V is the room volume in m³: floor area times height. Δt is the difference between the outside temperature and the temperature you want. K is the dispersion coefficient and expresses how fast the space loses heat.
For a well insulated building use K around 0.5; for an ordinary existing building 0.8; for a poorly insulated warehouse 1.2; for a marquee or building shell 2.
Example: a warehouse of 400 m² and 4 m high is 1,600 m³. To raise it 15 °C in a poorly insulated space: 1,600 × 15 × 1.2 = 28,800 Kcal/h, divided by 860 is roughly 33 kW.
Deliberately allow margin when doors open frequently or the space has to start from cold.