I'm looking for a way to partially power an applicance (a fridge freezer for argument's sake) using solar/renewable energy.
I don't expect the couple of hundred watts of solar power to be sufficient to fully power what I have in mind, but wondered if such a device as the following exists or is easily made so as to reduce energy bills but not have to interfere with the incoming mains power supply or investing in a bank of batteries.
What I'm imagining is a solar inverter which plugs into the mains (230V AC) and also takes 12V DC as input (from the panels), and only uses mains power to "top up" any requirements not met by solar, with a 3 pin UK plug socket as output to plug the appliance into?
Does such a device exist, and if it doesn't is there good reason for it not to exist?
 I've found this http://www.amazon.co.uk/dp/B00C0D8C3W but it's unclear whether it will flow back into the grid?