Interactive tool
What size heater do I need?
Enter the room, tell us how well it holds heat and how cold it gets, and we'll give you the watts and BTU you need, the class of heater that delivers it, and what it costs to run at your electricity rate. The honest takeaway for most rooms: a single 1,500W plug-in is enough to supplement the furnace, and not enough to replace it.
How this works. We start at 10 watts per square foot — the standard rule of thumb for holding a comfortable temperature in an average room in a cold climate — then adjust for ceiling height, insulation, climate, and whether the heater is doing the whole job or helping the furnace. Treat the result as a planning number, not a guarantee: a room with a big north-facing window or a leaky door will want more.