With solar energy. You can use solar panels, if you live somewhere with a reasonable amount of sunlight. You can even heat water a bit by pumping it through black hose when it's sunny Using solar energy for the pump). You really need to say what kind of climate you live in, whether you have access to new technology and just what kind of facilities or bits of "stuff" you have available that could be used. For instance, if you have old radiators (from an old central heating system), you can paint them black, join them into a circulation system with a tank and it should keep itself gong without any electricity, heating water.