If you happen to have an old Ferraris meter without a ratchet it can even run backwards when you produce more than you consume. And, of course the 800 Watt EU limit is a worst case limit. If you feed in on a dedicated (no other consumers or generators) line with its own fuse and sufficient wire crossection you can feed in up to about 2 kW. Be aware that the fuse will not trigger if you have a near-short while you generate peak or near peak which can cause overheating and is a fire hazard. While none of the above is legal and you will lose house insurance in case some electric-related fire event occurs it is safe technically.