AC Unit Size Calculator (BTU for Phoenix Climate)
Estimate the correct air conditioner size (BTU/hr and tons) for a home in the Phoenix, AZ metro area. Phoenix's extreme desert heat — with design temperatures reaching 110 °F — requires larger equipment than national averages. Enter your home's details below for a Manual J–style estimate.
Formula & Methodology
This calculator uses a simplified Manual J–style sensible heat load approach tuned for Phoenix:
- Base Load:
Base BTU = Floor Area (sq ft) × 35 BTU/sq ft
The 35 BTU/sq ft figure derives from the standard 20 BTU/sq ft national baseline scaled by the Phoenix design temperature differential (110 °F outdoor – 75 °F indoor = 35 °F ΔT) versus the national average ΔT of 20 °F → multiplier = 35/20 = 1.75. - Volume Correction:
× (Ceiling Height / 8 ft)— scales for air volume above the standard 8 ft reference. - Insulation Multiplier: Poor 1.30 · Average 1.00 · Good 0.85 · Excellent 0.72
- Window Solar Load: Added BTU/sq ft of floor area — Low +1 · Average +3 · High +6 · Very High +10 — reflecting Phoenix's intense solar irradiance (~2,300 kWh/m²/yr).
- Multi-Story Factor: 1-story 1.00 · 2-story 0.93 · 3-story 0.88 (less roof area per sq ft).
- Occupant Load:
Occupants × 600 BTU/hr(ASHRAE sensible + latent per person). - Shading Factor: None 1.00 · Partial 0.95 · Good 0.88
- Round Up to the nearest standard residential unit size (1.5 T – 5 T).
Tons to BTU: 1 ton of cooling = 12,000 BTU/hr
Assumptions & References
- Phoenix ASHRAE 99.6% summer design dry-bulb temperature: 110 °F (ASHRAE Fundamentals 2021, Climate Data).
- Indoor design setpoint: 75 °F (standard residential comfort per ACCA Manual J).
- Calculation follows the ACCA Manual J Residential Load Calculation (8th Edition) simplified method.
- Occupant sensible + latent heat: 600 BTU/hr per person (ASHRAE 62.2 / Manual J Table 1).
- Does not account for duct losses (add ~10–15% for ducts in unconditioned attic space, common in Phoenix).
- Does not include kitchen/appliance loads or infiltration beyond the insulation factor.
- Arizona minimum efficiency standard: SEER2 14.3 (≥ 45,000 BTU) or SEER2 15.2 (< 45,000 BTU) per DOE 2023 Southwest regional standards.
- Oversizing by more than 15% is discouraged — oversized units short-cycle, reducing dehumidification and equipment life.
- Source: ACCA Manual J, ASHRAE Handbook of Fundamentals, Arizona Department of Housing Energy Code, APS / SRP energy efficiency guidelines.