Sounds like a plan. Definitely, you'd want to use a propane or oil fueled heater.
As a comparison to what I mentioned earlier, a 5kW (17kBTU) electric space heater consumes around 5kW, the actual value is determined by your line voltage. If the heater is rated at 230V, and you actually have 230v, then 5kW it is. Some houses has as much as 245 volts, some, as low as 205 or so (if you happen to be connected to a 3 phase network, pretty unlikely. BGE still uses lots of 117/234 transformers. more voltage, more heat and power consumed. Reverse for lower voltages.
On the other hand, a mini-split, with 17kBTU of cooling and 19kBTU of heating uses 9.08 amps at 230 volts. Accounting for power factor, actual consumption is 2.09kW. This from a unit with a SEER number around 10.
The choice is yours: 17kBTU of heat from a unit heater for 5kW of electricity, or 19kBTU of heat for ~2.1kW of electricity.
Efficiency can be a misused word also. Unit heaters are efficient at converting electricity to heat. They are not efficient at heating a room, coupling the heat to a room uniformly. Electricity is not an efficient power to use for heat. There may be many variables and losses of efficiency, especially in a ducted system, one where a furnace and condensing unit are upgraded and the rest of the system left in place, as in a finished home, but heat pump use (an electrical system to transfer heat, as opposed to creating it) is much more effective at heating a space at a lower cost, until the temperature drops very low. It's all a moving target with lots of variables.
Still, the propane unit would be better if the flame is not in the workshop.
BTW, unit heaters do a pretty awful job of making a space comfortable. In warehouses, etc., they use them because they are cheap to install. Fans go bad, especially when they are circulating sawdust. For a space you need to live, work and function in, perimeter baseboard heaters are the way to go, electric or water. The upsides of a unit heater is that they can change room temperature fairly quickly, compared to baseboard units, they are about as cheap as you can get, and they are easy to install.
I've been directly involved in the design, installation, repair, removal and replacement of countless such units in over 40 years. I heat my garage with propane, and I can get a unit heater free. I like to be comfortable.
Ideally, use a propane (or oil!) fired water heater in another space, Teco/B&G circulating pump and water baseboard units; thermostat turns the pump on/off. Beats a fan forced heater any day, keeps the whole space warm and doesn't have an open flame in a room where combustible dust, flammable liquids and explosive gasses may be present during production.
I've gotten away with a propane heater in the garage, but can't help the feeling that things are going to explode any minute.
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