Cooling load calculator
Room cooling load from dimensions, design temperatures and envelope U-values — plus solar gain by window orientation and glazing type, internal gains and ventilation. Required cooling capacity in kW, broken down by source.
How to use this calculator
- Enter the room dimensions and indoor/outdoor design temperatures.
- Pick wall, glazing, door, ceiling and floor constructions, or enter your own U-values.
- Enter window area and orientation for solar gain, plus the glazing g-value.
- Read the total cooling load, broken down by transmission, solar and internal/ventilation gains.
What this calculates
A room's cooling load is the sum of four physically distinct gains — heat conducted through the envelope, solar radiation through glazing, heat given off by people and equipment inside, and the sensible heat carried in by outdoor air:
Q_transmission = Σ (Ui × Ai × ΔT) for walls, windows, doors, ceiling, floor
Q_solar = A_window × g × I_orientation
Q_internal = occupants × W_person + A_floor × (W_equip + W_light)
Q_ventilation = 0.34 × n × V × ΔT
Q_total = Q_transmission + Q_solar + Q_internal + Q_ventilation
ΔT = outdoor design temperature minus indoor cooling setpoint, K
g = glazing solar heat gain coefficient (g-value / SHGC)
I = peak design solar irradiance on the window's orientation, W/m²
This is the same steady-state relationship the Heat Loss / Heating Load calculator uses for transmission and ventilation — this tool loads that calculator's own U-value tables directly rather than duplicating them, so a wall, window, door, ceiling or floor construction picked here means exactly the same U-value it would in the heating calculation. ΔT runs the other way (outdoor warmer than indoor), so heat flows into the room instead of out of it.
Solar gain and orientation
Solar gain is usually the dominant, most variable term in a cooling load — and the one a simple U·A·ΔT calculation misses entirely. It depends on the window area, how much of the incident solar radiation the glazing actually transmits (the g-value — a highly glazed, uncoated window can pass 80%+ of incident solar energy, a solar-control coating as little as 30-35%), and how much radiation hits that facade in the first place, which is why orientation matters. West-facing glazing is the conventional worst case for cooling sizing: afternoon sun arrives at a low angle straight into the glass at the same time the day's outdoor temperature peaks.
Internal gains and ventilation
People, equipment and lighting all add heat that has nowhere to go but into the room — sized here from occupant count and per-m² power densities you can adjust for the actual space. Ventilation uses the same air-heat-capacity relationship as the heating calculator's ventilation term, just with the temperature difference running the other way.
Annual energy and cost
The design-point cooling load above is one worst-case moment — the same degree-day method used by the Heat Loss calculator turns it into an annual estimate, run in the cooling direction: Cooling Degree Days (CDD) sum the whole cooling season's warm weather into one number (a hot day contributes a lot, a mild day little, a winter day nothing), instead of assuming the design-day ΔT holds for the whole year.
Q/K = Q_total / ΔT_design [W/K]
Annual cooling energy = Q/K × CDD × 24 / 1000 [kWh/yr]
Annual electricity = Annual cooling energy / COP
Annual cost = Annual electricity × price
Cooling equipment is almost always electric, so unlike the heating calculator's heat source there's no fuel-unit conversion here — picking a cooling source just sets a typical COP/EER for that equipment type (a basic window AC and a water-cooled chiller can differ by close to 2× in electricity use for the same cooling delivered), and price is always entered per kWh.