In my experience with IR heaters, Those two 120 volt 1500 watt heaters will not be enough for that patio. I recently did a job and installed (2) 3000 watt 240 volt IR heaters on a covered 12x16 screened porch, and that was border line for being enough. Four would have been better. The thing about IR heaters is that they don't heat the air, they heat objects. If you don't have one aimed at you, you will not feel it. IR heaters can get expensive real quick trying to heat your size of patio, and need a lot of power. There are some calculators online that will help figure what you need
Personal experience with 1500W IR heaters here. We did a family Thanksgiving outside last year in my back yard. We had 8 people sitting around a large table (no tent or anything, but we did have some wind breaks), withfour 1500W IR heaters aimed at it from all sides. After sundown, it was below 40F, and while we were wearing sweaters, we all had a comfortable meal.
They don't make the space warm, but they make it quite bearable. When you stepped away from the table, it felt pretty cold (we used steam pans to keep the food from getting immediately chilled). Yes, they're expensive to run, but there's no fuel to store, and
you only pay when you're running them. I figure that I spent $6 on electric to run the heaters. I spent more on sterno to keep the food warm. If I were trying to condition a space with them, the budget would be crazy. For the limited hours a month they get used, they're well worth the electrical cost.
So, here's what you need to know.
First, these small IR heaters have a sweet spot for reach. Figure 6-8 feet. Within 3' of mine, and you're cooking. Bigger 240V versions will heat a wider area, but on the 120V plug in stuff, figure that you can warm up two people per heater max.
Second, they heat surfaces, not objects. The distinction is important, because if you have a cushion on your chair and you're wearing a jacket and the IR is hitting you from behind, all of that stuff will block the heat, and only your exposed areas (ears) will be warmed by it. Now, I've got wrought iron patio furniture, so cushions are important to keep the metal from conducting heat away from you, but basically, if you're well covered up, it doesn't do too much. Still, if you aim it to warm your face and hands, which you'll want exposed anyway, it might be just what you need.
Third, 1500W heaters consume a 20A circuit by themselves. I happen to have four different outdoor outlets within reasonable extension cord reach of my back yard. One is on my garage. The other three were home runs back to a box right by my panel, so I was easily able to split them onto separate circuits, so I lucked out. Otherwise it would have been super expensive to do this. So, basically, plan it out so you don't get stuck with too little heat and no way to add more.
