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Source: https://outdoors.stackexchange.com/q/19187
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

<p>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.</p>

<p>Our heater is similar to <a href="https://www.walmart.com/ip/5-Gallon-Super-Solar-Sun-Backpacking-Camping-Hiking-Outdoor-Shower-Heats-Water/39805109?wmlspartner=wlpa&amp;selectedSellerId=2542&amp;adid=22222222227027556581&amp;wl0=&amp;wl1=g&amp;wl2=c&amp;wl3=54698439671&amp;wl4=pla-90025315031&amp;wl5=1014895&amp;wl6=&amp;wl7=&amp;wl8=&amp;wl9=pla&amp;wl10=113134860&amp;wl11=online&amp;wl12=39805109&amp;wl13=&amp;veh=sem" rel="noreferrer">this</a>, but is a different, earlier brand.</p>

<p>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.  </p>

<p>We have never used the heater in cold weather, and, <strong>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.</strong>  Assume no wind. The only time we used the heater at home was in the <a href="https://en.wikipedia.org/wiki/June_2012_North_American_derecho" rel="noreferrer">derecho</a> of June/July 2012, when the ambient temperature was in the 90s; it worked well then, obviously.</p>

<p>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.    </p>