I've used these for years on my airplanes. With inlet covers on and a blanket over the cowling of the RV-10, the entire engine compartment will be over 70 degrees, with oil between 85-95, in about 4-5 hours.
Tuesday, when I was parked outside in 15 degree weather at KFDK for 3.5 hours, I just plugged it in with the inlet covers installed (no blanket). The oil stayed over 100 degrees and CHT's were about 90 when I started up.
Yesterday morning it was 18 here again in Atlanta and I had to go to another airport for a couple of hours. Not knowing if there was power where I was parking, I grabbed a charged Odyssey PC925, an inverter, and an extension cord. I was on the ground for about 3.5 hours again and the engine was still very nice and warm with only the inlet covers.
I measured the battery draw and it appears to be around 7 amps, so the battery is good for about 3-4 hours. Works as a nice portable setup when needed.
Vic