🌡️What does an electric space heater cost to run?

None of them, and the question is the trap. At the same wattage, every electric resistance heater converts electricity to heat at exactly 100%, so a 1,500 W oil-filled radiator and a 1,500 W fan heater cost identically per hour. Marketing claims of an efficient heater are about how quickly the room feels warm, not about the energy used.

Often yes, if you genuinely stop heating the rest. One 1,500 W heater is far less than a central system warming five rooms. It stops being true when the space heater runs alongside the central heating rather than instead of it, or when the room is large — above about 150 sq ft a single plug-in heater cannot keep up in a cold room anyway.

It saves hours, which is the same thing. Once the room is at temperature a thermostatic heater cycles rather than running flat out, so a heater that is on for six hours may only draw power for three. When entering hours here, use the time it is actually heating, not the time it is switched on.

Eight hours at 1,500 W is 12 kWh — $2.04 at $0.17, £3.00 at £0.25, €4.20 at €0.35. Doing that every night through a winter is $180 to $380 depending on where you live, for one room, which is usually the point at which a heat pump starts to look like a purchase rather than an expense.

The supply voltage. A 15-amp circuit at 120 V carries 1,800 W on paper and 1,440 W as a continuous load; at 230 V the same current carries twice as much. It is the wiring, not the heater design — and it is why an American plug-in heater tops out where a European one is only half way up.

Not per unit of energy — it is still 100% resistance heating. What it does differently is heat objects and people directly rather than the air, so you can feel warm sooner and sometimes at a lower air temperature. If that lets you run it fewer hours, the bill falls; the rate per hour does not.