One unit means one failure takes the room out, and server rooms fail at the worst possible moment because that is when the load is highest. Two units each sized for the full load, alternating weekly, gives you a spare that you know works — which a spare that has never run does not.

Only by reducing the watts drawn. Efficiency changes how much work you get per watt, not where the watt ends up: it still leaves as heat. If new hardware does the same job on 2 kW instead of 3 kW, the cooling load falls by exactly that 1 kW — 3,412 BTU/h.

Size for the load you expect in three years and check the room’s electrical supply matches. In practice most rooms grow to fill their power feed, so if the room has a 32 A circuit, work out what that circuit can deliver and make sure the cooling could cope with it. That is the ceiling the room will drift towards.