Sense data buffer?

[RyanAtSense]

What happens if Sense loses Wi-Fi connection?

If Sense is unable to reconnect, the monitor can hold at least six hours of device-specific, per-second offline 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 a minimum of three months. Note that in cases where multiple power outages occur while Sense is disconnected from Wi-Fi, further data loss can occur.

So, 6 hours for real-time data. However, if multiple power outage occur, you’ll lose that data as it is on volatile memory.