docs(ups): document apcupsd test results and Energy Dashboard limitations

The APC Smart-UPS C 1500 (FW 02.2, mfg 2012) does not expose ups.load or
ups.realpower over USB HID. Tested apcupsd as an alternative — it reads the
same HID descriptor and returned COMMLOST. Debian's apcupsd package also
conflicts with nut-server (mutually exclusive).

Documented that the HA NUT integration provides battery/runtime/status sensors
only, not wattage for the Energy Dashboard. Practical path is a smart plug on
the UPS output. Updated daily-ops examples (removed ups.load reference since
the variable doesn't exist on this UPS).

NUT stack fully restored after the apcupsd test cycle.

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Assisted-by: Crush:glm-5.2

ups/README.md

@@ -184,8 +184,7 @@ upsc -l pfv-tsys1
 # Full variable dump for one UPS
 upsc apc-smartups-c1500@pfv-tsys1
 
-# Just the live load + runtime
-upsc apc-smartups-c1500@pfv-tsys1 ups.load
+# Battery runtime (the only runtime/charge data this UPS exposes)
 upsc apc-smartups-c1500@pfv-tsys1 battery.runtime
 ```
 
@@ -194,6 +193,18 @@ upsc apc-smartups-c1500@pfv-tsys1 battery.runtime
 - **No USB passthrough to the HA VM.** Keeping the UPS on the host preserves
   hypervisor graceful-shutdown capability and matches the `powerman/` pattern
   (PDU managed on the host where the adapter physically lives).
-- **Per-unit power (Watts):** `usbhid-ups` reports `ups.realpower` directly on
-  the APC Smart-UPS C 1500. If absent, derive Watts as
-  `load% × VA_rating × power_factor` in HA templates.
+- **No `ups.load` / `ups.realpower` on this UPS (FW 02.2, mfg 2012):** The
+  APC Smart-UPS C 1500 does not expose load or power data over USB HID.
+  Both NUT `usbhid-ups` and `apcupsd` (USB mode, tested 2026-07-30) read the
+  same HID descriptor — the variable simply isn't there. This means the HA
+  NUT integration provides **battery/runtime/status sensors only**, not
+  wattage for the Energy Dashboard.
+  - **apcupsd test note:** Debian's `apcupsd` package conflicts with
+    `nut-server` (mutually exclusive). apcupsd USB mode returned `COMMLOST`
+    even before we could check load. The APC Smart Serial protocol (serial
+    cable, AP940-1524C, ~$30) DOES report load%, but this requires a serial
+    port on the UPS and on the host.
+  - **Energy Dashboard path:** A smart plug (Shelly Plug S / TP-Link Kasa,
+    ~$15-25) on the UPS output reports real watts natively and feeds the
+    Energy Dashboard with zero UPS-driver hacking. The NUT sensors remain
+    valuable for outage detection and graceful-shutdown automations.