Stop drawing a stale meter as 0 W balanced

.changeset/stale-meter-no-fake-zero.md

@@ -0,0 +1,5 @@
+---
+"ftw": patch
+---
+
+A meter that has stopped reporting no longer shows as 0 W balanced. Grid and house load go blank, and solar or battery that went quiet with it stay on the diagram as no data instead of vanishing.

go/internal/api/api.go

@@ -1141,13 +1141,11 @@ func (s *Server) handleStatus(w http.ResponseWriter, r *http.Request) {
 		"mode":                  ctrl.Mode,
 		"troubleshooting_mode":  troubleshootingMode,
 		"plan_stale":            ctrl.PlanStale,
-		"grid_w":                gridW,
 		"pv_w":                  pvW,
 		"pv_w_predicted":        pvPredictW,
 		"bat_w":                 batW,
 		"ev_w":                  evW,
 		"v2x_w":                 v2xW,
-		"load_w":                loadW,
 		"load_w_predicted":      loadPredictW,
 		"bat_soc":               avgSoC,
 		"grid_target_w":         ctrl.GridTargetW,
@@ -1177,6 +1175,18 @@ func (s *Server) handleStatus(w http.ResponseWriter, r *http.Request) {
 		// (under). Idle slots (|planned| ≤ 50 Wh) are ignored.
 		"slot_delivery_stats": ctrl.SlotDeliveryStats,
 	}
+	// A stale or missing site meter is not 0 W. Publishing zero made the
+	// dashboard and the FTW app draw "balanced" / "0 W" as if the house
+	// were idle. JSON null is what the flow mapping already treats as
+	// "no data". Development setups with no configured meter keep the
+	// historical zero (haveGrid is forced true above).
+	if haveGrid {
+		resp["grid_w"] = gridW
+		resp["load_w"] = loadW
+	} else {
+		resp["grid_w"] = nil
+		resp["load_w"] = nil
+	}
 	if energyToday != nil || energyCurrentSlot != nil {
 		energy := map[string]any{}
 		if energyToday != nil {

go/internal/api/api_test.go

@@ -296,6 +296,59 @@ func TestHandleStatusKeepsFaultedSiteMeterReading(t *testing.T) {
 	}
 }
 
+func TestHandleStatusOmitsWattsWhenSiteMeterIsOffline(t *testing.T) {
+	tel := telemetry.NewStore()
+	ctrl := &control.State{SiteMeterDriver: "ferroamp"}
+
+	tel.Update("ferroamp", telemetry.DerMeter, 2400, nil, nil)
+	soc := 0.55
+	tel.Update("ferroamp", telemetry.DerBattery, -500, &soc, nil)
+	tel.Update("ferroamp", telemetry.DerPV, -1800, nil, nil)
+	tel.RecordDriverSuccess("ferroamp")
+	tel.DriverHealthMut("ferroamp").SetOffline()
+
+	srv := New(&Deps{
+		Tel:        tel,
+		Ctrl:       ctrl,
+		CtrlMu:     &sync.Mutex{},
+		CapMu:      &sync.RWMutex{},
+		Capacities: map[string]float64{"ferroamp": 10_000},
+		CfgMu:      &sync.RWMutex{},
+		Cfg:        &config.Config{},
+	})
+	req := httptest.NewRequest(http.MethodGet, "/api/status", nil)
+	rr := httptest.NewRecorder()
+	srv.Handler().ServeHTTP(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, body: %s", rr.Code, rr.Body.String())
+	}
+
+	var raw map[string]any
+	if err := json.Unmarshal(rr.Body.Bytes(), &raw); err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := raw["grid_w"]; !ok {
+		t.Fatal("grid_w key missing; want explicit JSON null, not an omitted field")
+	}
+	if raw["grid_w"] != nil {
+		t.Fatalf("grid_w = %v, want null so the UI cannot draw 0 W balanced", raw["grid_w"])
+	}
+	if raw["load_w"] != nil {
+		t.Fatalf("load_w = %v, want null", raw["load_w"])
+	}
+	drivers, _ := raw["drivers"].(map[string]any)
+	ferroamp, _ := drivers["ferroamp"].(map[string]any)
+	if ferroamp["status"] != "offline" {
+		t.Fatalf("driver status = %v, want offline", ferroamp["status"])
+	}
+	if ferroamp["pv_w"] != -1800.0 {
+		t.Fatalf("pv_w = %v, want last-known -1800 so the UI can keep a solar node", ferroamp["pv_w"])
+	}
+	if ferroamp["bat_w"] != -500.0 {
+		t.Fatalf("bat_w = %v, want last-known -500 so the UI can keep a battery node", ferroamp["bat_w"])
+	}
+}
+
 func TestHandleV2XPolicyReturnsLiveEnvelope(t *testing.T) {
 	tel := telemetry.NewStore()
 	tel.Update("meter", telemetry.DerMeter, 1500, nil, nil)