1500 watts of electrical power converts to 5120 BTU no matter how you slice it.
You are absolutely correct. Some units may deliver the heat in a somewhat more
desirable manner, but it is still 5120 BTU/1500 watts.
Obviously, we are talking about resistance heating. A heat pump will deliver
more than 3.414 BTUs per watt (ie, 5121BTUs from 1500 watts). And obviously
the portable unit being advertised is not a heat pump and you were not
including heatpumps when making your comment.
FYI: Sometime I can't get a furnace working immediately and I have to
give some advise to a homeowner who faces cold outdoor temps and no
furnace for a while. It amazes me how difficult it is to convince people
that every watt of electricity consumed in the average home is going to
generate 3.414 BTUs of heat. Yes, there is a very slight bit of BTU loss
due to light that escapes out of a window, etc. There is also a small
transient BTU gain to the living space from a fridge or freezer since these
are "heat pump" devices. But that is splitting hairs.
It is very easy to do a quick walk through a house to estimate how much
heating can be produced by leaving on every reasonable electrical device.
Then, that can be compared with the heat output of the furnace and the
anticipated cycling of the furnace under current ambient temps.
Toss in one or two small electric heaters and a bathtub full of max temp
hot water and the homeowner can protect his home and insure some
comfort until the furnace is working again. Obviously this isn't the most
economical way to heat a home, but it works in an emergency. Let's
see - burn $0.50 to $1.00 per hour to heat the house overnight or risk
getting frozen pipes, cracked walls, etc? Easy decision.
The laws of physics don't really care if it is a ceramic heater or a non-
ceramic heater. They don't care if it is a heater, an incandescent light bulb,
a fluorescent light bulb, a TV set, a PC, etc, etc. One watt of indoor
electrical usage is going to ultimately generate 3.414 BTUs of heat.