As others have said, the solar showers work quite well but tend to lose their heat quickly. 20l is enough for a decent shower for a couple. Just rember to rinse, turn off, lather, rinse and you'll be fine. If you're anything like my wife, take a metal garbage bin, put it on the fire to warm up and have your 20 min shower
By the way, 20l of water is 20kg, 40l is 40kg. I'd avoid the 40l. 40kg is a task for some people to lift over their head and attempt to hook up to a pole.
The hand held battery powered ones with heated water are the way to go. I've seen some really interesting camp showers during my short time camping. One of the best I've seen is a cheap boat bilge pump sitting in a bucket with a basin shower head, extended with a hose... the shower heads are the ones you buy from Woolies and use for baby showers or dog showers! The guy I saw using it had it for years and had no issues. The boat bilge pumps are made to be in water and run on a 12v car battery. They won't draw enough power to drain your car battery unless you're using it for hours a day.