For outdoor heating short wave infrared radiant heat is usually the most efficient heating solution. This is because short wave infrared heat works like the sun and heats objects and not the air. In outdoor settings heating the air is often inefficent as any wind will blow away the warm air.
Infrared heaters are most well known for keeping punters warm in pub gardens, but they work quickly and are relatively cheap to run. They can be used indoors or outdoors equally well.
The different types of portable infrared heaters come with different heat outputs. The most common outputs are 1.5 kilowatt (kW), 2 kW and 3kW. The higher the kilowatt, the more powerful the heater is but it will also be more expensive to run.