Get a Genuinely Hot Shower From a 20L Solar Shower Bag

A black or dark-coloured solar shower bag left in direct sun for a few hours can genuinely reach a properly hot temperature, hot enough to scald if you're not careful, which is a big step up from the lukewarm trickle most people expect from camp showers. The trick is timing, positioning and a bit of insulation, not a better bag.

This is worth using for car camping, a static off-grid camp, or a garden shower during a power cut in warm weather. It is not a fast solution: on a cool or overcast day it may only ever reach tepid, and it's not a substitute for washing water when you need it immediately.

1. 01

   Fill the bag with the coldest water you have first thing in the morning. Starting early gives the bag the maximum hours of sun to work with; filling at midday wastes the strongest heating window.

2. 02

   Lay it flat, dark side up, in full unobstructed sun. Flat and exposed heats faster than hanging, because more of the water mass sits close to the sun-facing surface; hang it only once it's warm and you're ready to use it.

3. 03

   Add a reflective or dark backing under the bag if you have one. A dark tarp or foil-backed mat underneath boosts the heat reaching the underside of the bag, especially useful on cooler days.

4. 04

   Check the water by hand periodically through the afternoon, never assume. Solar bags can get hot enough to scald, particularly the water nearest the black surface, so test before anyone showers.

5. 05

   Hang the bag once warm, an hour or so before use, at a height that gives decent pressure. Higher hanging gives more flow force through the nozzle, at the cost of a slightly awkward reach.

6. 06

   Mix in cool water at the tap or nozzle if it's too hot. Most bags have a simple on/off valve rather than a mixer, so temper by briefly adding cooler water from a second container if needed, or let it cool a few minutes before use.

The black or dark PVC absorbs solar radiation far more efficiently than a light-coloured surface would, converting sunlight to heat inside a fairly thin water layer. Because the water volume is shallow relative to its surface area when laid flat, that heat transfers through the whole 20 litres reasonably evenly over a few hours, rather than just warming a thin surface layer the way a still pond would.

Take the scald risk seriously. Water in a dark bag in strong sun can climb well above a comfortable shower temperature and cause real burns, especially to children or anyone with reduced sensation. Always test the water on the inside of your wrist before anyone uses it, and consider a temporary shade cover once it's reached the temperature you want, so it doesn't keep climbing unattended.

The other real hazard is bacterial growth, particularly Legionella, which thrives in stagnant water sitting in the lukewarm range for extended periods, roughly body temperature up to around 45°C. A bag used the same day it's filled and then emptied is low risk. A bag left partially full, refilled on top of old water, or left standing for days in warm weather is a genuine risk, especially if the water is later used in a way that creates a fine mist or spray you might inhale. Empty, rinse and dry the bag between uses rather than topping it up, and don't leave standing water in it for extended periods in warm conditions.

Whatever temperature it reaches, water from a solar shower bag should not be treated as drinking water. These bags are not food-grade sealed systems, they sit in the sun accumulating whatever is in the source water and anything that gets into the fill opening, and there's no disinfection step. If you need drinking water in an emergency, follow proper treatment methods instead, such as those covered in our guide to emergency water purification.

Filling with already-warm water and expecting it hotter, hanging the bag in partial shade, and leaving it topped up and unemptied for days are the most common ways people either get a disappointing wash or create the stagnant-water risk above.

A dark bin bag or dark container of water left flat in the sun works on the same principle, though without the built-in nozzle you'll need to decant carefully. For a genuinely cold snap, look instead at heating one room without power and skip washing priorities until things warm up.

A solar shower bag cannot deliver hot water reliably on a cold, overcast day, cannot provide continuous flow the way a plumbed shower does, and should never double as a water storage or drinking vessel.

• US CDC on Legionella and water systems: CDC
• General outdoor and camping safety: ready.gov

If you're setting up a longer static camp, pair this with our notes on tarp rainwater catchment for topping up your wash-water supply without touching drinking stores.