I'm looking at doing almost the same thing but via openenergymonitor.org hardware rather than RPi. OEM can monitor the power being generated by the PV, and that being consumed by the rest of the house, and (with a little extra code) turn on the immersion heater if there is sufficient surplus.
RPi will need some interfacing to be able to monitor power (unless your inverter already has Bluetooth or similar for remote monitoring). It does have onboard GPIO so that it could drive a relay directly (no USB interface required), although a Gertboard (search the RPi blog) may make life easier.
As a further trick, drive your immersion heater via a high powered dimmer circuit (triac), and you can make use of all the surplus power from your PV, even if it doesn't exceed the threshold to turn your immersion on at full power. Read the OEM forums for various discussions on that.