Free HVAC Load Estimate calculator with clear step-by-step results.
Splits the load into the three components that dominate: fabric conduction through the envelope, glazing which loses far more per square metre than wall, and ventilation air that must be heated from outside to inside temperature. Cooling adds occupant and equipment gains plus solar through the glass.
Fabric loss
Watts = envelope area x U-value x design temperature difference
Ventilation loss
Watts = volume x air changes per hour x 0.33 x temperature difference
Simplified estimate only. Heating and cooling equipment must be sized using a recognised load calculation method and installed to applicable codes and manufacturer requirements.
It is a sanity check, not a design. A proper room-by-room calculation to a recognised method is needed before specifying equipment, particularly for a heat pump where oversizing causes cycling.
It benchmarks the building. Above roughly 100 W/m2 suggests a poorly insulated envelope; a well-built modern house sits nearer 30-50 W/m2.