Solar power enthusiast Robin Emley has devised a simple yet effective system that diverts excess energy from his photovoltaic (solar) power system to the water heater. However energy is only diverted once their is a pre-determined excess after every cycle which avoids leaving the household without PV energy. The power is diverted by an Arduino-controlled Motorola MOC3041 and a triac bolted to a huge heatsink:
Remember when working with mains current - even if off the grid, check the relevant local licensing regulations. For more information on the project, visit Robin's detailed page with code, schematic and other notes. And we're on twitter, so follow us for news and product updates as well.
This type of project lends itself to not only Arduino-based control, but also monitoring over a network or the Internet. To take advantage of these features, consider the Freetronics EtherTen. Apart from being fully Arduino Uno-compatible, it has onboard Ethernet, microSD socket and supports optional Power-over-Ethernet: