I think we're getting the absolute best from them at present because of the sunny weather. I'm not going to assume this is typical.

I haven't come across a good system for battery charging. The thing is that you've got to be careful that the sun doesn't suddenly vanish, leaving you charging the battery from the mains. If someone invents a gadget that can use the surplus to either charge a battery or heat water, then I'll jump on it (if it's a sensible price)

electic showers are an invention of the devil as far as I'm concerned. Way too expensive for anyone to use if they're actually aware of the cost.

Towel cookery probably works even better than a slow cooker. Insulation achieves wonders. Though I might look into a slow cooker.