If backup power wasn’t a concern then the enphase battery would be the most logical option being long life, high quality, modular and uses the same monitoring system.
If backup is a must then any AC battery solution would work of which there are a variety of options such as Tesla Powerwall 2, TrinaBESS AC or Sonnen ECO. The Tesla is far too large at 14kWh so the Sonnen or TrinaBESS are modular and both offer backup.
The Sonnen is the premium option with a very long battery life but also the most expensive while the TrinaBESS Powercube AC has a very good backup power rating. See the review below for more info