How Much Cooling Power (BTU/kW) Does Your Room Need? A Simple Sizing Guide

Too small won't cool, too big wastes power. Here's how to find the right BTU or kW rating for your room in a few minutes.

6 min read · Updated July 2026

Buying a cooling unit on price or brand alone is the fastest way to end up disappointed. A device that's too weak runs all day and never reaches a comfortable temperature. One that's too strong cools in short bursts, leaves the air clammy, and costs more than it should. The good news: matching cooling power to your room takes about five minutes and a bit of arithmetic.

BTU, kW and watts: what the numbers mean

Cooling capacity is labelled in two units, and both describe the same thing: how much heat the unit can move out of a room per hour.

• BTU/h (British Thermal Units per hour) is the number you'll see most often on portable and window units.
• kW (kilowatts) is the metric equivalent used on many spec sheets and energy labels.
• The conversion is simple: 1 kW is about 3,412 BTU/h. So a 9,000 BTU unit is roughly 2.6 kW, and 12,000 BTU is about 3.5 kW.
• Don't confuse cooling capacity with power draw. A unit can output 2.6 kW of cooling while only consuming around 0.9 kW of electricity. The ratio between the two is its efficiency (EER/SEER).

Rule of thumb: cooling power per square metre

For a rough first estimate, size by floor area. Insulated flat with a standard ceiling height of around 2.5 m? Then plan for roughly 60 watts of cooling per square metre with a sealed split unit. Portable monoblock units need more, closer to 100 watts per square metre, because their exhaust hose pushes some already-cooled air out of the room and pulls warm air back in. That single hose is why monoblocks look weaker on paper than a split of the same rating.

Cooling power by room size: the quick table

Use these ranges as a starting point for a normal living space under typical conditions. Read on for the factors that push you toward the upper end.

• 10–15 m²: about 5,000–7,000 BTU (1.5–2.0 kW)
• 15–25 m²: about 7,000–9,000 BTU (2.0–2.6 kW)
• 25–35 m²: about 9,000–12,000 BTU (2.6–3.5 kW)
• 35–45 m²: about 12,000–14,000 BTU (3.5–4.1 kW)

Factors that raise your requirement

The table assumes an average room. Each of these conditions adds heat, so add roughly 10–20% for every one that applies to you:

• Top floor or directly under the roof (a Dachgeschoss heats up hard in summer).
• Large windows facing south or west, especially without external shading.
• Several people using the room regularly, or a room open to a kitchen.
• Lots of electronics, or strong lighting that runs for hours.
• Poor insulation, older single-glazing, or a ceiling higher than 2.5 m.

Choosing a permission-free unit (and what the rules say)

In Germany, the type of device decides how much paperwork you face. Mobile monoblock units, window units, and mobile split units with a permanently sealed refrigerant circuit are generally permission-free: they arrive pre-filled, you don't handle any refrigerant, and you don't drill a fixed hole through the building. Fixed split systems are a different story. They involve a wall penetration and a refrigerant circuit that must be installed by a certified technician under the EU F-Gas regulation, and because that counts as a structural change, your landlord's consent is usually required. For most renters, a correctly sized permission-free unit is the practical route, and if you'd rather not buy and store a device for one summer, a seasonal rental keeps things simple.

So sizing comes down to three steps: convert to a single unit, apply the per-square-metre rule for your device type, then nudge upward for top-floor, sun, and occupancy. Get within range of your real need and the room cools quickly, the air stays comfortable, and you won't pay for capacity you never use.

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