I am new to this forum, but not new to solar and off grid living. We have an off-grid cottage in northern Canada and we spend 5 or 6 months there in the summer. A few years ago I upgraded the solar power to get the cottage power system to operate more like on grid than off grid. This means we have the dishwasher, clothes washing machine, 2 large refrigerators, a mid sized deep freeze and electric appliances like coffee makers, kettle, microwave, really everything you would have at home including a couple of big LED TV's and satellite receiver. The exceptions are the kitchen stove/oven and the hot water heater. For hot water I use a 6 gallon RV style propane water heater. This works pretty well. We always have hot water and the little tank recovers very quickly. I have a 3800w solar array and in the summer there is lots of hours of sunshine so I have power to spare. I was looking for a way to use the excess power to heat water but importantly, I needed to protect my batteries and do this more or less automatically. I thought about diverting power from the panels to an electric water heater to preheat water for the propane heater but then I came across a very inexpensive and easy to install solution. It is a solid state relay that will turn on/off a 120 vac outlet based (loosely) on the state of charge of the battery bank. I have a 120 v -10 gallon water heater (1500w) plugged into that outlet. Of course there is a caveat - you need a charge controller or an inverter with an AUX DC power output that is programmable. Many of the inverters and charge controllers have this feature. I use a Schneider CC and inverter - both have programable AUX DC output capability either of which I can program to turn the outlet on/off based on the voltage of the battery bank. The following link is to a product sold by Sparkfun:
A solid state relay (SSR) allows you to control high-curent AC loads from lower voltage DC control circuitry. Solid state relays have several advantages ov
www.sparkfun.com
Check it out if you have the equipment to use it and don't let the cost discourage you. This set up with a $10 relay provides around half of our hot water each season. Simple to operate and protects the most important part of the power system, the batteries....bank account.
Brian