Sounds like your requirement is exactly the same as ours.  We originally purchased the Sunamp PV specifically because it did this, although it's capacity was the main limit, at just 4.5 kWh.  In practice this didn't matter too much, as it would accept any charge that was available, so tended to top up pretty quickly.   As an example, this is the data for the past week, the first week of energy monitoring that has included measuring the energy used to charge the Sunamp from the grid, th