research: battery is backup-only by design

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U8EHjVrf5ffw1Vjn5bPGJu

research/energy/README.md

@@ -29,4 +29,7 @@ SDM630 house meter: 645 W vs 640 W):
 | 526 | today load | /10 kWh | 24.2 |
 | 529 | today PV production | /10 kWh | 29.9 |
 
+The battery is configured as **backup only** (power cuts), on purpose: day and night the house
+runs on PV and the grid, so a full battery with ~0 W flow is the normal state.
+
 Fronius Symo 3.7 (`192.168.0.143`, Solar API JSON) produced another 4.4 kWh that day.