You want 120 Volt plug in? And a 1500 Watt unit is no good.
Power (in Watts) = V x I
That's basic physics and the only way to get more heat than that from electricity is to use a heat pump.
You are limiting this to 120 Volts and the largest 120 Volt circuit that you are likely to have is going to be 20 Amps, many are only 15 Amps. So...
P = 120 V x 20 A = 2400 Watts.
That is a total maximum amount of heat that is possible on a single 120 Volt circuit. But 120 Volts is not often what you get. 115 Volts is more like it. And drawing a full 20 Amps is a good way to have the breaker pop and no manufacturer wants returns because your breaker is a bit weak. They do not make appliances that draw a full 20 Amps. So lets say you find a unit that draws 18 Amps. Now you are down to:
P = 115 V x 18 A = 2070 Watts.
You would be better off (3000 Watts) with two of the "household" heaters that you don't want. Or you will have to give up that 115 V thing and get something that runs on 230 V.
Beyond that, something troubles me here. You say you have TWO propane, stand alone heaters that you use all the time. AND you are still alive. Alive, as in not a victim of CO or CO2.
Therefore you must have a LOT of outside air coming into the shop. In the winter that would be COLD air that is coming in. You are fighting things here.
I would first talk to a real HVAC tech. about a real heating system. And then, after that is installed, talk to a contractor about sealing up your building and adding some effective insulation. That is the real solution here. Not just adding a band-aid on top of a totally wrong situation.
If you must have a temporary solution, get two or even three of the radiator style, free standing, electric heaters. They are fairly inexpensive and can be plugged into 115 Volt outlets. They do work. I have used them for emergency heat in my trailer when I was living in Iowa. I have one in my bedroom and another one in my shop right now. They work. And nothing you can buy, other than a heat pump, will give you more heat on a given electrical circuit. Black holes can break the laws of physics, but your shop heater can not.
Under certain cold conditions I like to pump some auxiliary heat into the shop. For doing this, I typically use two freestanding 20-lb propane tanks, each with a heating head; the faces of these heads glow redhot and put out a lot of heat. I would like to find a 3rd heater, for emergency use when a tank runs out of propane, to add to the mix, and I would like a heater that is electrical, with a 120v plugin. I had one, an old fan forced GE baseboard heater that had excellent heat output, but it quit and I tossed it. In modern times, I think that thing would be judged dangerous and kept off the market. If anyone knows of an electric heater that might fit the bill, I would appreciate recommendations. I am not interested in the many "1500 watt" household heaters that are pictured keeping mommy and baby warm and cozy in the playroom. Also would like to avoid big dog heaters, like salamanders, that use kero or similar and need better ventilation that I will have in the space where the heater will be used.
-Marty-