Updated · 3 min read
In short
- A 12,000 BTU (3.5 kW) inverter unit uses about 0.4–0.6 kWh per hour of real operation.
- About 100–150 kWh per month at 8 hours a day in summer.
- Formula: cooling capacity in kW ÷ SEER × hours = kWh.
How to calculate it
SEER is a seasonal average: in a heatwave hourly use is higher, on mild days lower.
- Convert BTU/h to kW: BTU × 0.293 ÷ 1,000. 12,000 BTU ≈ 3.5 kW of cooling.
- Take the SEER from the energy label (for example 6.5 for A++).
- Average draw ≈ 3.5 ÷ 6.5 ≈ 0.54 kWh per hour.
- Multiply by real running hours and by your own tariff.
Typical values by size
- 9,000 BTU (about 2.6 kW): 0.3–0.45 kWh/h, 75–110 kWh/month at 8 h/day;
- 12,000 BTU (about 3.5 kW): 0.4–0.6 kWh/h, 100–150 kWh/month;
- 18,000 BTU (about 5.3 kW): 0.65–0.9 kWh/h, 160–220 kWh/month;
- 24,000 BTU (about 7 kW): 0.9–1.2 kWh/h, 210–290 kWh/month.
What pushes consumption up
- a very low set temperature — each degree lower adds a few percent;
- an undersized unit running flat out;
- direct sun on windows without external shading;
- dirty filters and an outdoor unit in the sun without airflow;
- doors and windows open in the cooled room.
Frequently asked questions
Is it cheaper to switch it off when I go out?
For inverter units, switching off for long periods (hours away, days) saves energy. Switching on and off every half hour does not — the unit loses time recovering at full power. A slightly higher set point (25–26°C) works better.