A "back-boiler" in these parts (USA) are offered as an accessory to an existing stove.
You can make one very inexpensively from a coil of copper tubing (mine was recycled from my old lightning still after I built a more efficient still)piped to a tank mounted above the coil. Bigger the diameter the more flow. One half inch inside diameter works well.
The heated water in the copper will rise into the above tank replacing the cold water. No electricity just physics.
If you care to have more reserve hot water, add more holding tanks in series connecting them to each other with large diameter auto radiator hose and use auto thermostadts installed in the hoses to control the temperature. Take care to position the thermostadts active side. Warm in to the top, cold out the bottom.
Happy showers!