How do I use a 680 Volt (DC) solar panel with an off the shelf system?
240V from a decent H bridge needs a 350V DC supply roughly and I would think about making 350V DC (from the 680 volts) with another H bridge driver circuit dedicated to converting 680 volts DC to 350 volts DC using a transformer, rectifer and smoothing capacitor.

I'm chickening out a bit here because, I feel, using a H bridge to convert straight to 240V AC may be too big of a step but, others may offer advice to the contrary and I will look forward to their suggestions.