We have long carried a backpacker's solar water heater on our trips. The bliss of warm -- nearly hot -- water in quantity for washing one's filthy self is worth the small extra weight -- especially when a packer is carrying all our gear up the first day -- the day of maximum elevation gain and minimum fitness.
Our heater is similar to this, but is a different, earlier brand.
The heater works very well when the ambient temperature is roughly 50 degrees F (10 degrees C) and heats cold stream water to about 100 degrees F after a few hours lying on a sunny rock.
We have never used the heater in cold weather, and, looking ahead to a major power failure this winter (we are overdue), we wonder if we could get warm water when the ambient T is in the 20s (F) by laying the heater on our sunny patio table (cleared of snow), and on top of an insulating sleeping pad. Assume no wind. The only time we used the heater at home was in the derecho of June/July 2012, when the ambient temperature was in the 90s; it worked well then, obviously.
If you think a patio table makes this Q off topic, assume a winter trip, a flattish rock brushed clear of snow, insulated sleeping pad, and water taken from a flowing stream. No wind.