Concluded the experiment at 15:50.
I was just curious about the results so I won't call it a success. The keg was so hot I couldn't touch it, and I couldn't hold my hand under the water when I opened the tap (which works fine). The flowing water was steaming btw. I can't say when the water reached the top temperature. Maybe when I wrote it hurt to touch it was done (at about 12:30, 2 hours after start).
I did not do anything particular with the water I'm glad with the experiment disinfect the keg. But in the future 5 liters of hot water can be used for do the dishes for example, the temperature compares very well against electric boilers. If I'd had an insulated box (maybe with polystyrene?) I could store the keg in and preserve the temperature for couple of hours.
I have a shallow metal pot, with a glass lid. Black the outside, blue in the inside. Can hold 2 liters of water, topped out, so I'll try with 1,5 liters tomorrow, although expecting some cloudy weather. If it boils (don't need rolling boil), then this thing can be used to cook. Now I'm thinking about shades and regulating power output.