With a high efficiency light source with 50% efficiency, half the power is radiated as heat and the other half as light. Almost all light is converted to heat after it hits the walls and plants. Only a small fraction of light is used by plants for photosynthesis.
The BTU requirement for a grow space can be calculated using the formula below. Only for non forced air duct cooled lamps.
P(BTU/hr) = 3.412141633 × P(W)