About 10W/sq ft: a 400 sq ft room needs ~4000W (use hardwired/gas or multiple zones); a standard 1500W plug unit covers ~150 sq ft well.

Not efficiently — they are for zone heating one room. Use them to supplement a weak central system, not replace it.

Electric is safe and simple up to your circuit limit; gas/propane delivers more heat for very large or uninsulated spaces but needs venting.

Drafts, high ceilings and poor insulation eat heat; seal gaps, use ceiling fans to push warm air down, and size up.

High-output units need stable placement, tip-over/overheat cutoffs and clearances; hardwired models should be installed by a pro.