BTU tells you the cooling job, not the bill
A 5,000 BTU/h room AC is in a different capacity class from a 12,000 BTU/h model. The rating describes cooling output per hour under the applicable test method. It does not mean the smaller unit draws 5,000 electrical watts or that the larger one always costs 2.4 times as much.
Use room size and conditions to identify a suitable capacity. Then use model-specific watts, annual kWh or a representative measurement to estimate electricity cost. An undersized unit that runs continuously can be a poor comparison with a suitable larger model that cycles.
Compare four transparent scenarios
The table below uses invented average inputs only to show the arithmetic. Every row assumes eight hours per day, 30 days and $0.18/kWh. It is not a database of typical models and does not attribute the watts to every AC in that capacity class.
| Capacity label | Assumed average input | Monthly energy | Monthly cost |
|---|---|---|---|
| 5,000 BTU/h | 450 W | 108 kWh | $19.44 |
| 6,000 BTU/h | 550 W | 132 kWh | $23.76 |
| 8,000 BTU/h | 700 W | 168 kWh | $30.24 |
| 12,000 BTU/h | 950 W | 228 kWh | $41.04 |
Formula: watts ÷ 1,000 × 8 hours × 30 days × $0.18/kWh. A different model, climate, thermostat setting or schedule can move the result outside this range.
Find a better input for the exact model
If the EnergyGuide or certified product data gives annual kWh, preserve its test assumptions when comparing models. For a personal schedule, a cumulative kWh reading over representative weather is often more useful than one momentary watt reading because it includes compressor cycling.
If you use a nameplate maximum, label the result as a constant-power upper scenario rather than normal consumption. Do not apply an extra cycling discount to an average that was already measured over on-and-off operation.
Size first, then compare efficiency and cost
ENERGY STAR’s room-AC guidance starts with floor area and adjusts for factors such as sun, shade, kitchens and occupancy. After suitable capacity is established, compare CEER, annual energy information, installation requirements and full purchase cost.
A lower monthly estimate is useful only if the appliance can cool the intended room and be installed correctly. Keep the room measurement, capacity source, watt or kWh source, schedule and electricity rate beside every estimate so the result can be checked later.
Choose your next step
Continue with the question that matches your next decision:
Sources & further reading
Numerical examples are illustrative unless explicitly described as measured. This guide does not claim hands-on product testing. Read the methodology and editorial disclosure.