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My PV Thor with Multipls II inverter


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 My question is, does anyone have successful experience of diverting excess solar to hot water in an off-grid system ?  

     I am trying to find a way to use off the shelf components to divert excess solar to hot water in Off Grid systems. There are many products that do this for grid tied solar, that rely on sensing power exported to the grid, and divert proportionally to the immersion element. I have used the Marlec iBoost, which works well.

     For my own Off Grid system I have done this using an Arduino and a Conversol 5 Kw inverter charger. The Conversol provides a serial data string to the Arduino, including data for battery volts, battery current, load current etc. Using battery current as an alternative to grid export, means I can power the immersion via a proportional 240v relay. When the battery reaches absorbtion voltage, the Arduino starts to divert current to the immersion. Power to the immersion increases when the batteries are being charged, and decreases if power is being drawn from the batteries.  This is not something I am happy to install for other people, it is a bit too home made !  

   The "My PV Thor" is quite expensive, but it works very well when coupled with a grid tie inverter. It responds to frequency shift by the Multiplus inverter. When surplus power is produced by the PV inverter it charges the batteries through the AC output of the multi.  When the batteries are at float, the Multi increases the grid frequency to prevent over charging. The Thor starts to dump power to the immersion element as soon as the frequency rises. The PV inverter will shut down if the frequency continues to rise. There are draw backs to this because the battery charging is not very accurate, and the return to 50 Hz is slow. That means the Thor will continue to draw power, even when there is no surplus, potentially overloading the Multi and draining the batteries.  

   What I would like to do is couple the My PV Thor to the Multi directly, without the frequency shift function. I do not know of a way to do that. Any ideas or useful tips ?



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