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 WiFi. 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.