What air conditioner power do I need for the room's m²?

One of the most expensive mistakes when quoting an air conditioner is relying only on the manufacturer's generic table ("up to 25 m²") without correcting for ceiling height, orientation or climate zone. A wrongly sized unit doesn't just perform worse: if it's too big, it starts and stops constantly, uses more and cools with less comfort.

The base rule: frigories per cubic metre

The standard for Spain in a medium climate zone is 50 frigories per m³, not per m². That means ceiling height matters as much as floor area:

1. Calculate the volume: area × ceiling height.
2. Multiply the volume by 50 to get the frigories needed.
3. Divide by 860 to convert to kW of cooling power.

Example: a 20 m² room with a 2.5 m ceiling has a volume of 50 m³. 50 m³ × 50 frig = 2,500 frigories, equivalent to 2.91 kW. This is the same calculation the PresupuestoYa calculator applies automatically for the AC Technician trade.

When to raise power above the base rule

The 50 frig/m³ rule is a starting point, not the final answer:

• Very hot areas (Seville, Alicante, Murcia): go up to 60 frig/m³.
• South or west orientation: a room with a lot of summer sun may need 25-30% more power than a north-facing one.
• Lots of glazing or little insulation: raise the margin the same as for orientation.

What type of unit suits each case

• Split 1×1 — the standard solution for one room, 3-4 hours to install.
• Multisplit — a single outdoor unit feeds 2-3 indoor units; saves facade space, but if the outdoor unit fails all the indoor units lose service at once. Always size the outdoor unit by adding up the real power of the indoor units, not the maximum nominal.
• Ceiling cassette — 360° air distribution from the false ceiling, usual in shops and offices; requires enough space in the false ceiling and almost always a condensate pump.
• Ducted unit — the most discreet option (no visible units) and also the most expensive, with more false-ceiling work.
• Portable — no fixed installation, but performs considerably less than a fixed split of the same nominal power; reserve it for temporary solutions or rentals.

What is usually forgotten when quoting

Beyond the unit itself, a complete AC installation includes:

• Insulated copper refrigerant line: measure the real distance (not in a straight line) and add 15% waste. Most domestic splits allow up to 10-15 metres of pipework; beyond that limit, power drops or gas has to be added.
• Dedicated electrical circuit: never shared with other appliances, because compressor start-up draws several times its rated current. 2.5 mm² cable with a 16A circuit breaker for units up to 3 kW, or 4 mm² with 20A for 3-5 kW units.
• Condensate drain: thin PVC pipe (Ø16-20 mm); if the drain ends up higher than the indoor unit (typical with ceiling cassettes), a mini condensate pump is needed.
• F-Gas certificate: mandatory by regulation for any installation, maintenance or removal that touches the refrigerant circuit. It's not an optional extra: always include it as a quote item.

Frequently asked questions

Is power calculated by m² or m³? By m³ (area × height), using 50 frigories/m³ as a reference in a medium climate zone.

How much more power does a south-facing room need? Between 25% and 30% more than an equivalent north-facing one, because of the summer solar load.

What energy efficiency should I look for in a new unit? At least A++ (SEER above 6.1 for cooling and SCOP above 4 for heating) for real savings on the electricity bill.

Conclusion

Sizing the power properly —correcting for height, orientation and zone— avoids two expensive problems: an insufficient unit that doesn't cool on the hottest days, or an oversized one that uses more and wears out sooner from constant starts. Automating this calculation, together with the refrigerant line and electrical circuit, gives a complete quote with no forgotten items.