AC Size Calculator: Cooling Load for Hot Climates

Simultaneous room heat gains

The calculation follows the component energy balances taught in NPTEL IIT Kharagpur, Lessons 34–35, with a deliberately simplified, steady design-day solar model. It estimates heat gains as cooling loads: radiant cooling-load factors are 1 and thermal storage is omitted. It does not run EnergyPlus.

Opaque surfaces use $Q=UA\Delta T_{eff}$. In normal mode, $\Delta T_{eff}=T_o-T_i+\alpha I/h_o$. This is a sol-air approximation with zero separate net long-wave correction. EnergyPlus 26.2, Outside Surface Heat Balance, explains the surface balance and sol-air reduction; its full conduction and radiation algorithms are more detailed. The user supplies U-values and the outside combined film coefficient; no construction catalogue is implied.

Solar direction is derived by projecting the solar vector onto true east, north and up, using latitude, declination and solar hour angle (15 degrees per hour from solar noon). Vertical N/E/S/W irradiance is positive beam incidence plus half the diffuse horizontal irradiance and half the ground-reflected horizontal irradiance. Ground reflectance is assumed 0.2. Roof irradiance is beam times solar-altitude sine plus diffuse. Irradiance is zero with the sun below the horizon. Window solar gain is area × SHGC × shading fraction × incident irradiance. Window conduction is separate, and glazing is removed from opaque wall area.

The entire day is sampled every 15 minutes at constant outdoor conditions, internal loads and DNI/DHI. Sizing uses the largest simultaneous sum, not the sum of individual orientation maxima. The component table reports contributions at that same peak. The sun animation uses a separate time input and does not alter sizing. In known-ETD mode, the supplied ETDs and window solar gain are one simultaneous design case, constant across the chart; a peak time has no meaning.

Humidity and outdoor air

FAO-56 Chapter 3 equation 11 supplies the approximate saturation-pressure curve. NPTEL Lesson 27 supplies $w=0.622p_v/(p-p_v)$ and ideal-gas dry-air density. ACH is converted to outdoor volumetric flow, then to dry-air mass flow. Sensible gain is $\dot m_{da}(c_{pa}+w_oc_{pv})(T_o-T_i)$. Latent gain uses $\dot m_{da}(h_{fg}+c_{pv}t_i)\max(w_o-w_i,0)$, where $t_i$ is temperature above 0 °C. Constants are 1005 and 1880 J/(kg·K), and 2501000 J/kg. Drier outdoor air receives no latent credit. Occupant sensible and latent loads, lights and appliances are added explicitly.

Capacity and cost

Capacity retention is 1 up to 35 °C and decreases linearly according to the user's retained fraction at 45 °C; the same line is extrapolated to 50 °C. This is a user scenario, not a universal AC curve. Minimum nominal capacity is room peak × (1 + allowance) / retention. One refrigeration ton is 12000 BTU/h.

The cost scenario is monthly delivered cooling divided by (entered ISEER × entered efficiency adjustment), multiplied by the entered tariff. BEE's 24 August 2016 methodology describes seasonal temperature weighting; ISEER is not an instantaneous COP at 45–50 °C. The set-temperature sweep recalculates envelope and outdoor-air gains at fixed indoor RH while retaining the same efficiency and mean-load fractions. In known-ETD mode, the supplied design gains are shifted by the change in set temperature, with negative envelope gains clipped at zero.

• One rectangular room with walls aligned to true cardinal directions. Adjacent surfaces disabled by the user adjoin equally cooled space. No ground-contact floor model, thermal bridges, skylights or hot adjoining rooms.
• U-values, irradiance, absorptivity, shading, infiltration and equipment performance are user inputs. Construction and weather defaults are illustrative scenarios, not measured local design data.
• Constant outdoor dry bulb and coincident RH over the sampled day. Constant DNI/DHI while the sun is up; isotropic diffuse sky and 0.2 ground reflectance. No sun obstruction, wall shading, thermal lag or radiant delay.
• People and appliance gains are simultaneous at the room peak. The default standing-person gain of 75 W sensible plus 75 W latent comes from NPTEL Lesson 35, Table 35.3, p.12.
• Untreated air is counted once as ACH at outdoor volumetric conditions. No heat recovery, exhaust imbalance, duct losses or duct-fan selection. No credit for drying by outdoor air.
• The default 85% capacity retention at 45 °C, 0.75 efficiency adjustment, 0.55 mean-load fraction, 5% allowance and tariff 8 are editable assumptions. They are not measured product performance or BEE-prescribed factors.
• Regional class lists use historical Haier product examples and Hitachi's 2026 E-series nominal capacities. They are illustrative capacity labels, with no claim about present availability or tropical suitability. Pi is a cooling class, not mechanical horsepower; tatami is a market class here, not an area conversion.

Preliminary cooling estimates for 30–50 °C outdoor dry bulb, 18–30 °C indoors, outdoor warmer than indoors, latitude −40° to +40°, and pressure 75–105 kPa. The intended hot-climate use is 40–50 °C. The permitted fields are calculation bounds, not a guarantee that every extreme combination represents a normal occupied room.

No prediction of indoor comfort, achievable humidity, pull-down time, compressor cycling, electrical circuit size or refrigerant selection. The steady solar model can overstate a heavy wall's instantaneous load and cannot reproduce its delayed evening peak. For detailed design, use local design-weather data and a transient heat-balance or appropriate cooling-load method.

Before purchase, verify delivered total AND sensible/latent capacity at the actual ambient temperature and indoor dry/wet bulb, minimum modulation, airflow and maximum permitted outdoor temperature. A high operating-temperature limit is not a promise of retained cooling capacity. Avoid accumulating independent oversizing allowances. Installation, refrigerant handling and electrical work belong to qualified people and must follow local rules.

ISEER comparisons use the 2016 BEE definition as background, not a current star-rating compliance assessment. No current star bands, rebate eligibility or regulated tariff are assigned. Prices are user-entered; the default tariff is illustrative as of 27 October 2026.