How this air conditioner calculator works
Most people ask two things about an air conditioner, one before buying and one after: what size do I need? and what will it cost to run? This calculator answers both.
Sizing. It starts from roughly 80 W of cooling per square metre of floor area (about 25 BTU/h per square foot), which matches the ENERGY STAR room-size table for an average room. It then scales for ceiling height, climate (from mild northern European summers to Gulf-state heat), sun exposure (shaded, average, west-facing glass, top floor under a roof), extra occupants (each person beyond two adds about 600 BTU/h, or 175 W of body heat) and kitchens (+4,000 BTU/h). The result is rounded up to the next standard unit size: 5,000, 6,000, 7,000, 9,000, 12,000, 18,000, 24,000 BTU and so on.
Running cost. Electrical input is cooling capacity divided by efficiency. You can enter efficiency in any of the common formats: European SEER (W/W), US SEER or SEER2 (BTU/Wh), EER (BTU/Wh at full load) or a full-load COP. Because inverter units spend most of their time at part load, the calculator multiplies the full-load input by an average load factor (55% by default) and then by your hours per day, days per season and electricity price. An optional heating mode adds the cost of using a reverse-cycle unit as a heat pump.
The formula
P_input (kW) = Q_unit (kW) ÷ efficiency (W/W) [US SEER or EER ÷ 3.412 → W/W] kWh/season = P_input × load factor × hours/day × days Cost = kWh × electricity price
Worked example: a 25 m² living room
With the defaults (25 m², 2.5 m ceiling, warm-summer climate, average sun, two people), the room needs about 6,800 BTU/h (2.0 kW) of cooling, so the calculator picks a standard 7,000 BTU (2.05 kW) unit.
At an EU SEER of 6.1 (A++), the unit’s maximum electrical input is 2.05 ÷ 6.1 = 0.34 kW. At a 55% average load it draws about 0.18 kW, which costs €0.046 per hour at €0.25/kWh. Running 8 hours a day costs €0.37 per day or about €11 for a month of use. Over a 75-day cooling season that is 111 kWh and roughly €28, and about 39 kg of CO₂ on a 350 g/kWh grid.
The efficiency chart shows why the label matters: the same cooling from a 3 W/W unit (typical of portable and old window units) would cost €56 a season, twice as much, while an 8.5 W/W premium unit would cost about €20.
How to read the results
- Cooling cost per season is the headline number for budgeting. Change days of use to match your climate; the climate selector fills in a typical value.
- Cost per hour, day and month help decide whether running the AC overnight or all day is affordable.
- Recommended size is in BTU/h, kW and refrigeration tons (1 ton = 12,000 BTU/h). If you enter your own unit capacity, the verdict tells you whether it is undersized, oversized or about right.
- Efficiency (W/W) converts whatever rating you entered so you can compare units across European and US labels.
Typical values
| Unit type | Typical efficiency | Input for 3.5 kW of cooling |
|---|---|---|
| Portable single-hose | EER 2.0–2.6 W/W | 1.3–1.8 kW |
| Window unit (older) | EER 2.6–3.2 W/W (9–11 BTU/Wh) | 1.1–1.3 kW |
| Split, basic (A+) | SEER 5.6–6.1 W/W | 0.6–0.7 kW (seasonal avg. lower) |
| Split, efficient (A++/A+++) | SEER 6.1–8.5+ W/W | 0.4–0.6 kW |
| US central AC | SEER2 13.4–20 BTU/Wh | 0.6–0.9 kW |
Tips to cut air-conditioning costs
- Set 24–26 °C, not 20 °C. Each degree lower raises cooling energy by roughly 6–10%.
- Block the sun before it enters. External shutters or awnings on west and south windows can reduce the cooling load more than any upgrade to the unit.
- Use inverter units and let them run steadily rather than switching on and off at full blast.
- Clean filters monthly in the cooling season; clogged filters cut efficiency noticeably.
- Pair with solar. Cooling demand peaks when solar output does; see the solar panel calculator.
- Use it for heating too. In winter a split unit is an efficient heat pump; compare it with your current system in the heat pump calculator.
Limitations
The sizing rule assumes a reasonably insulated room with normal window areas. Rooms with large glazed facades, poor insulation or high internal gains (servers, many people) need a proper heat-gain calculation (for example Manual J in the US or EN 12831-style methods in Europe). Real consumption depends on thermostat setpoint, humidity and how often doors are opened; the load factor is a single average. Cooling-season length varies widely between years. Standby consumption of the indoor unit (typically 1–5 W) is not included — see the standby power calculator.
Method & assumptions
Required capacity Q = area × 80 W/m² × (ceiling ÷ 2.5 m) × climate factor × sun factor + 175 W per extra person (+1.17 kW for kitchens), rounded up to the next standard size. Electrical input = Q ÷ efficiency (US SEER/EER in BTU/Wh ÷ 3.412 = W/W). Energy = input × load factor × hours × days. Heating mode: energy = 50% of rated capacity × hours × days ÷ SCOP.
Frequently asked questions
How much does it cost to run an air conditioner per hour?
What size air conditioner do I need for my room?
What is the difference between SEER and EER?
Is a bigger air conditioner better?
How many kWh does an air conditioner use per day?
Is it cheaper to heat with an air conditioner?
Sources & references
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