AC Wattage Calculator
Calculate the exact AC wattage, BTU capacity, tonnage, amperage, and energy cost needed to cool any room based on dimensions, insulation, climate, and heat sources.
About
Undersizing an air conditioner forces the compressor into continuous duty, inflating electricity bills by 20 - 40% while failing to dehumidify. Oversizing causes short-cycling, which degrades the compressor and leaves latent moisture in the space. Both errors shorten equipment life. This calculator uses the ASHRAE-derived manual-J simplified method: base cooling load equals room area in ft² multiplied by a climate-zone factor (Fc), then adjusted for insulation quality (Fi), solar exposure (Fs), ceiling height deviation, occupant count (600 BTU/hr per person beyond two), and internal heat gains from appliances. The result is a net cooling load in BTU/hr, converted to watts, tons, and amps at the selected supply voltage.
Accuracy depends on honest insulation assessment. A single-pane west-facing window can add 1,500 - 3,000 BTU/hr that no rule-of-thumb captures. This tool approximates residential cooling loads assuming standard construction. It does not replace a professional Manual J load calculation for commercial or multi-zone HVAC design. Pro tip: add 10% margin if ductwork runs through an unconditioned attic.
Formulas
The net cooling load is the sum of the structural load and internal heat gains:
where the structural load accounts for room area, climate, insulation, sun exposure, and ceiling height:
The ceiling height factor normalises to a standard 8 ft room:
Occupant heat gain beyond two people:
Conversion from BTU/hr to electrical units:
where A = room area (ft²), Fc = climate zone factor (20 - 40 BTU/ft²), Fi = insulation multiplier (0.8 - 1.3), Fs = sun exposure multiplier (0.9 - 1.15), h = ceiling height (ft), n = number of occupants, V = supply voltage (V), T = tonnage, I = current draw (A).
Reference Data
| Room Type | Typical Area | Recommended BTU | AC Tonnage | Wattage (approx) |
|---|---|---|---|---|
| Small Bedroom | 100 - 150 ft² | 5,000 - 6,000 | 0.5 | 1,465 - 1,758 W |
| Medium Bedroom | 150 - 250 ft² | 6,000 - 8,000 | 0.5 - 0.67 | 1,758 - 2,344 W |
| Large Bedroom / Office | 250 - 350 ft² | 8,000 - 10,000 | 0.67 - 0.83 | 2,344 - 2,931 W |
| Living Room | 350 - 500 ft² | 10,000 - 14,000 | 0.83 - 1.17 | 2,931 - 4,103 W |
| Master Suite | 300 - 450 ft² | 10,000 - 12,000 | 0.83 - 1.0 | 2,931 - 3,516 W |
| Open Kitchen | 200 - 350 ft² | 8,000 - 12,000 | 0.67 - 1.0 | 2,344 - 3,516 W |
| Garage / Workshop | 400 - 600 ft² | 12,000 - 18,000 | 1.0 - 1.5 | 3,516 - 5,275 W |
| Small Apartment | 500 - 700 ft² | 14,000 - 18,000 | 1.17 - 1.5 | 4,103 - 5,275 W |
| Large Open-Plan | 700 - 1,000 ft² | 18,000 - 24,000 | 1.5 - 2.0 | 5,275 - 7,034 W |
| Server Room | 100 - 200 ft² | 12,000 - 24,000 | 1.0 - 2.0 | 3,516 - 7,034 W |
| Small Office | 150 - 300 ft² | 7,000 - 10,000 | 0.58 - 0.83 | 2,051 - 2,931 W |
| Restaurant Dining | 500 - 1,000 ft² | 24,000 - 36,000 | 2.0 - 3.0 | 7,034 - 10,551 W |