What are you using to voltage control the relay? I was thinking of doing this for the watermaker to make use of surplus solar power.
(I sometimes have a relay in line with the thermostat, when the bateries hit 13.5 volts, the compressor runs and at high speed. It works great in the summer to store extra energy. When the relay is connected, it means I have to pull the fuse. I ought to make a switch for the thermostat/relay variations, but I'm still not done designing it so the fridge doesn't get too cold.)