If Sense loses its connection to the network and is unable to reconnect, the monitor can hold up to six hours of device-specific, per-second data. After that space is exhausted, the data may be lost, but the monitor will try to preserve per-minute data for whole home consumption and solar production for up to three months.
This assumes that the Sense is still receiving power while it is disconnected from Wi-Fi. In cases where multiple power outages occur while Sense is disconnected from Wi-Fi, or if the Sense is rebooted before it can reconnect to the server, cached data will not be retained.