Propane or electric?
This is the first call to make, and it comes down to where the heater will live. Propane heaters are the portable workhorses. They run off a tank in the base, so you can roll one wherever the crowd ends up, and they put out enough heat to warm an open deck in real cold. The trade off is wind: because propane warms the surrounding air, a stiff breeze carries that warmth away, and you should never run one in an enclosed space.
Electric infrared heaters work the other way around. Instead of heating the air, they warm the people and objects in front of them, the way the sun does, so wind barely touches them. They give off no fumes, which makes them the only safe pick for a screened porch or a covered patio. The catch is that they need an outlet or hardwiring, so they stay put once mounted.
Sizing it to your patio
Heat is measured in BTUs, and a quick way to estimate what you need is to multiply your patio square footage by 20. A 10 by 20 foot patio works out to about 4,000 BTUs. Electric models are usually listed in watts, so multiply the wattage by 3.41 to compare: a 1,500 watt heater lands around 5,100 BTUs.
Placing it safely
Placement is the part most people skip. Keep at least three feet of clearance above and around the element, away from walls, railings, and anything fabric, and give propane units extra room overhead for exhaust. Choose a portable heater with a tip over switch that cuts the fuel or power if it gets knocked over, which matters on a windy night or a busy patio. Keep propane outdoors only, since it needs open airflow, and use electric for anything covered. When in doubt, the manual lists the exact clearances for your model.