Watts per plant is not an accurate measurement really. Watts per square foot or square meter is a better way to gauge. Some ppl have SOG set ups with plants taking up a 6”x6” area and some ppl have plants that fill a whole 4x4.
HPS may be the way to go for that heat factor.. but ultimately it’s not best to rely on the light to get you an additional 20 degrees C for your space. Is it gonna drop to 5 with lights off? That’s bad news for the plant.
I’d look at bar light type set ups like mars Hydro FC3000 or FC4800 and maybe supplement heat